Gcc сообщения об ошибках

Check the code for syntax errors, but don’t do anything beyond that.

Limits the maximum number of error messages to n, at which point
GCC bails out rather than attempting to continue processing the source
code. If n is 0 (the default), there is no limit on the number
of error messages produced. If -Wfatal-errors is also
specified, then -Wfatal-errors takes precedence over this
option.

Inhibit all warning messages.

Make all warnings into errors.

Make the specified warning into an error. The specifier for a warning
is appended; for example -Werror=switch turns the warnings
controlled by -Wswitch into errors. This switch takes a
negative form, to be used to negate -Werror for specific
warnings; for example -Wno-error=switch makes
-Wswitch warnings not be errors, even when -Werror
is in effect.

The warning message for each controllable warning includes the
option that controls the warning. That option can then be used with
-Werror= and -Wno-error= as described above.
(Printing of the option in the warning message can be disabled using the
-fno-diagnostics-show-option flag.)

Note that specifying -Werror=foo automatically implies
-Wfoo. However, -Wno-error=foo does not
imply anything.

This option causes the compiler to abort compilation on the first error
occurred rather than trying to keep going and printing further error
messages.

Issue all the warnings demanded by strict ISO C and ISO C++;
diagnose all programs that use forbidden extensions, and some other
programs that do not follow ISO C and ISO C++. This follows the version
of the ISO C or C++ standard specified by any -std option used.

Valid ISO C and ISO C++ programs should compile properly with or without
this option (though a rare few require -ansi or a
-std option specifying the version of the standard). However,
without this option, certain GNU extensions and traditional C and C++
features are supported as well. With this option, they are diagnosed
(or rejected with -pedantic-errors).

-Wpedantic does not cause warning messages for use of the
alternate keywords whose names begin and end with ‘__’. This alternate
format can also be used to disable warnings for non-ISO ‘__intN’ types,
i.e. ‘__intN__’.
Pedantic warnings are also disabled in the expression that follows
__extension__. However, only system header files should use
these escape routes; application programs should avoid them.
See Alternate Keywords.

Some warnings about non-conforming programs are controlled by options
other than -Wpedantic; in many cases they are implied by
-Wpedantic but can be disabled separately by their specific
option, e.g. -Wpedantic -Wno-pointer-sign.

Where the standard specified with -std represents a GNU
extended dialect of C, such as ‘gnu90’ or ‘gnu99’, there is a
corresponding base standard, the version of ISO C on which the GNU
extended dialect is based. Warnings from -Wpedantic are given
where they are required by the base standard. (It does not make sense
for such warnings to be given only for features not in the specified GNU
C dialect, since by definition the GNU dialects of C include all
features the compiler supports with the given option, and there would be
nothing to warn about.)

Give an error whenever the base standard (see -Wpedantic)
requires a diagnostic, in some cases where there is undefined behavior
at compile-time and in some other cases that do not prevent compilation
of programs that are valid according to the standard. This is not
equivalent to -Werror=pedantic: the latter option is unlikely to be
useful, as it only makes errors of the diagnostics that are controlled by
-Wpedantic, whereas this option also affects required diagnostics that
are always enabled or controlled by options other than -Wpedantic.

If you want the required diagnostics that are warnings by default to
be errors instead, but don’t also want to enable the -Wpedantic
diagnostics, you can specify -pedantic-errors -Wno-pedantic
(or -pedantic-errors -Wno-error=pedantic to enable them but
only as warnings).

Some required diagnostics are errors by default, but can be reduced to
warnings using -fpermissive or their specific warning option,
e.g. -Wno-error=narrowing.

Some diagnostics for non-ISO practices are controlled by specific
warning options other than -Wpedantic, but are also made
errors by -pedantic-errors. For instance:

-Wattributes (for standard attributes)
-Wchanges-meaning (C++)
-Wcomma-subscript (C++23 or later)
-Wdeclaration-after-statement (C90 or earlier)
-Welaborated-enum-base (C++11 or later)
-Wimplicit-int (C99 or later)
-Wimplicit-function-declaration (C99 or later)
-Wincompatible-pointer-types
-Wint-conversion
-Wlong-long (C90 or earlier)
-Wmain
-Wnarrowing (C++11 or later)
-Wpointer-arith
-Wpointer-sign
-Wincompatible-pointer-types
-Wregister (C++17 or later)
-Wvla (C90 or earlier)
-Wwrite-strings (C++11 or later)

This enables all the warnings about constructions that some users
consider questionable, and that are easy to avoid (or modify to
prevent the warning), even in conjunction with macros. This also
enables some language-specific warnings described in Options Controlling C++ Dialect and Options Controlling Objective-C and Objective-C++ Dialects.

-Wall turns on the following warning flags:

-Waddress
-Warray-bounds=1 (only with -O2)
-Warray-compare
-Warray-parameter=2 (C and Objective-C only)
-Wbool-compare
-Wbool-operation
-Wc++11-compat  -Wc++14-compat
-Wcatch-value (C++ and Objective-C++ only)
-Wchar-subscripts
-Wcomment
-Wdangling-pointer=2
-Wduplicate-decl-specifier (C and Objective-C only)
-Wenum-compare (in C/ObjC; this is on by default in C++)
-Wenum-int-mismatch (C and Objective-C only)
-Wformat
-Wformat-overflow
-Wformat-truncation
-Wint-in-bool-context
-Wimplicit (C and Objective-C only)
-Wimplicit-int (C and Objective-C only)
-Wimplicit-function-declaration (C and Objective-C only)
-Winit-self (only for C++)
-Wlogical-not-parentheses
-Wmain (only for C/ObjC and unless -ffreestanding)
-Wmaybe-uninitialized
-Wmemset-elt-size
-Wmemset-transposed-args
-Wmisleading-indentation (only for C/C++)
-Wmismatched-dealloc
-Wmismatched-new-delete (only for C/C++)
-Wmissing-attributes
-Wmissing-braces (only for C/ObjC)
-Wmultistatement-macros
-Wnarrowing (only for C++)
-Wnonnull
-Wnonnull-compare
-Wopenmp-simd
-Wparentheses
-Wpessimizing-move (only for C++)
-Wpointer-sign
-Wrange-loop-construct (only for C++)
-Wreorder
-Wrestrict
-Wreturn-type
-Wself-move (only for C++)
-Wsequence-point
-Wsign-compare (only in C++)
-Wsizeof-array-div
-Wsizeof-pointer-div
-Wsizeof-pointer-memaccess
-Wstrict-aliasing
-Wstrict-overflow=1
-Wswitch
-Wtautological-compare
-Wtrigraphs
-Wuninitialized
-Wunknown-pragmas
-Wunused-function
-Wunused-label
-Wunused-value
-Wunused-variable
-Wuse-after-free=2
-Wvla-parameter (C and Objective-C only)
-Wvolatile-register-var
-Wzero-length-bounds

Note that some warning flags are not implied by -Wall. Some of
them warn about constructions that users generally do not consider
questionable, but which occasionally you might wish to check for;
others warn about constructions that are necessary or hard to avoid in
some cases, and there is no simple way to modify the code to suppress
the warning. Some of them are enabled by -Wextra but many of
them must be enabled individually.

This enables some extra warning flags that are not enabled by
-Wall. (This option used to be called -W. The older
name is still supported, but the newer name is more descriptive.)

-Wclobbered
-Wcast-function-type
-Wdeprecated-copy (C++ only)
-Wempty-body
-Wenum-conversion (C only)
-Wignored-qualifiers
-Wimplicit-fallthrough=3
-Wmissing-field-initializers
-Wmissing-parameter-type (C only)
-Wold-style-declaration (C only)
-Woverride-init
-Wsign-compare (C only)
-Wstring-compare
-Wredundant-move (only for C++)
-Wtype-limits
-Wuninitialized
-Wshift-negative-value (in C++11 to C++17 and in C99 and newer)
-Wunused-parameter (only with -Wunused or -Wall)
-Wunused-but-set-parameter (only with -Wunused or -Wall)

The option -Wextra also prints warning messages for the
following cases:

• A pointer is compared against integer zero with <, <=,
  >, or >=.

• (C++ only) An enumerator and a non-enumerator both appear in a
  conditional expression.
• (C++ only) Ambiguous virtual bases.
• (C++ only) Subscripting an array that has been declared register.
• (C++ only) Taking the address of a variable that has been declared
  register.

• (C++ only) A base class is not initialized in the copy constructor
  of a derived class.

Warn about code affected by ABI changes. This includes code that may
not be compatible with the vendor-neutral C++ ABI as well as the psABI
for the particular target.

Since G++ now defaults to updating the ABI with each major release,
normally -Wabi warns only about C++ ABI compatibility
problems if there is a check added later in a release series for an
ABI issue discovered since the initial release. -Wabi warns
about more things if an older ABI version is selected (with
-fabi-version=n).

-Wabi can also be used with an explicit version number to
warn about C++ ABI compatibility with a particular -fabi-version
level, e.g. -Wabi=2 to warn about changes relative to
-fabi-version=2.

If an explicit version number is provided and
-fabi-compat-version is not specified, the version number
from this option is used for compatibility aliases. If no explicit
version number is provided with this option, but
-fabi-compat-version is specified, that version number is
used for C++ ABI warnings.

Although an effort has been made to warn about
all such cases, there are probably some cases that are not warned about,
even though G++ is generating incompatible code. There may also be
cases where warnings are emitted even though the code that is generated
is compatible.

You should rewrite your code to avoid these warnings if you are
concerned about the fact that code generated by G++ may not be binary
compatible with code generated by other compilers.

Known incompatibilities in -fabi-version=2 (which was the
default from GCC 3.4 to 4.9) include:

• A template with a non-type template parameter of reference type was
  mangled incorrectly:

  extern int N;
  template <int &> struct S {};
  void n (S<N>) {2}
  

  This was fixed in -fabi-version=3.

• SIMD vector types declared using __attribute ((vector_size)) were
  mangled in a non-standard way that does not allow for overloading of
  functions taking vectors of different sizes.

  The mangling was changed in -fabi-version=4.

• __attribute ((const)) and noreturn were mangled as type
  qualifiers, and decltype of a plain declaration was folded away.

  These mangling issues were fixed in -fabi-version=5.

• Scoped enumerators passed as arguments to a variadic function are
  promoted like unscoped enumerators, causing va_arg to complain.
  On most targets this does not actually affect the parameter passing
  ABI, as there is no way to pass an argument smaller than int.

  Also, the ABI changed the mangling of template argument packs,
  const_cast, static_cast, prefix increment/decrement, and
  a class scope function used as a template argument.

  These issues were corrected in -fabi-version=6.

• Lambdas in default argument scope were mangled incorrectly, and the
  ABI changed the mangling of nullptr_t.

  These issues were corrected in -fabi-version=7.
• When mangling a function type with function-cv-qualifiers, the
  un-qualified function type was incorrectly treated as a substitution
  candidate.

  This was fixed in -fabi-version=8, the default for GCC 5.1.

• decltype(nullptr) incorrectly had an alignment of 1, leading to
  unaligned accesses. Note that this did not affect the ABI of a
  function with a nullptr_t parameter, as parameters have a
  minimum alignment.

  This was fixed in -fabi-version=9, the default for GCC 5.2.

• Target-specific attributes that affect the identity of a type, such as
  ia32 calling conventions on a function type (stdcall, regparm, etc.),
  did not affect the mangled name, leading to name collisions when
  function pointers were used as template arguments.

  This was fixed in -fabi-version=10, the default for GCC 6.1.

This option also enables warnings about psABI-related changes.
The known psABI changes at this point include:

• For SysV/x86-64, unions with long double members are
  passed in memory as specified in psABI. Prior to GCC 4.4, this was not
  the case. For example:

  union U {
    long double ld;
    int i;
  };
  

  union U is now always passed in memory.

C++ requires that unqualified uses of a name within a class have the
same meaning in the complete scope of the class, so declaring the name
after using it is ill-formed:

struct A;
struct B1 { A a; typedef A A; }; // warning, 'A' changes meaning
struct B2 { A a; struct A { }; }; // error, 'A' changes meaning

By default, the B1 case is only a warning because the two declarations
have the same type, while the B2 case is an error. Both diagnostics
can be disabled with -Wno-changes-meaning. Alternately, the
error case can be reduced to a warning with
-Wno-error=changes-meaning or -fpermissive.

Both diagnostics are also suppressed by -fms-extensions.

Warn if an array subscript has type char. This is a common cause
of error, as programmers often forget that this type is signed on some
machines.
This warning is enabled by -Wall.

Warn if feedback profiles do not match when using the
-fprofile-use option.
If a source file is changed between compiling with -fprofile-generate
and with -fprofile-use, the files with the profile feedback can fail
to match the source file and GCC cannot use the profile feedback
information. By default, this warning is enabled and is treated as an
error. -Wno-coverage-mismatch can be used to disable the
warning or -Wno-error=coverage-mismatch can be used to
disable the error. Disabling the error for this warning can result in
poorly optimized code and is useful only in the
case of very minor changes such as bug fixes to an existing code-base.
Completely disabling the warning is not recommended.

Warn in case a function ends earlier than it begins due
to an invalid linenum macros. The warning is emitted only
with —coverage enabled.

By default, this warning is enabled and is treated as an
error. -Wno-coverage-invalid-line-number can be used to disable the
warning or -Wno-error=coverage-invalid-line-number can be used to
disable the error.

Suppress warning messages emitted by #warning directives.

Give a warning when a value of type float is implicitly
promoted to double. CPUs with a 32-bit “single-precision”
floating-point unit implement float in hardware, but emulate
double in software. On such a machine, doing computations
using double values is much more expensive because of the
overhead required for software emulation.

It is easy to accidentally do computations with double because
floating-point literals are implicitly of type double. For
example, in:

float area(float radius)
{
   return 3.14159 * radius * radius;
}

the compiler performs the entire computation with double
because the floating-point literal is a double.

Warn if a declaration has duplicate const, volatile,
restrict or _Atomic specifier. This warning is enabled by
-Wall.

Check calls to printf and scanf, etc., to make sure that
the arguments supplied have types appropriate to the format string
specified, and that the conversions specified in the format string make
sense. This includes standard functions, and others specified by format
attributes (see Declaring Attributes of Functions), in the printf,
scanf, strftime and strfmon (an X/Open extension,
not in the C standard) families (or other target-specific families).
Which functions are checked without format attributes having been
specified depends on the standard version selected, and such checks of
functions without the attribute specified are disabled by
-ffreestanding or -fno-builtin.

The formats are checked against the format features supported by GNU
libc version 2.2. These include all ISO C90 and C99 features, as well
as features from the Single Unix Specification and some BSD and GNU
extensions. Other library implementations may not support all these
features; GCC does not support warning about features that go beyond a
particular library’s limitations. However, if -Wpedantic is used
with -Wformat, warnings are given about format features not
in the selected standard version (but not for strfmon formats,
since those are not in any version of the C standard). See Options Controlling C Dialect.

-Wformat=1 ¶

-Wformat

Option -Wformat is equivalent to -Wformat=1, and
-Wno-format is equivalent to -Wformat=0. Since
-Wformat also checks for null format arguments for several
functions, -Wformat also implies -Wnonnull. Some
aspects of this level of format checking can be disabled by the
options: -Wno-format-contains-nul,
-Wno-format-extra-args, and -Wno-format-zero-length.
-Wformat is enabled by -Wall.

-Wformat=2 ¶

Enable -Wformat plus additional format checks. Currently
equivalent to -Wformat -Wformat-nonliteral -Wformat-security
-Wformat-y2k.

If -Wformat is specified, do not warn about format strings that
contain NUL bytes.

If -Wformat is specified, do not warn about excess arguments to a
printf or scanf format function. The C standard specifies
that such arguments are ignored.

Where the unused arguments lie between used arguments that are
specified with ‘$’ operand number specifications, normally
warnings are still given, since the implementation could not know what
type to pass to va_arg to skip the unused arguments. However,
in the case of scanf formats, this option suppresses the
warning if the unused arguments are all pointers, since the Single
Unix Specification says that such unused arguments are allowed.

Warn about calls to formatted input/output functions such as sprintf
and vsprintf that might overflow the destination buffer. When the
exact number of bytes written by a format directive cannot be determined
at compile-time it is estimated based on heuristics that depend on the
level argument and on optimization. While enabling optimization
will in most cases improve the accuracy of the warning, it may also
result in false positives.

-Wformat-overflow ¶

-Wformat-overflow=1

Level 1 of -Wformat-overflow enabled by -Wformat
employs a conservative approach that warns only about calls that most
likely overflow the buffer. At this level, numeric arguments to format
directives with unknown values are assumed to have the value of one, and
strings of unknown length to be empty. Numeric arguments that are known
to be bounded to a subrange of their type, or string arguments whose output
is bounded either by their directive’s precision or by a finite set of
string literals, are assumed to take on the value within the range that
results in the most bytes on output. For example, the call to sprintf
below is diagnosed because even with both a and b equal to zero,
the terminating NUL character ('\0') appended by the function
to the destination buffer will be written past its end. Increasing
the size of the buffer by a single byte is sufficient to avoid the
warning, though it may not be sufficient to avoid the overflow.

void f (int a, int b)
{
  char buf [13];
  sprintf (buf, "a = %i, b = %i\n", a, b);
}

-Wformat-overflow=2

Level 2 warns also about calls that might overflow the destination
buffer given an argument of sufficient length or magnitude. At level
2, unknown numeric arguments are assumed to have the minimum
representable value for signed types with a precision greater than 1, and
the maximum representable value otherwise. Unknown string arguments whose
length cannot be assumed to be bounded either by the directive’s precision,
or by a finite set of string literals they may evaluate to, or the character
array they may point to, are assumed to be 1 character long.

At level 2, the call in the example above is again diagnosed, but
this time because with a equal to a 32-bit INT_MIN the first
%i directive will write some of its digits beyond the end of
the destination buffer. To make the call safe regardless of the values
of the two variables, the size of the destination buffer must be increased
to at least 34 bytes. GCC includes the minimum size of the buffer in
an informational note following the warning.

An alternative to increasing the size of the destination buffer is to
constrain the range of formatted values. The maximum length of string
arguments can be bounded by specifying the precision in the format
directive. When numeric arguments of format directives can be assumed
to be bounded by less than the precision of their type, choosing
an appropriate length modifier to the format specifier will reduce
the required buffer size. For example, if a and b in the
example above can be assumed to be within the precision of
the short int type then using either the %hi format
directive or casting the argument to short reduces the maximum
required size of the buffer to 24 bytes.

void f (int a, int b)
{
  char buf [23];
  sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
}

If -Wformat is specified, do not warn about zero-length formats.
The C standard specifies that zero-length formats are allowed.

If -Wformat is specified, also warn if the format string is not a
string literal and so cannot be checked, unless the format function
takes its format arguments as a va_list.

If -Wformat is specified, also warn about uses of format
functions that represent possible security problems. At present, this
warns about calls to printf and scanf functions where the
format string is not a string literal and there are no format arguments,
as in printf (foo);. This may be a security hole if the format
string came from untrusted input and contains ‘%n’. (This is
currently a subset of what -Wformat-nonliteral warns about, but
in future warnings may be added to -Wformat-security that are not
included in -Wformat-nonliteral.)

If -Wformat is specified, also warn if the format string
requires an unsigned argument and the argument is signed and vice versa.

Warn about calls to formatted input/output functions such as snprintf
and vsnprintf that might result in output truncation. When the exact
number of bytes written by a format directive cannot be determined at
compile-time it is estimated based on heuristics that depend on
the level argument and on optimization. While enabling optimization
will in most cases improve the accuracy of the warning, it may also result
in false positives. Except as noted otherwise, the option uses the same
logic -Wformat-overflow.

-Wformat-truncation ¶

-Wformat-truncation=1

Level 1 of -Wformat-truncation enabled by -Wformat
employs a conservative approach that warns only about calls to bounded
functions whose return value is unused and that will most likely result
in output truncation.

-Wformat-truncation=2

Level 2 warns also about calls to bounded functions whose return
value is used and that might result in truncation given an argument of
sufficient length or magnitude.

If -Wformat is specified, also warn about strftime
formats that may yield only a two-digit year.

Warn about passing a null pointer for arguments marked as
requiring a non-null value by the nonnull function attribute.

-Wnonnull is included in -Wall and -Wformat. It
can be disabled with the -Wno-nonnull option.

Warn when comparing an argument marked with the nonnull
function attribute against null inside the function.

-Wnonnull-compare is included in -Wall. It
can be disabled with the -Wno-nonnull-compare option.

Warn if the compiler detects paths that trigger erroneous or
undefined behavior due to dereferencing a null pointer. This option
is only active when -fdelete-null-pointer-checks is active,
which is enabled by optimizations in most targets. The precision of
the warnings depends on the optimization options used.

Warn if the compiler does not elide the copy from a local variable to
the return value of a function in a context where it is allowed by
[class.copy.elision]. This elision is commonly known as the Named
Return Value Optimization. For instance, in the example below the
compiler cannot elide copies from both v1 and v2, so it elides neither.

std::vector<int> f()
{
  std::vector<int> v1, v2;
  // ...
  if (cond) return v1;
  else return v2; // warning: not eliding copy
}

Warn about infinitely recursive calls. The warning is effective at all
optimization levels but requires optimization in order to detect infinite
recursion in calls between two or more functions.
-Winfinite-recursion is included in -Wall.

Compare with -Wanalyzer-infinite-recursion which provides a
similar diagnostic, but is implemented in a different way (as part of
-fanalyzer).

Warn about uninitialized variables that are initialized with themselves.
Note this option can only be used with the -Wuninitialized option.

For example, GCC warns about i being uninitialized in the
following snippet only when -Winit-self has been specified:

int f()
{
  int i = i;
  return i;
}

This warning is enabled by -Wall in C++.

This option controls warnings when a declaration does not specify a type.
This warning is enabled by default in C99 and later dialects of C,
and also by -Wall.

This warning is upgraded to an error by -pedantic-errors.

This option controls warnings when a function is used before being declared.
This warning is enabled by default in C99 and later dialects of C,
and also by -Wall.

This warning is upgraded to an error by -pedantic-errors.

Same as -Wimplicit-int and -Wimplicit-function-declaration.
This warning is enabled by -Wall.

-Wimplicit-fallthrough is the same as -Wimplicit-fallthrough=3
and -Wno-implicit-fallthrough is the same as
-Wimplicit-fallthrough=0.

Warn when a switch case falls through. For example:

switch (cond)
  {
  case 1:
    a = 1;
    break;
  case 2:
    a = 2;
  case 3:
    a = 3;
    break;
  }

This warning does not warn when the last statement of a case cannot
fall through, e.g. when there is a return statement or a call to function
declared with the noreturn attribute. -Wimplicit-fallthrough=
also takes into account control flow statements, such as ifs, and only
warns when appropriate. E.g.

switch (cond)
  {
  case 1:
    if (i > 3) {
      bar (5);
      break;
    } else if (i < 1) {
      bar (0);
    } else
      return;
  default:
    …
  }

Since there are occasions where a switch case fall through is desirable,
GCC provides an attribute, __attribute__ ((fallthrough)), that is
to be used along with a null statement to suppress this warning that
would normally occur:

switch (cond)
  {
  case 1:
    bar (0);
    __attribute__ ((fallthrough));
  default:
    …
  }

C++17 provides a standard way to suppress the -Wimplicit-fallthrough
warning using [[fallthrough]]; instead of the GNU attribute. In C++11
or C++14 users can use [[gnu::fallthrough]];, which is a GNU extension.
Instead of these attributes, it is also possible to add a fallthrough comment
to silence the warning. The whole body of the C or C++ style comment should
match the given regular expressions listed below. The option argument n
specifies what kind of comments are accepted:

• -Wimplicit-fallthrough=0 disables the warning altogether.
• -Wimplicit-fallthrough=1 matches .* regular
  expression, any comment is used as fallthrough comment.

• -Wimplicit-fallthrough=2 case insensitively matches
  .*falls?[ \t-]*thr(ough|u).* regular expression.

• -Wimplicit-fallthrough=3 case sensitively matches one of the
  following regular expressions:
  • -fallthrough
  • @fallthrough@
  • lint -fallthrough[ \t]*
  • [ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?
FALL(S | |-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?
  • [ \t.!]*(Else,? |Intentional(ly)? )?
Fall((s | |-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?
  • [ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?
fall(s | |-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?
• -Wimplicit-fallthrough=4 case sensitively matches one of the
  following regular expressions:
  • -fallthrough
  • @fallthrough@
  • lint -fallthrough[ \t]*
  • [ \t]*FALLTHR(OUGH|U)[ \t]*
• -Wimplicit-fallthrough=5 doesn’t recognize any comments as
  fallthrough comments, only attributes disable the warning.

The comment needs to be followed after optional whitespace and other comments
by case or default keywords or by a user label that precedes some
case or default label.

switch (cond)
  {
  case 1:
    bar (0);
    /* FALLTHRU */
  default:
    …
  }

The -Wimplicit-fallthrough=3 warning is enabled by -Wextra.

Control if warnings triggered by the warn_if_not_aligned attribute
should be issued. These warnings are enabled by default.

Warn if the return type of a function has a type qualifier
such as const. For ISO C such a type qualifier has no effect,
since the value returned by a function is not an lvalue.
For C++, the warning is only emitted for scalar types or void.
ISO C prohibits qualified void return types on function
definitions, so such return types always receive a warning
even without this option.

This warning is also enabled by -Wextra.

This option controls warnings when an attribute is ignored.
This is different from the
-Wattributes option in that it warns whenever the compiler decides
to drop an attribute, not that the attribute is either unknown, used in a
wrong place, etc. This warning is enabled by default.

Warn if the type of main is suspicious. main should be
a function with external linkage, returning int, taking either zero
arguments, two, or three arguments of appropriate types. This warning
is enabled by default in C++ and is enabled by either -Wall
or -Wpedantic.

This warning is upgraded to an error by -pedantic-errors.

Warn when the indentation of the code does not reflect the block structure.
Specifically, a warning is issued for if, else, while, and
for clauses with a guarded statement that does not use braces,
followed by an unguarded statement with the same indentation.

In the following example, the call to “bar” is misleadingly indented as
if it were guarded by the “if” conditional.

  if (some_condition ())
    foo ();
    bar ();  /* Gotcha: this is not guarded by the "if".  */

In the case of mixed tabs and spaces, the warning uses the
-ftabstop= option to determine if the statements line up
(defaulting to 8).

The warning is not issued for code involving multiline preprocessor logic
such as the following example.

  if (flagA)
    foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
  if (flagB)
#endif
    foo (1);

The warning is not issued after a #line directive, since this
typically indicates autogenerated code, and no assumptions can be made
about the layout of the file that the directive references.

This warning is enabled by -Wall in C and C++.

Warn when a declaration of a function is missing one or more attributes
that a related function is declared with and whose absence may adversely
affect the correctness or efficiency of generated code. For example,
the warning is issued for declarations of aliases that use attributes
to specify less restrictive requirements than those of their targets.
This typically represents a potential optimization opportunity.
By contrast, the -Wattribute-alias=2 option controls warnings
issued when the alias is more restrictive than the target, which could
lead to incorrect code generation.
Attributes considered include alloc_align, alloc_size,
cold, const, hot, leaf, malloc,
nonnull, noreturn, nothrow, pure,
returns_nonnull, and returns_twice.

In C++, the warning is issued when an explicit specialization of a primary
template declared with attribute alloc_align, alloc_size,
assume_aligned, format, format_arg, malloc,
or nonnull is declared without it. Attributes deprecated,
error, and warning suppress the warning.
(see Declaring Attributes of Functions).

You can use the copy attribute to apply the same
set of attributes to a declaration as that on another declaration without
explicitly enumerating the attributes. This attribute can be applied
to declarations of functions (see Common Function Attributes),
variables (see Common Variable Attributes), or types
(see Common Type Attributes).

-Wmissing-attributes is enabled by -Wall.

For example, since the declaration of the primary function template
below makes use of both attribute malloc and alloc_size
the declaration of the explicit specialization of the template is
diagnosed because it is missing one of the attributes.

template <class T>
T* __attribute__ ((malloc, alloc_size (1)))
allocate (size_t);

template <>
void* __attribute__ ((malloc))   // missing alloc_size
allocate<void> (size_t);

Warn if an aggregate or union initializer is not fully bracketed. In
the following example, the initializer for a is not fully
bracketed, but that for b is fully bracketed.

int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };

This warning is enabled by -Wall.

Warn if a user-supplied include directory does not exist. This option is disabled
by default for C, C++, Objective-C and Objective-C++. For Fortran, it is partially
enabled by default by warning for -I and -J, only.

This option controls warnings if feedback profiles are missing when using the
-fprofile-use option.
This option diagnoses those cases where a new function or a new file is added
between compiling with -fprofile-generate and with
-fprofile-use, without regenerating the profiles.
In these cases, the profile feedback data files do not contain any
profile feedback information for
the newly added function or file respectively. Also, in the case when profile
count data (.gcda) files are removed, GCC cannot use any profile feedback
information. In all these cases, warnings are issued to inform you that a
profile generation step is due.
Ignoring the warning can result in poorly optimized code.
-Wno-missing-profile can be used to
disable the warning, but this is not recommended and should be done only
when non-existent profile data is justified.

Warn for calls to deallocation functions with pointer arguments returned
from from allocations functions for which the former isn’t a suitable
deallocator. A pair of functions can be associated as matching allocators
and deallocators by use of attribute malloc. Unless disabled by
the -fno-builtin option the standard functions calloc,
malloc, realloc, and free, as well as the corresponding
forms of C++ operator new and operator delete are implicitly
associated as matching allocators and deallocators. In the following
example mydealloc is the deallocator for pointers returned from
myalloc.

void mydealloc (void*);

__attribute__ ((malloc (mydealloc, 1))) void*
myalloc (size_t);

void f (void)
{
  void *p = myalloc (32);
  // …use p…
  free (p);   // warning: not a matching deallocator for myalloc
  mydealloc (p);   // ok
}

In C++, the related option -Wmismatched-new-delete diagnoses
mismatches involving either operator new or operator delete.

Option -Wmismatched-dealloc is included in -Wall.

Warn about unsafe multiple statement macros that appear to be guarded
by a clause such as if, else, for, switch, or
while, in which only the first statement is actually guarded after
the macro is expanded.

For example:

#define DOIT x++; y++
if (c)
  DOIT;

will increment y unconditionally, not just when c holds.
The can usually be fixed by wrapping the macro in a do-while loop:

#define DOIT do { x++; y++; } while (0)
if (c)
  DOIT;

This warning is enabled by -Wall in C and C++.

Warn if parentheses are omitted in certain contexts, such
as when there is an assignment in a context where a truth value
is expected, or when operators are nested whose precedence people
often get confused about.

Also warn if a comparison like x<=y<=z appears; this is
equivalent to (x<=y ? 1 : 0) <= z, which is a different
interpretation from that of ordinary mathematical notation.

Also warn for dangerous uses of the GNU extension to
?: with omitted middle operand. When the condition
in the ?: operator is a boolean expression, the omitted value is
always 1. Often programmers expect it to be a value computed
inside the conditional expression instead.

For C++ this also warns for some cases of unnecessary parentheses in
declarations, which can indicate an attempt at a function call instead
of a declaration:

{
  // Declares a local variable called mymutex.
  std::unique_lock<std::mutex> (mymutex);
  // User meant std::unique_lock<std::mutex> lock (mymutex);
}

This warning is enabled by -Wall.

This warning warns when a value is moved to itself with std::move.
Such a std::move typically has no effect.

struct T {
…
};
void fn()
{
  T t;
  …
  t = std::move (t);
}

This warning is enabled by -Wall.

Warn about code that may have undefined semantics because of violations
of sequence point rules in the C and C++ standards.

The C and C++ standards define the order in which expressions in a C/C++
program are evaluated in terms of sequence points, which represent
a partial ordering between the execution of parts of the program: those
executed before the sequence point, and those executed after it. These
occur after the evaluation of a full expression (one which is not part
of a larger expression), after the evaluation of the first operand of a
&&, ||, ? : or , (comma) operator, before a
function is called (but after the evaluation of its arguments and the
expression denoting the called function), and in certain other places.
Other than as expressed by the sequence point rules, the order of
evaluation of subexpressions of an expression is not specified. All
these rules describe only a partial order rather than a total order,
since, for example, if two functions are called within one expression
with no sequence point between them, the order in which the functions
are called is not specified. However, the standards committee have
ruled that function calls do not overlap.

It is not specified when between sequence points modifications to the
values of objects take effect. Programs whose behavior depends on this
have undefined behavior; the C and C++ standards specify that “Between
the previous and next sequence point an object shall have its stored
value modified at most once by the evaluation of an expression.
Furthermore, the prior value shall be read only to determine the value
to be stored.”. If a program breaks these rules, the results on any
particular implementation are entirely unpredictable.

Examples of code with undefined behavior are a = a++;, a[n]
= b[n++] and a[i++] = i;. Some more complicated cases are not
diagnosed by this option, and it may give an occasional false positive
result, but in general it has been found fairly effective at detecting
this sort of problem in programs.

The C++17 standard will define the order of evaluation of operands in
more cases: in particular it requires that the right-hand side of an
assignment be evaluated before the left-hand side, so the above
examples are no longer undefined. But this option will still warn
about them, to help people avoid writing code that is undefined in C
and earlier revisions of C++.

The standard is worded confusingly, therefore there is some debate
over the precise meaning of the sequence point rules in subtle cases.
Links to discussions of the problem, including proposed formal
definitions, may be found on the GCC readings page, at
https://gcc.gnu.org/readings.html.

This warning is enabled by -Wall for C and C++.

Do not warn about returning a pointer (or in C++, a reference) to a
variable that goes out of scope after the function returns.

Warn whenever a function is defined with a return type that defaults
to int. Also warn about any return statement with no
return value in a function whose return type is not void
(falling off the end of the function body is considered returning
without a value).

For C only, warn about a return statement with an expression in a
function whose return type is void, unless the expression type is
also void. As a GNU extension, the latter case is accepted
without a warning unless -Wpedantic is used. Attempting
to use the return value of a non-void function other than main
that flows off the end by reaching the closing curly brace that terminates
the function is undefined.

Unlike in C, in C++, flowing off the end of a non-void function other
than main results in undefined behavior even when the value of
the function is not used.

This warning is enabled by default in C++ and by -Wall otherwise.

Controls warnings if a shift count is negative.
This warning is enabled by default.

Controls warnings if a shift count is greater than or equal to the bit width
of the type. This warning is enabled by default.

Warn if left shifting a negative value. This warning is enabled by
-Wextra in C99 (and newer) and C++11 to C++17 modes.

These options control warnings about left shift overflows.

-Wshift-overflow=1

This is the warning level of -Wshift-overflow and is enabled
by default in C99 and C++11 modes (and newer). This warning level does
not warn about left-shifting 1 into the sign bit. (However, in C, such
an overflow is still rejected in contexts where an integer constant expression
is required.) No warning is emitted in C++20 mode (and newer), as signed left
shifts always wrap.

-Wshift-overflow=2

This warning level also warns about left-shifting 1 into the sign bit,
unless C++14 mode (or newer) is active.

Warn whenever a switch statement has an index of enumerated type
and lacks a case for one or more of the named codes of that
enumeration. (The presence of a default label prevents this
warning.) case labels outside the enumeration range also
provoke warnings when this option is used (even if there is a
default label).
This warning is enabled by -Wall.

Warn whenever a switch statement does not have a default
case.

Warn whenever a switch statement has an index of enumerated type
and lacks a case for one or more of the named codes of that
enumeration. case labels outside the enumeration range also
provoke warnings when this option is used. The only difference
between -Wswitch and this option is that this option gives a
warning about an omitted enumeration code even if there is a
default label.

Do not warn when a switch statement has an index of boolean type
and the case values are outside the range of a boolean type.
It is possible to suppress this warning by casting the controlling
expression to a type other than bool. For example:

switch ((int) (a == 4))
  {
  …
  }

This warning is enabled by default for C and C++ programs.

This option controls warnings when a switch case has a value
that is outside of its
respective type range. This warning is enabled by default for
C and C++ programs.

Do not warn when a switch statement contains statements between the
controlling expression and the first case label, which will never be
executed. For example:

switch (cond)
  {
   i = 15;
  …
   case 5:
  …
  }

-Wswitch-unreachable does not warn if the statement between the
controlling expression and the first case label is just a declaration:

switch (cond)
  {
   int i;
  …
   case 5:
   i = 5;
  …
  }

This warning is enabled by default for C and C++ programs.

Warn when __sync_fetch_and_nand and __sync_nand_and_fetch
built-in functions are used. These functions changed semantics in GCC 4.4.

Warn when -ftrivial-auto-var-init cannot initialize the automatic
variable. A common situation is an automatic variable that is declared
between the controlling expression and the first case label of a switch
statement.

Warn whenever a function parameter is assigned to, but otherwise unused
(aside from its declaration).

To suppress this warning use the unused attribute
(see Specifying Attributes of Variables).

This warning is also enabled by -Wunused together with
-Wextra.

Warn whenever a local variable is assigned to, but otherwise unused
(aside from its declaration).
This warning is enabled by -Wall.

To suppress this warning use the unused attribute
(see Specifying Attributes of Variables).

This warning is also enabled by -Wunused, which is enabled
by -Wall.

Warn whenever a static function is declared but not defined or a
non-inline static function is unused.
This warning is enabled by -Wall.

Warn whenever a label is declared but not used.
This warning is enabled by -Wall.

To suppress this warning use the unused attribute
(see Specifying Attributes of Variables).

Warn when a typedef locally defined in a function is not used.
This warning is enabled by -Wall.

Warn whenever a function parameter is unused aside from its declaration.

To suppress this warning use the unused attribute
(see Specifying Attributes of Variables).

Do not warn if a caller of a function marked with attribute
warn_unused_result (see Declaring Attributes of Functions) does not use
its return value. The default is -Wunused-result.

Warn whenever a local or static variable is unused aside from its
declaration. This option implies -Wunused-const-variable=1 for C,
but not for C++. This warning is enabled by -Wall.

To suppress this warning use the unused attribute
(see Specifying Attributes of Variables).

Warn whenever a constant static variable is unused aside from its declaration.
-Wunused-const-variable=1 is enabled by -Wunused-variable
for C, but not for C++. In C this declares variable storage, but in C++ this
is not an error since const variables take the place of #defines.

To suppress this warning use the unused attribute
(see Specifying Attributes of Variables).

-Wunused-const-variable=1

This is the warning level that is enabled by -Wunused-variable for
C. It warns only about unused static const variables defined in the main
compilation unit, but not about static const variables declared in any
header included.

-Wunused-const-variable=2

This warning level also warns for unused constant static variables in
headers (excluding system headers). This is the warning level of
-Wunused-const-variable and must be explicitly requested since
in C++ this isn’t an error and in C it might be harder to clean up all
headers included.

Warn whenever a statement computes a result that is explicitly not
used. To suppress this warning cast the unused expression to
void. This includes an expression-statement or the left-hand
side of a comma expression that contains no side effects. For example,
an expression such as x[i,j] causes a warning, while
x[(void)i,j] does not.

This warning is enabled by -Wall.

All the above -Wunused options combined.

In order to get a warning about an unused function parameter, you must
either specify -Wextra -Wunused (note that -Wall implies
-Wunused), or separately specify -Wunused-parameter.

Warn if an object with automatic or allocated storage duration is used
without having been initialized. In C++, also warn if a non-static
reference or non-static const member appears in a class without
constructors.

In addition, passing a pointer (or in C++, a reference) to an uninitialized
object to a const-qualified argument of a built-in function known to
read the object is also diagnosed by this warning.
(-Wmaybe-uninitialized is issued for ordinary functions.)

If you want to warn about code that uses the uninitialized value of the
variable in its own initializer, use the -Winit-self option.

These warnings occur for individual uninitialized elements of
structure, union or array variables as well as for variables that are
uninitialized as a whole. They do not occur for variables or elements
declared volatile. Because these warnings depend on
optimization, the exact variables or elements for which there are
warnings depend on the precise optimization options and version of GCC
used.

Note that there may be no warning about a variable that is used only
to compute a value that itself is never used, because such
computations may be deleted by data flow analysis before the warnings
are printed.

In C++, this warning also warns about using uninitialized objects in
member-initializer-lists. For example, GCC warns about b being
uninitialized in the following snippet:

struct A {
  int a;
  int b;
  A() : a(b) { }
};

This option controls warnings
for invocations of Built-in Functions for Memory Model Aware Atomic Operations, Legacy __sync Built-in Functions for Atomic Memory Access,
and the C11 atomic generic functions with a memory consistency argument
that is either invalid for the operation or outside the range of values
of the memory_order enumeration. For example, since the
__atomic_store and __atomic_store_n built-ins are only
defined for the relaxed, release, and sequentially consistent memory
orders the following code is diagnosed:

void store (int *i)
{
  __atomic_store_n (i, 0, memory_order_consume);
}

-Winvalid-memory-model is enabled by default.

For an object with automatic or allocated storage duration, if there exists
a path from the function entry to a use of the object that is initialized,
but there exist some other paths for which the object is not initialized,
the compiler emits a warning if it cannot prove the uninitialized paths
are not executed at run time.

In addition, passing a pointer (or in C++, a reference) to an uninitialized
object to a const-qualified function argument is also diagnosed by
this warning. (-Wuninitialized is issued for built-in functions
known to read the object.) Annotating the function with attribute
access (none) indicates that the argument isn’t used to access
the object and avoids the warning (see Common Function Attributes).

These warnings are only possible in optimizing compilation, because otherwise
GCC does not keep track of the state of variables.

These warnings are made optional because GCC may not be able to determine when
the code is correct in spite of appearing to have an error. Here is one
example of how this can happen:

{
  int x;
  switch (y)
    {
    case 1: x = 1;
      break;
    case 2: x = 4;
      break;
    case 3: x = 5;
    }
  foo (x);
}

If the value of y is always 1, 2 or 3, then x is
always initialized, but GCC doesn’t know this. To suppress the
warning, you need to provide a default case with assert(0) or
similar code.

This option also warns when a non-volatile automatic variable might be
changed by a call to longjmp.
The compiler sees only the calls to setjmp. It cannot know
where longjmp will be called; in fact, a signal handler could
call it at any point in the code. As a result, you may get a warning
even when there is in fact no problem because longjmp cannot
in fact be called at the place that would cause a problem.

Some spurious warnings can be avoided if you declare all the functions
you use that never return as noreturn. See Declaring Attributes of Functions.

This warning is enabled by -Wall or -Wextra.

Warn when a #pragma directive is encountered that is not understood by
GCC. If this command-line option is used, warnings are even issued
for unknown pragmas in system header files. This is not the case if
the warnings are only enabled by the -Wall command-line option.

Do not warn about misuses of pragmas, such as incorrect parameters,
invalid syntax, or conflicts between pragmas. See also
-Wunknown-pragmas.

Do not warn if a priority from 0 to 100 is used for constructor or destructor.
The use of constructor and destructor attributes allow you to assign a
priority to the constructor/destructor to control its order of execution
before main is called or after it returns. The priority values must be
greater than 100 as the compiler reserves priority values between 0–100 for
the implementation.

This option is only active when -fstrict-aliasing is active.
It warns about code that might break the strict aliasing rules that the
compiler is using for optimization. The warning does not catch all
cases, but does attempt to catch the more common pitfalls. It is
included in -Wall.
It is equivalent to -Wstrict-aliasing=3

This option is only active when -fstrict-aliasing is active.
It warns about code that might break the strict aliasing rules that the
compiler is using for optimization.
Higher levels correspond to higher accuracy (fewer false positives).
Higher levels also correspond to more effort, similar to the way -O
works.
-Wstrict-aliasing is equivalent to -Wstrict-aliasing=3.

Level 1: Most aggressive, quick, least accurate.
Possibly useful when higher levels
do not warn but -fstrict-aliasing still breaks the code, as it has very few
false negatives. However, it has many false positives.
Warns for all pointer conversions between possibly incompatible types,
even if never dereferenced. Runs in the front end only.

Level 2: Aggressive, quick, not too precise.
May still have many false positives (not as many as level 1 though),
and few false negatives (but possibly more than level 1).
Unlike level 1, it only warns when an address is taken. Warns about
incomplete types. Runs in the front end only.

Level 3 (default for -Wstrict-aliasing):
Should have very few false positives and few false
negatives. Slightly slower than levels 1 or 2 when optimization is enabled.
Takes care of the common pun+dereference pattern in the front end:
*(int*)&some_float.
If optimization is enabled, it also runs in the back end, where it deals
with multiple statement cases using flow-sensitive points-to information.
Only warns when the converted pointer is dereferenced.
Does not warn about incomplete types.

This option is only active when signed overflow is undefined.
It warns about cases where the compiler optimizes based on the
assumption that signed overflow does not occur. Note that it does not
warn about all cases where the code might overflow: it only warns
about cases where the compiler implements some optimization. Thus
this warning depends on the optimization level.

An optimization that assumes that signed overflow does not occur is
perfectly safe if the values of the variables involved are such that
overflow never does, in fact, occur. Therefore this warning can
easily give a false positive: a warning about code that is not
actually a problem. To help focus on important issues, several
warning levels are defined. No warnings are issued for the use of
undefined signed overflow when estimating how many iterations a loop
requires, in particular when determining whether a loop will be
executed at all.

-Wstrict-overflow=1

Warn about cases that are both questionable and easy to avoid. For
example the compiler simplifies
x + 1 > x to 1. This level of
-Wstrict-overflow is enabled by -Wall; higher levels
are not, and must be explicitly requested.

-Wstrict-overflow=2

Also warn about other cases where a comparison is simplified to a
constant. For example: abs (x) >= 0. This can only be
simplified when signed integer overflow is undefined, because
abs (INT_MIN) overflows to INT_MIN, which is less than
zero. -Wstrict-overflow (with no level) is the same as
-Wstrict-overflow=2.

-Wstrict-overflow=3

Also warn about other cases where a comparison is simplified. For
example: x + 1 > 1 is simplified to x > 0.

-Wstrict-overflow=4

Also warn about other simplifications not covered by the above cases.
For example: (x * 10) / 5 is simplified to x * 2.

-Wstrict-overflow=5

Also warn about cases where the compiler reduces the magnitude of a
constant involved in a comparison. For example: x + 2 > y is
simplified to x + 1 >= y. This is reported only at the
highest warning level because this simplification applies to many
comparisons, so this warning level gives a very large number of
false positives.

Warn for calls to strcmp and strncmp whose result is
determined to be either zero or non-zero in tests for such equality
owing to the length of one argument being greater than the size of
the array the other argument is stored in (or the bound in the case
of strncmp). Such calls could be mistakes. For example,
the call to strcmp below is diagnosed because its result is
necessarily non-zero irrespective of the contents of the array a.

extern char a[4];
void f (char *d)
{
  strcpy (d, "string");
  …
  if (0 == strcmp (a, d))   // cannot be true
    puts ("a and d are the same");
}

-Wstring-compare is enabled by -Wextra.

Warn for calls to string manipulation functions such as memcpy and
strcpy that are determined to overflow the destination buffer. The
optional argument is one greater than the type of Object Size Checking to
perform to determine the size of the destination. See Object Size Checking.
The argument is meaningful only for functions that operate on character arrays
but not for raw memory functions like memcpy which always make use
of Object Size type-0. The option also warns for calls that specify a size
in excess of the largest possible object or at most SIZE_MAX / 2 bytes.
The option produces the best results with optimization enabled but can detect
a small subset of simple buffer overflows even without optimization in
calls to the GCC built-in functions like __builtin_memcpy that
correspond to the standard functions. In any case, the option warns about
just a subset of buffer overflows detected by the corresponding overflow
checking built-ins. For example, the option issues a warning for
the strcpy call below because it copies at least 5 characters
(the string "blue" including the terminating NUL) into the buffer
of size 4.

enum Color { blue, purple, yellow };
const char* f (enum Color clr)
{
  static char buf [4];
  const char *str;
  switch (clr)
    {
      case blue: str = "blue"; break;
      case purple: str = "purple"; break;
      case yellow: str = "yellow"; break;
    }

  return strcpy (buf, str);   // warning here
}

Option -Wstringop-overflow=2 is enabled by default.

-Wstringop-overflow ¶

-Wstringop-overflow=1

The -Wstringop-overflow=1 option uses type-zero Object Size Checking
to determine the sizes of destination objects. At this setting the option
does not warn for writes past the end of subobjects of larger objects accessed
by pointers unless the size of the largest surrounding object is known. When
the destination may be one of several objects it is assumed to be the largest
one of them. On Linux systems, when optimization is enabled at this setting
the option warns for the same code as when the _FORTIFY_SOURCE macro
is defined to a non-zero value.

-Wstringop-overflow=2

The -Wstringop-overflow=2 option uses type-one Object Size Checking
to determine the sizes of destination objects. At this setting the option
warns about overflows when writing to members of the largest complete
objects whose exact size is known. However, it does not warn for excessive
writes to the same members of unknown objects referenced by pointers since
they may point to arrays containing unknown numbers of elements. This is
the default setting of the option.

-Wstringop-overflow=3

The -Wstringop-overflow=3 option uses type-two Object Size Checking
to determine the sizes of destination objects. At this setting the option
warns about overflowing the smallest object or data member. This is the
most restrictive setting of the option that may result in warnings for safe
code.

-Wstringop-overflow=4

The -Wstringop-overflow=4 option uses type-three Object Size Checking
to determine the sizes of destination objects. At this setting the option
warns about overflowing any data members, and when the destination is
one of several objects it uses the size of the largest of them to decide
whether to issue a warning. Similarly to -Wstringop-overflow=3 this
setting of the option may result in warnings for benign code.

Warn for calls to string manipulation functions such as memchr, or
strcpy that are determined to read past the end of the source
sequence.

Option -Wstringop-overread is enabled by default.

Do not warn for calls to bounded string manipulation functions
such as strncat,
strncpy, and stpncpy that may either truncate the copied string
or leave the destination unchanged.

In the following example, the call to strncat specifies a bound that
is less than the length of the source string. As a result, the copy of
the source will be truncated and so the call is diagnosed. To avoid the
warning use bufsize - strlen (buf) - 1) as the bound.

void append (char *buf, size_t bufsize)
{
  strncat (buf, ".txt", 3);
}

As another example, the following call to strncpy results in copying
to d just the characters preceding the terminating NUL, without
appending the NUL to the end. Assuming the result of strncpy is
necessarily a NUL-terminated string is a common mistake, and so the call
is diagnosed. To avoid the warning when the result is not expected to be
NUL-terminated, call memcpy instead.

void copy (char *d, const char *s)
{
  strncpy (d, s, strlen (s));
}

In the following example, the call to strncpy specifies the size
of the destination buffer as the bound. If the length of the source
string is equal to or greater than this size the result of the copy will
not be NUL-terminated. Therefore, the call is also diagnosed. To avoid
the warning, specify sizeof buf - 1 as the bound and set the last
element of the buffer to NUL.

void copy (const char *s)
{
  char buf[80];
  strncpy (buf, s, sizeof buf);
  …
}

In situations where a character array is intended to store a sequence
of bytes with no terminating NUL such an array may be annotated
with attribute nonstring to avoid this warning. Such arrays,
however, are not suitable arguments to functions that expect
NUL-terminated strings. To help detect accidental misuses of
such arrays GCC issues warnings unless it can prove that the use is
safe. See Common Variable Attributes.

Warn about inproper usages of flexible array members
according to the level of the strict_flex_array (level)
attribute attached to the trailing array field of a structure if it’s
available, otherwise according to the level of the option
-fstrict-flex-arrays=level.

This option is effective only when level is bigger than 0. Otherwise,
it will be ignored with a warning.

when level=1, warnings will be issued for a trailing array reference
of a structure that have 2 or more elements if the trailing array is referenced
as a flexible array member.

when level=2, in addition to level=1, additional warnings will be
issued for a trailing one-element array reference of a structure
if the array is referenced as a flexible array member.

when level=3, in addition to level=2, additional warnings will be
issued for a trailing zero-length array reference of a structure
if the array is referenced as a flexible array member.

Warn for cases where adding an attribute may be beneficial. The
attributes currently supported are listed below.

-Wsuggest-attribute=pure ¶

-Wsuggest-attribute=const

-Wsuggest-attribute=noreturn

-Wmissing-noreturn

-Wsuggest-attribute=malloc

Warn about functions that might be candidates for attributes
pure, const or noreturn or malloc. The compiler
only warns for functions visible in other compilation units or (in the case of
pure and const) if it cannot prove that the function returns
normally. A function returns normally if it doesn’t contain an infinite loop or
return abnormally by throwing, calling abort or trapping. This analysis
requires option -fipa-pure-const, which is enabled by default at
-O and higher. Higher optimization levels improve the accuracy
of the analysis.

-Wsuggest-attribute=format ¶

-Wmissing-format-attribute

Warn about function pointers that might be candidates for format
attributes. Note these are only possible candidates, not absolute ones.
GCC guesses that function pointers with format attributes that
are used in assignment, initialization, parameter passing or return
statements should have a corresponding format attribute in the
resulting type. I.e. the left-hand side of the assignment or
initialization, the type of the parameter variable, or the return type
of the containing function respectively should also have a format
attribute to avoid the warning.

GCC also warns about function definitions that might be
candidates for format attributes. Again, these are only
possible candidates. GCC guesses that format attributes
might be appropriate for any function that calls a function like
vprintf or vscanf, but this might not always be the
case, and some functions for which format attributes are
appropriate may not be detected.

-Wsuggest-attribute=cold ¶

Warn about functions that might be candidates for cold attribute. This
is based on static detection and generally only warns about functions which
always leads to a call to another cold function such as wrappers of
C++ throw or fatal error reporting functions leading to abort.

Warn about calls to allocation functions decorated with attribute
alloc_size that specify zero bytes, including those to the built-in
forms of the functions aligned_alloc, alloca, calloc,
malloc, and realloc. Because the behavior of these functions
when called with a zero size differs among implementations (and in the case
of realloc has been deprecated) relying on it may result in subtle
portability bugs and should be avoided.

Warn about calls to functions decorated with attribute alloc_size
that attempt to allocate objects larger than the specified number of bytes,
or where the result of the size computation in an integer type with infinite
precision would exceed the value of ‘PTRDIFF_MAX’ on the target.
-Walloc-size-larger-than=‘PTRDIFF_MAX’ is enabled by default.
Warnings controlled by the option can be disabled either by specifying
byte-size of ‘SIZE_MAX’ or more or by
-Wno-alloc-size-larger-than.
See Declaring Attributes of Functions.

Disable -Walloc-size-larger-than= warnings. The option is
equivalent to -Walloc-size-larger-than=‘SIZE_MAX’ or
larger.

This option warns on all uses of alloca in the source.

This option warns on calls to alloca with an integer argument whose
value is either zero, or that is not bounded by a controlling predicate
that limits its value to at most byte-size. It also warns for calls
to alloca where the bound value is unknown. Arguments of non-integer
types are considered unbounded even if they appear to be constrained to
the expected range.

For example, a bounded case of alloca could be:

void func (size_t n)
{
  void *p;
  if (n <= 1000)
    p = alloca (n);
  else
    p = malloc (n);
  f (p);
}

In the above example, passing -Walloca-larger-than=1000 would not
issue a warning because the call to alloca is known to be at most
1000 bytes. However, if -Walloca-larger-than=500 were passed,
the compiler would emit a warning.

Unbounded uses, on the other hand, are uses of alloca with no
controlling predicate constraining its integer argument. For example:

void func ()
{
  void *p = alloca (n);
  f (p);
}

If -Walloca-larger-than=500 were passed, the above would trigger
a warning, but this time because of the lack of bounds checking.

Note, that even seemingly correct code involving signed integers could
cause a warning:

void func (signed int n)
{
  if (n < 500)
    {
      p = alloca (n);
      f (p);
    }
}

In the above example, n could be negative, causing a larger than
expected argument to be implicitly cast into the alloca call.

This option also warns when alloca is used in a loop.

-Walloca-larger-than=‘PTRDIFF_MAX’ is enabled by default
but is usually only effective when -ftree-vrp is active (default
for -O2 and above).

See also -Wvla-larger-than=‘byte-size’.

Disable -Walloca-larger-than= warnings. The option is
equivalent to -Walloca-larger-than=‘SIZE_MAX’ or larger.

Do warn about implicit conversions from arithmetic operations even
when conversion of the operands to the same type cannot change their
values. This affects warnings from -Wconversion,
-Wfloat-conversion, and -Wsign-conversion.

void f (char c, int i)
{
  c = c + i; // warns with -Wconversion
  c = c + 1; // only warns with -Warith-conversion
}

Warn about out of bounds subscripts or offsets into arrays. This warning
is enabled by -Wall. It is more effective when -ftree-vrp
is active (the default for -O2 and above) but a subset of instances
are issued even without optimization.

By default, the trailing array of a structure will be treated as a flexible
array member by -Warray-bounds or -Warray-bounds=n
if it is declared as either a flexible array member per C99 standard onwards
(‘[]’), a GCC zero-length array extension (‘[0]’), or an one-element
array (‘[1]’). As a result, out of bounds subscripts or offsets into
zero-length arrays or one-element arrays are not warned by default.

You can add the option -fstrict-flex-arrays or
-fstrict-flex-arrays=level to control how this
option treat trailing array of a structure as a flexible array member:

when level<=1, no change to the default behavior.

when level=2, additional warnings will be issued for out of bounds
subscripts or offsets into one-element arrays;

when level=3, in addition to level=2, additional warnings will be
issued for out of bounds subscripts or offsets into zero-length arrays.

-Warray-bounds=1

This is the default warning level of -Warray-bounds and is enabled
by -Wall; higher levels are not, and must be explicitly requested.

-Warray-bounds=2

This warning level also warns about the intermediate results of pointer
arithmetic that may yield out of bounds values. This warning level may
give a larger number of false positives and is deactivated by default.

Warn about equality and relational comparisons between two operands of array
type. This comparison was deprecated in C++20. For example:

int arr1[5];
int arr2[5];
bool same = arr1 == arr2;

-Warray-compare is enabled by -Wall.

Warn about redeclarations of functions involving arguments of array or
pointer types of inconsistent kinds or forms, and enable the detection
of out-of-bounds accesses to such parameters by warnings such as
-Warray-bounds.

If the first function declaration uses the array form the bound specified
in the array is assumed to be the minimum number of elements expected to
be provided in calls to the function and the maximum number of elements
accessed by it. Failing to provide arguments of sufficient size or accessing
more than the maximum number of elements may be diagnosed by warnings such
as -Warray-bounds. At level 1 the warning diagnoses inconsistencies
involving array parameters declared using the T[static N] form.

For example, the warning triggers for the following redeclarations because
the first one allows an array of any size to be passed to f while
the second one with the keyword static specifies that the array
argument must have at least four elements.

void f (int[static 4]);
void f (int[]);           // warning (inconsistent array form)

void g (void)
{
  int *p = (int *)malloc (4);
  f (p);                  // warning (array too small)
  …
}

At level 2 the warning also triggers for redeclarations involving any other
inconsistency in array or pointer argument forms denoting array sizes.
Pointers and arrays of unspecified bound are considered equivalent and do
not trigger a warning.

void g (int*);
void g (int[]);     // no warning
void g (int[8]);    // warning (inconsistent array bound)

-Warray-parameter=2 is included in -Wall. The
-Wvla-parameter option triggers warnings for similar inconsistencies
involving Variable Length Array arguments.

Warn about declarations using the alias and similar attributes whose
target is incompatible with the type of the alias.
See Declaring Attributes of Functions.

-Wattribute-alias=1

The default warning level of the -Wattribute-alias option diagnoses
incompatibilities between the type of the alias declaration and that of its
target. Such incompatibilities are typically indicative of bugs.

-Wattribute-alias=2

At this level -Wattribute-alias also diagnoses cases where
the attributes of the alias declaration are more restrictive than the
attributes applied to its target. These mismatches can potentially
result in incorrect code generation. In other cases they may be
benign and could be resolved simply by adding the missing attribute to
the target. For comparison, see the -Wmissing-attributes
option, which controls diagnostics when the alias declaration is less
restrictive than the target, rather than more restrictive.

Attributes considered include alloc_align, alloc_size,
cold, const, hot, leaf, malloc,
nonnull, noreturn, nothrow, pure,
returns_nonnull, and returns_twice.

-Wattribute-alias is equivalent to -Wattribute-alias=1.
This is the default. You can disable these warnings with either
-Wno-attribute-alias or -Wattribute-alias=0.

Warn about possibly misleading UTF-8 bidirectional control characters in
comments, string literals, character constants, and identifiers. Such
characters can change left-to-right writing direction into right-to-left
(and vice versa), which can cause confusion between the logical order and
visual order. This may be dangerous; for instance, it may seem that a piece
of code is not commented out, whereas it in fact is.

There are three levels of warning supported by GCC. The default is
-Wbidi-chars=unpaired, which warns about improperly terminated
bidi contexts. -Wbidi-chars=none turns the warning off.
-Wbidi-chars=any warns about any use of bidirectional control
characters.

By default, this warning does not warn about UCNs. It is, however, possible
to turn on such checking by using -Wbidi-chars=unpaired,ucn or
-Wbidi-chars=any,ucn. Using -Wbidi-chars=ucn is valid,
and is equivalent to -Wbidi-chars=unpaired,ucn, if no previous
-Wbidi-chars=any was specified.

Warn about boolean expression compared with an integer value different from
true/false. For instance, the following comparison is
always false:

int n = 5;
…
if ((n > 1) == 2) { … }

This warning is enabled by -Wall.

Warn about suspicious operations on expressions of a boolean type. For
instance, bitwise negation of a boolean is very likely a bug in the program.
For C, this warning also warns about incrementing or decrementing a boolean,
which rarely makes sense. (In C++, decrementing a boolean is always invalid.
Incrementing a boolean is invalid in C++17, and deprecated otherwise.)

This warning is enabled by -Wall.

Warn when an if-else has identical branches. This warning detects cases like

if (p != NULL)
  return 0;
else
  return 0;

It doesn’t warn when both branches contain just a null statement. This warning
also warn for conditional operators:

Warn about duplicated conditions in an if-else-if chain. For instance,
warn for the following code:

if (p->q != NULL) { … }
else if (p->q != NULL) { … }

Warn when the ‘__builtin_frame_address’ or ‘__builtin_return_address’
is called with an argument greater than 0. Such calls may return indeterminate
values or crash the program. The warning is included in -Wall.

Do not warn if type qualifiers on pointers are being discarded.
Typically, the compiler warns if a const char * variable is
passed to a function that takes a char * parameter. This option
can be used to suppress such a warning.

Do not warn if type qualifiers on arrays which are pointer targets
are being discarded. Typically, the compiler warns if a
const int (*)[] variable is passed to a function that
takes a int (*)[] parameter. This option can be used to
suppress such a warning.

Do not warn when there is a conversion between pointers that have incompatible
types. This warning is for cases not covered by -Wno-pointer-sign,
which warns for pointer argument passing or assignment with different
signedness.

This warning is upgraded to an error by -pedantic-errors.

Do not warn about incompatible integer to pointer and pointer to integer
conversions. This warning is about implicit conversions; for explicit
conversions the warnings -Wno-int-to-pointer-cast and
-Wno-pointer-to-int-cast may be used.

This warning is upgraded to an error by -pedantic-errors.

Warn about accesses to elements of zero-length array members that might
overlap other members of the same object. Declaring interior zero-length
arrays is discouraged because accesses to them are undefined.
See Arrays of Length Zero.

For example, the first two stores in function bad are diagnosed
because the array elements overlap the subsequent members b and
c. The third store is diagnosed by -Warray-bounds
because it is beyond the bounds of the enclosing object.

struct X { int a[0]; int b, c; };
struct X x;

void bad (void)
{
  x.a[0] = 0;   // -Wzero-length-bounds
  x.a[1] = 1;   // -Wzero-length-bounds
  x.a[2] = 2;   // -Warray-bounds
}

Option -Wzero-length-bounds is enabled by -Warray-bounds.

Do not warn about compile-time integer division by zero. Floating-point
division by zero is not warned about, as it can be a legitimate way of
obtaining infinities and NaNs.

Print warning messages for constructs found in system header files.
Warnings from system headers are normally suppressed, on the assumption
that they usually do not indicate real problems and would only make the
compiler output harder to read. Using this command-line option tells
GCC to emit warnings from system headers as if they occurred in user
code. However, note that using -Wall in conjunction with this
option does not warn about unknown pragmas in system
headers—for that, -Wunknown-pragmas must also be used.

Warn if a self-comparison always evaluates to true or false. This
warning detects various mistakes such as:

int i = 1;
…
if (i > i) { … }

This warning also warns about bitwise comparisons that always evaluate
to true or false, for instance:

if ((a & 16) == 10) { … }

will always be false.

This warning is enabled by -Wall.

Warn about trampolines generated for pointers to nested functions.
A trampoline is a small piece of data or code that is created at run
time on the stack when the address of a nested function is taken, and is
used to call the nested function indirectly. For some targets, it is
made up of data only and thus requires no special treatment. But, for
most targets, it is made up of code and thus requires the stack to be
made executable in order for the program to work properly.

Warn if floating-point values are used in equality comparisons.

The idea behind this is that sometimes it is convenient (for the
programmer) to consider floating-point values as approximations to
infinitely precise real numbers. If you are doing this, then you need
to compute (by analyzing the code, or in some other way) the maximum or
likely maximum error that the computation introduces, and allow for it
when performing comparisons (and when producing output, but that’s a
different problem). In particular, instead of testing for equality, you
should check to see whether the two values have ranges that overlap; and
this is done with the relational operators, so equality comparisons are
probably mistaken.

Warn about certain constructs that behave differently in traditional and
ISO C. Also warn about ISO C constructs that have no traditional C
equivalent, and/or problematic constructs that should be avoided.

• Macro parameters that appear within string literals in the macro body.
  In traditional C macro replacement takes place within string literals,
  but in ISO C it does not.

• In traditional C, some preprocessor directives did not exist.
  Traditional preprocessors only considered a line to be a directive
  if the ‘#’ appeared in column 1 on the line. Therefore
  -Wtraditional warns about directives that traditional C
  understands but ignores because the ‘#’ does not appear as the
  first character on the line. It also suggests you hide directives like
  #pragma not understood by traditional C by indenting them. Some
  traditional implementations do not recognize #elif, so this option
  suggests avoiding it altogether.

• A function-like macro that appears without arguments.
• The unary plus operator.
• The ‘U’ integer constant suffix, or the ‘F’ or ‘L’ floating-point
  constant suffixes. (Traditional C does support the ‘L’ suffix on integer
  constants.) Note, these suffixes appear in macros defined in the system
  headers of most modern systems, e.g. the ‘_MIN’/‘_MAX’ macros in <limits.h>.
  Use of these macros in user code might normally lead to spurious
  warnings, however GCC’s integrated preprocessor has enough context to
  avoid warning in these cases.

• A function declared external in one block and then used after the end of
  the block.

• A switch statement has an operand of type long.
• A non-static function declaration follows a static one.
  This construct is not accepted by some traditional C compilers.

• The ISO type of an integer constant has a different width or
  signedness from its traditional type. This warning is only issued if
  the base of the constant is ten. I.e. hexadecimal or octal values, which
  typically represent bit patterns, are not warned about.

• Usage of ISO string concatenation is detected.
• Initialization of automatic aggregates.
• Identifier conflicts with labels. Traditional C lacks a separate
  namespace for labels.

• Initialization of unions. If the initializer is zero, the warning is
  omitted. This is done under the assumption that the zero initializer in
  user code appears conditioned on e.g. __STDC__ to avoid missing
  initializer warnings and relies on default initialization to zero in the
  traditional C case.

• Conversions by prototypes between fixed/floating-point values and vice
  versa. The absence of these prototypes when compiling with traditional
  C causes serious problems. This is a subset of the possible
  conversion warnings; for the full set use -Wtraditional-conversion.

• Use of ISO C style function definitions. This warning intentionally is
  not issued for prototype declarations or variadic functions
  because these ISO C features appear in your code when using
  libiberty’s traditional C compatibility macros, PARAMS and
  VPARAMS. This warning is also bypassed for nested functions
  because that feature is already a GCC extension and thus not relevant to
  traditional C compatibility.

Warn if a prototype causes a type conversion that is different from what
would happen to the same argument in the absence of a prototype. This
includes conversions of fixed point to floating and vice versa, and
conversions changing the width or signedness of a fixed-point argument
except when the same as the default promotion.

Warn when a declaration is found after a statement in a block. This
construct, known from C++, was introduced with ISO C99 and is by default
allowed in GCC. It is not supported by ISO C90. See Mixed Declarations, Labels and Code.

This warning is upgraded to an error by -pedantic-errors.

Warn whenever a local variable or type declaration shadows another
variable, parameter, type, class member (in C++), or instance variable
(in Objective-C) or whenever a built-in function is shadowed. Note
that in C++, the compiler warns if a local variable shadows an
explicit typedef, but not if it shadows a struct/class/enum.
If this warning is enabled, it includes also all instances of
local shadowing. This means that -Wno-shadow=local
and -Wno-shadow=compatible-local are ignored when
-Wshadow is used.
Same as -Wshadow=global.

Do not warn whenever a local variable shadows an instance variable in an
Objective-C method.

Warn for any shadowing.
Same as -Wshadow.

Warn when a local variable shadows another local variable or parameter.

Warn when a local variable shadows another local variable or parameter
whose type is compatible with that of the shadowing variable. In C++,
type compatibility here means the type of the shadowing variable can be
converted to that of the shadowed variable. The creation of this flag
(in addition to -Wshadow=local) is based on the idea that when
a local variable shadows another one of incompatible type, it is most
likely intentional, not a bug or typo, as shown in the following example:

for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
  for (int i = 0; i < N; ++i)
  {
    ...
  }
  ...
}

Since the two variable i in the example above have incompatible types,
enabling only -Wshadow=compatible-local does not emit a warning.
Because their types are incompatible, if a programmer accidentally uses one
in place of the other, type checking is expected to catch that and emit an
error or warning. Use of this flag instead of -Wshadow=local can
possibly reduce the number of warnings triggered by intentional shadowing.
Note that this also means that shadowing const char *i by
char *i does not emit a warning.

This warning is also enabled by -Wshadow=local.

Warn whenever an object is defined whose size exceeds byte-size.
-Wlarger-than=‘PTRDIFF_MAX’ is enabled by default.
Warnings controlled by the option can be disabled either by specifying
byte-size of ‘SIZE_MAX’ or more or by -Wno-larger-than.

Also warn for calls to bounded functions such as memchr or
strnlen that specify a bound greater than the largest possible
object, which is ‘PTRDIFF_MAX’ bytes by default. These warnings
can only be disabled by -Wno-larger-than.

Disable -Wlarger-than= warnings. The option is equivalent
to -Wlarger-than=‘SIZE_MAX’ or larger.

Warn if the size of a function frame exceeds byte-size.
The computation done to determine the stack frame size is approximate
and not conservative.
The actual requirements may be somewhat greater than byte-size
even if you do not get a warning. In addition, any space allocated
via alloca, variable-length arrays, or related constructs
is not included by the compiler when determining
whether or not to issue a warning.
-Wframe-larger-than=‘PTRDIFF_MAX’ is enabled by default.
Warnings controlled by the option can be disabled either by specifying
byte-size of ‘SIZE_MAX’ or more or by
-Wno-frame-larger-than.

Disable -Wframe-larger-than= warnings. The option is equivalent
to -Wframe-larger-than=‘SIZE_MAX’ or larger.

Warn when attempting to deallocate an object that was either not allocated
on the heap, or by using a pointer that was not returned from a prior call
to the corresponding allocation function. For example, because the call
to stpcpy returns a pointer to the terminating nul character and
not to the beginning of the object, the call to free below is
diagnosed.

void f (char *p)
{
  p = stpcpy (p, "abc");
  // ...
  free (p);   // warning
}

-Wfree-nonheap-object is included in -Wall.

Warn if the stack usage of a function might exceed byte-size.
The computation done to determine the stack usage is conservative.
Any space allocated via alloca, variable-length arrays, or related
constructs is included by the compiler when determining whether or not to
issue a warning.

The message is in keeping with the output of -fstack-usage.

• If the stack usage is fully static but exceeds the specified amount, it’s:

    warning: stack usage is 1120 bytes
  

• If the stack usage is (partly) dynamic but bounded, it’s:

    warning: stack usage might be 1648 bytes
  

• If the stack usage is (partly) dynamic and not bounded, it’s:

    warning: stack usage might be unbounded
  

-Wstack-usage=‘PTRDIFF_MAX’ is enabled by default.
Warnings controlled by the option can be disabled either by specifying
byte-size of ‘SIZE_MAX’ or more or by
-Wno-stack-usage.

Disable -Wstack-usage= warnings. The option is equivalent
to -Wstack-usage=‘SIZE_MAX’ or larger.

Warn if the loop cannot be optimized because the compiler cannot
assume anything on the bounds of the loop indices. With
-funsafe-loop-optimizations warn if the compiler makes
such assumptions.

When used in combination with -Wformat
and -pedantic without GNU extensions, this option
disables the warnings about non-ISO printf / scanf format
width specifiers I32, I64, and I used on Windows targets,
which depend on the MS runtime.

Warn about anything that depends on the “size of” a function type or
of void. GNU C assigns these types a size of 1, for
convenience in calculations with void * pointers and pointers
to functions. In C++, warn also when an arithmetic operation involves
NULL. This warning is also enabled by -Wpedantic.

This warning is upgraded to an error by -pedantic-errors.

Do not warn if a pointer is compared with a zero character constant.
This usually
means that the pointer was meant to be dereferenced. For example:

const char *p = foo ();
if (p == '\0')
  return 42;

Note that the code above is invalid in C++11.

This warning is enabled by default.

Warn about unsupported features in ThreadSanitizer.

ThreadSanitizer does not support std::atomic_thread_fence and
can report false positives.

This warning is enabled by default.

Warn if a comparison is always true or always false due to the limited
range of the data type, but do not warn for constant expressions. For
example, warn if an unsigned variable is compared against zero with
< or >=. This warning is also enabled by
-Wextra.

Warn for calls to standard functions that compute the absolute value
of an argument when a more appropriate standard function is available.
For example, calling abs(3.14) triggers the warning because the
appropriate function to call to compute the absolute value of a double
argument is fabs. The option also triggers warnings when the
argument in a call to such a function has an unsigned type. This
warning can be suppressed with an explicit type cast and it is also
enabled by -Wextra.

Warn whenever a comment-start sequence ‘/*’ appears in a ‘/*’
comment, or whenever a backslash-newline appears in a ‘//’ comment.
This warning is enabled by -Wall.

Warn if any trigraphs are encountered that might change the meaning of
the program. Trigraphs within comments are not warned about,
except those that would form escaped newlines.

This option is implied by -Wall. If -Wall is not
given, this option is still enabled unless trigraphs are enabled. To
get trigraph conversion without warnings, but get the other
-Wall warnings, use ‘-trigraphs -Wall -Wno-trigraphs’.

Warn if an undefined identifier is evaluated in an #if directive.
Such identifiers are replaced with zero.

Warn whenever ‘defined’ is encountered in the expansion of a macro
(including the case where the macro is expanded by an ‘#if’ directive).
Such usage is not portable.
This warning is also enabled by -Wpedantic and -Wextra.

Warn about macros defined in the main file that are unused. A macro
is used if it is expanded or tested for existence at least once.
The preprocessor also warns if the macro has not been used at the
time it is redefined or undefined.

Built-in macros, macros defined on the command line, and macros
defined in include files are not warned about.

Note: If a macro is actually used, but only used in skipped
conditional blocks, then the preprocessor reports it as unused. To avoid the
warning in such a case, you might improve the scope of the macro’s
definition by, for example, moving it into the first skipped block.
Alternatively, you could provide a dummy use with something like:

#if defined the_macro_causing_the_warning
#endif

Do not warn whenever an #else or an #endif are followed by text.
This sometimes happens in older programs with code of the form

#if FOO
…
#else FOO
…
#endif FOO

The second and third FOO should be in comments.
This warning is on by default.

Warn when a function call is cast to a non-matching type.
For example, warn if a call to a function returning an integer type
is cast to a pointer type.

Warn about features not present in ISO C90, but present in ISO C99.
For instance, warn about use of variable length arrays, long long
type, bool type, compound literals, designated initializers, and so
on. This option is independent of the standards mode. Warnings are disabled
in the expression that follows __extension__.

Warn about features not present in ISO C99, but present in ISO C11.
For instance, warn about use of anonymous structures and unions,
_Atomic type qualifier, _Thread_local storage-class specifier,
_Alignas specifier, Alignof operator, _Generic keyword,
and so on. This option is independent of the standards mode. Warnings are
disabled in the expression that follows __extension__.

Warn about features not present in ISO C11, but present in ISO C2X.
For instance, warn about omitting the string in _Static_assert,
use of ‘[[]]’ syntax for attributes, use of decimal
floating-point types, and so on. This option is independent of the
standards mode. Warnings are disabled in the expression that follows
__extension__.

When not compiling in C2X mode, these warnings are upgraded to errors
by -pedantic-errors.

Warn about ISO C constructs that are outside of the common subset of
ISO C and ISO C++, e.g. request for implicit conversion from
void * to a pointer to non-void type.

Warn about C++ constructs whose meaning differs between ISO C++ 1998
and ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that are keywords
in ISO C++ 2011. This warning turns on -Wnarrowing and is
enabled by -Wall.

Warn about C++ constructs whose meaning differs between ISO C++ 2011
and ISO C++ 2014. This warning is enabled by -Wall.

Warn about C++ constructs whose meaning differs between ISO C++ 2014
and ISO C++ 2017. This warning is enabled by -Wall.

Warn about C++ constructs whose meaning differs between ISO C++ 2017
and ISO C++ 2020. This warning is enabled by -Wall.

Do not warn about C++11 constructs in code being compiled using
an older C++ standard. Even without this option, some C++11 constructs
will only be diagnosed if -Wpedantic is used.

Do not warn about C++14 constructs in code being compiled using
an older C++ standard. Even without this option, some C++14 constructs
will only be diagnosed if -Wpedantic is used.

Do not warn about C++17 constructs in code being compiled using
an older C++ standard. Even without this option, some C++17 constructs
will only be diagnosed if -Wpedantic is used.

Do not warn about C++20 constructs in code being compiled using
an older C++ standard. Even without this option, some C++20 constructs
will only be diagnosed if -Wpedantic is used.

Do not warn about C++23 constructs in code being compiled using
an older C++ standard. Even without this option, some C++23 constructs
will only be diagnosed if -Wpedantic is used.

Warn whenever a pointer is cast so as to remove a type qualifier from
the target type. For example, warn if a const char * is cast
to an ordinary char *.

Also warn when making a cast that introduces a type qualifier in an
unsafe way. For example, casting char ** to const char **
is unsafe, as in this example:

  /* p is char ** value.  */
  const char **q = (const char **) p;
  /* Assignment of readonly string to const char * is OK.  */
  *q = "string";
  /* Now char** pointer points to read-only memory.  */
  **p = 'b';

Warn whenever a pointer is cast such that the required alignment of the
target is increased. For example, warn if a char * is cast to
an int * on machines where integers can only be accessed at
two- or four-byte boundaries.

Warn whenever a pointer is cast such that the required alignment of the
target is increased. For example, warn if a char * is cast to
an int * regardless of the target machine.

Warn when a function pointer is cast to an incompatible function pointer.
In a cast involving function types with a variable argument list only
the types of initial arguments that are provided are considered.
Any parameter of pointer-type matches any other pointer-type. Any benign
differences in integral types are ignored, like int vs. long
on ILP32 targets. Likewise type qualifiers are ignored. The function
type void (*) (void) is special and matches everything, which can
be used to suppress this warning.
In a cast involving pointer to member types this warning warns whenever
the type cast is changing the pointer to member type.
This warning is enabled by -Wextra.

When compiling C, give string constants the type const
char[length] so that copying the address of one into a
non-const char * pointer produces a warning. These
warnings help you find at compile time code that can try to write
into a string constant, but only if you have been very careful about
using const in declarations and prototypes. Otherwise, it is
just a nuisance. This is why we did not make -Wall request
these warnings.

When compiling C++, warn about the deprecated conversion from string
literals to char *. This warning is enabled by default for C++
programs.

This warning is upgraded to an error by -pedantic-errors in
C++11 mode or later.

Warn for variables that might be changed by longjmp or
vfork. This warning is also enabled by -Wextra.

By default, language front ends complain when a command-line option is
valid, but not applicable to that front end.
This may be disabled with -Wno-complain-wrong-lang,
which is mostly useful when invoking a single compiler driver for
multiple source files written in different languages, for example:

$ g++ -fno-rtti a.cc b.f90

The driver g++ invokes the C++ front end to compile a.cc
and the Fortran front end to compile b.f90.
The latter front end diagnoses
‘f951: Warning: command-line option ‘-fno-rtti’ is valid for C++/D/ObjC++ but not for Fortran’,
which may be disabled with -Wno-complain-wrong-lang.

Warn if pointers of distinct types are compared without a cast. This
warning is enabled by default.

Warn for implicit conversions that may alter a value. This includes
conversions between real and integer, like abs (x) when
x is double; conversions between signed and unsigned,
like unsigned ui = -1; and conversions to smaller types, like
sqrtf (M_PI). Do not warn for explicit casts like abs
((int) x) and ui = (unsigned) -1, or if the value is not
changed by the conversion like in abs (2.0). Warnings about
conversions between signed and unsigned integers can be disabled by
using -Wno-sign-conversion.

For C++, also warn for confusing overload resolution for user-defined
conversions; and conversions that never use a type conversion
operator: conversions to void, the same type, a base class or a
reference to them. Warnings about conversions between signed and
unsigned integers are disabled by default in C++ unless
-Wsign-conversion is explicitly enabled.

Warnings about conversion from arithmetic on a small type back to that
type are only given with -Warith-conversion.

Warn about constructions where there may be confusion to which
if statement an else branch belongs. Here is an example of
such a case:

{
  if (a)
    if (b)
      foo ();
  else
    bar ();
}

In C/C++, every else branch belongs to the innermost possible
if statement, which in this example is if (b). This is
often not what the programmer expected, as illustrated in the above
example by indentation the programmer chose. When there is the
potential for this confusion, GCC issues a warning when this flag
is specified. To eliminate the warning, add explicit braces around
the innermost if statement so there is no way the else
can belong to the enclosing if. The resulting code
looks like this:

{
  if (a)
    {
      if (b)
        foo ();
      else
        bar ();
    }
}

This warning is enabled by -Wparentheses.

Warn about uses of pointers (or C++ references) to objects with automatic
storage duration after their lifetime has ended. This includes local
variables declared in nested blocks, compound literals and other unnamed
temporary objects. In addition, warn about storing the address of such
objects in escaped pointers. The warning is enabled at all optimization
levels but may yield different results with optimization than without.

-Wdangling-pointer=1

At level 1 the warning diagnoses only unconditional uses of dangling pointers.
For example

int f (int c1, int c2, x)
{
  char *p = strchr ((char[]){ c1, c2 }, c3);
  // warning: dangling pointer to a compound literal
  return p ? *p : 'x';
}

In the following function the store of the address of the local variable
x in the escaped pointer *p also triggers the warning.

void g (int **p)
{
  int x = 7;
  // warning: storing the address of a local variable in *p
  *p = &x;
}

-Wdangling-pointer=2

At level 2, in addition to unconditional uses the warning also diagnoses
conditional uses of dangling pointers.

For example, because the array a in the following function is out of
scope when the pointer s that was set to point is used, the warning
triggers at this level.

void f (char *s)
{
  if (!s)
    {
      char a[12] = "tmpname";
      s = a;
    }
  // warning: dangling pointer to a may be used
  strcat (s, ".tmp");
  ...
}

-Wdangling-pointer=2 is included in -Wall.

Warn when macros __TIME__, __DATE__ or __TIMESTAMP__
are encountered as they might prevent bit-wise-identical reproducible
compilations.

Warn if an empty body occurs in an if, else or do
while statement. This warning is also enabled by -Wextra.

Do not warn about stray tokens after #else and #endif.

Warn about a comparison between values of different enumerated types.
In C++ enumerated type mismatches in conditional expressions are also
diagnosed and the warning is enabled by default. In C this warning is
enabled by -Wall.

Warn when a value of enumerated type is implicitly converted to a
different enumerated type. This warning is enabled by -Wextra
in C.

Warn about mismatches between an enumerated type and an integer type in
declarations. For example:

enum E { l = -1, z = 0, g = 1 };
int foo(void);
enum E foo(void);

In C, an enumerated type is compatible with char, a signed
integer type, or an unsigned integer type. However, since the choice
of the underlying type of an enumerated type is implementation-defined,
such mismatches may cause portability issues. In C++, such mismatches
are an error. In C, this warning is enabled by -Wall and
-Wc++-compat.

Warn if a goto statement or a switch statement jumps
forward across the initialization of a variable, or jumps backward to a
label after the variable has been initialized. This only warns about
variables that are initialized when they are declared. This warning is
only supported for C and Objective-C; in C++ this sort of branch is an
error in any case.

-Wjump-misses-init is included in -Wc++-compat. It
can be disabled with the -Wno-jump-misses-init option.

Warn when a comparison between signed and unsigned values could produce
an incorrect result when the signed value is converted to unsigned.
In C++, this warning is also enabled by -Wall. In C, it is
also enabled by -Wextra.

Warn for implicit conversions that may change the sign of an integer
value, like assigning a signed integer expression to an unsigned
integer variable. An explicit cast silences the warning. In C, this
option is enabled also by -Wconversion.

Warn when a structure containing a C99 flexible array member as the last
field is not at the end of another structure.
This warning warns e.g. about

struct flex  { int length; char data[]; };
struct mid_flex { int m; struct flex flex_data; int n; };

Warn for implicit conversions that reduce the precision of a real value.
This includes conversions from real to integer, and from higher precision
real to lower precision real values. This option is also enabled by
-Wconversion.

Do not warn on suspicious constructs involving reverse scalar storage order.

Warn about divisions of two sizeof operators when the first one is applied
to an array and the divisor does not equal the size of the array element.
In such a case, the computation will not yield the number of elements in the
array, which is likely what the user intended. This warning warns e.g. about

int fn ()
{
  int arr[10];
  return sizeof (arr) / sizeof (short);
}

This warning is enabled by -Wall.

Warn for suspicious divisions of two sizeof expressions that divide
the pointer size by the element size, which is the usual way to compute
the array size but won’t work out correctly with pointers. This warning
warns e.g. about sizeof (ptr) / sizeof (ptr[0]) if ptr is
not an array, but a pointer. This warning is enabled by -Wall.

Warn for suspicious length parameters to certain string and memory built-in
functions if the argument uses sizeof. This warning triggers for
example for memset (ptr, 0, sizeof (ptr)); if ptr is not
an array, but a pointer, and suggests a possible fix, or about
memcpy (&foo, ptr, sizeof (&foo));. -Wsizeof-pointer-memaccess
also warns about calls to bounded string copy functions like strncat
or strncpy that specify as the bound a sizeof expression of
the source array. For example, in the following function the call to
strncat specifies the size of the source string as the bound. That
is almost certainly a mistake and so the call is diagnosed.

void make_file (const char *name)
{
  char path[PATH_MAX];
  strncpy (path, name, sizeof path - 1);
  strncat (path, ".text", sizeof ".text");
  …
}

The -Wsizeof-pointer-memaccess option is enabled by -Wall.

Do not warn when the sizeof operator is applied to a parameter that is
declared as an array in a function definition. This warning is enabled by
default for C and C++ programs.

Warn for suspicious calls to the memset built-in function, if the
first argument references an array, and the third argument is a number
equal to the number of elements, but not equal to the size of the array
in memory. This indicates that the user has omitted a multiplication by
the element size. This warning is enabled by -Wall.

Warn for suspicious calls to the memset built-in function where
the second argument is not zero and the third argument is zero. For
example, the call memset (buf, sizeof buf, 0) is diagnosed because
memset (buf, 0, sizeof buf) was meant instead. The diagnostic
is only emitted if the third argument is a literal zero. Otherwise, if
it is an expression that is folded to zero, or a cast of zero to some
type, it is far less likely that the arguments have been mistakenly
transposed and no warning is emitted. This warning is enabled
by -Wall.

Warn about suspicious uses of address expressions. These include comparing
the address of a function or a declared object to the null pointer constant
such as in

void f (void);
void g (void)
{
  if (!f)   // warning: expression evaluates to false
    abort ();
}

comparisons of a pointer to a string literal, such as in

void f (const char *x)
{
  if (x == "abc")   // warning: expression evaluates to false
    puts ("equal");
}

and tests of the results of pointer addition or subtraction for equality
to null, such as in

void f (const int *p, int i)
{
  return p + i == NULL;
}

Such uses typically indicate a programmer error: the address of most
functions and objects necessarily evaluates to true (the exception are
weak symbols), so their use in a conditional might indicate missing
parentheses in a function call or a missing dereference in an array
expression. The subset of the warning for object pointers can be
suppressed by casting the pointer operand to an integer type such
as intptr_t or uintptr_t.
Comparisons against string literals result in unspecified behavior
and are not portable, and suggest the intent was to call strcmp.
The warning is suppressed if the suspicious expression is the result
of macro expansion.
-Waddress warning is enabled by -Wall.

Do not warn when the address of packed member of struct or union is taken,
which usually results in an unaligned pointer value. This is
enabled by default.

Warn about suspicious uses of logical operators in expressions.
This includes using logical operators in contexts where a
bit-wise operator is likely to be expected. Also warns when
the operands of a logical operator are the same:

extern int a;
if (a < 0 && a < 0) { … }

Warn about logical not used on the left hand side operand of a comparison.
This option does not warn if the right operand is considered to be a boolean
expression. Its purpose is to detect suspicious code like the following:

int a;
…
if (!a > 1) { … }

It is possible to suppress the warning by wrapping the LHS into
parentheses:

This warning is enabled by -Wall.

Warn if any functions that return structures or unions are defined or
called. (In languages where you can return an array, this also elicits
a warning.)

Warn if in a loop with constant number of iterations the compiler detects
undefined behavior in some statement during one or more of the iterations.

Do not warn if an unexpected __attribute__ is used, such as
unrecognized attributes, function attributes applied to variables,
etc. This does not stop errors for incorrect use of supported
attributes.

Warnings about ill-formed uses of standard attributes are upgraded to
errors by -pedantic-errors.

Additionally, using -Wno-attributes=, it is possible to suppress
warnings about unknown scoped attributes (in C++11 and C2X). For example,
-Wno-attributes=vendor::attr disables warning about the following
declaration:

[[vendor::attr]] void f();

It is also possible to disable warning about all attributes in a namespace
using -Wno-attributes=vendor:: which prevents warning about both
of these declarations:

[[vendor::safe]] void f();
[[vendor::unsafe]] void f2();

Note that -Wno-attributes= does not imply -Wno-attributes.

Warn if a built-in function is declared with an incompatible signature
or as a non-function, or when a built-in function declared with a type
that does not include a prototype is called with arguments whose promoted
types do not match those expected by the function. When -Wextra
is specified, also warn when a built-in function that takes arguments is
declared without a prototype. The -Wbuiltin-declaration-mismatch
warning is enabled by default. To avoid the warning include the appropriate
header to bring the prototypes of built-in functions into scope.

For example, the call to memset below is diagnosed by the warning
because the function expects a value of type size_t as its argument
but the type of 32 is int. With -Wextra,
the declaration of the function is diagnosed as well.

extern void* memset ();
void f (void *d)
{
  memset (d, '\0', 32);
}

Do not warn if certain built-in macros are redefined. This suppresses
warnings for redefinition of __TIMESTAMP__, __TIME__,
__DATE__, __FILE__, and __BASE_FILE__.

Warn if a function is declared or defined without specifying the
argument types. (An old-style function definition is permitted without
a warning if preceded by a declaration that specifies the argument
types.)

Warn for obsolescent usages, according to the C Standard, in a
declaration. For example, warn if storage-class specifiers like
static are not the first things in a declaration. This warning
is also enabled by -Wextra.

Warn if an old-style function definition is used. A warning is given
even if there is a previous prototype. A definition using ‘()’
is not considered an old-style definition in C2X mode, because it is
equivalent to ‘(void)’ in that case, but is considered an
old-style definition for older standards.

A function parameter is declared without a type specifier in K&R-style
functions:

This warning is also enabled by -Wextra.

Warn if a global function is defined without a previous prototype
declaration. This warning is issued even if the definition itself
provides a prototype. Use this option to detect global functions
that do not have a matching prototype declaration in a header file.
This option is not valid for C++ because all function declarations
provide prototypes and a non-matching declaration declares an
overload rather than conflict with an earlier declaration.
Use -Wmissing-declarations to detect missing declarations in C++.

Warn if a global variable is defined without a previous declaration.
Use this option to detect global variables that do not have a matching
extern declaration in a header file.

Warn if a global function is defined without a previous declaration.
Do so even if the definition itself provides a prototype.
Use this option to detect global functions that are not declared in
header files. In C, no warnings are issued for functions with previous
non-prototype declarations; use -Wmissing-prototypes to detect
missing prototypes. In C++, no warnings are issued for function templates,
or for inline functions, or for functions in anonymous namespaces.

Warn if a structure’s initializer has some fields missing. For
example, the following code causes such a warning, because
x.h is implicitly zero:

struct s { int f, g, h; };
struct s x = { 3, 4 };

In C this option does not warn about designated initializers, so the
following modification does not trigger a warning:

struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };

In C this option does not warn about the universal zero initializer
‘{ 0 }’:

struct s { int f, g, h; };
struct s x = { 0 };

Likewise, in C++ this option does not warn about the empty { }
initializer, for example:

struct s { int f, g, h; };
s x = { };

This warning is included in -Wextra. To get other -Wextra
warnings without this one, use -Wextra -Wno-missing-field-initializers.

By default, the compiler warns about a concept-id appearing as a C++20 simple-requirement:

bool satisfied = requires { C<T> };

Here ‘satisfied’ will be true if ‘C<T>’ is a valid
expression, which it is for all T. Presumably the user meant to write

bool satisfied = requires { requires C<T> };

so ‘satisfied’ is only true if concept ‘C’ is satisfied for
type ‘T’.

This warning can be disabled with -Wno-missing-requires.

The member access tokens ., -> and :: must be followed by the template
keyword if the parent object is dependent and the member being named is a
template.

template <class X>
void DoStuff (X x)
{
  x.template DoSomeOtherStuff<X>(); // Good.
  x.DoMoreStuff<X>(); // Warning, x is dependent.
}

In rare cases it is possible to get false positives. To silence this, wrap
the expression in parentheses. For example, the following is treated as a
template, even where m and N are integers:

void NotATemplate (my_class t)
{
  int N = 5;

  bool test = t.m < N > (0); // Treated as a template.
  test = (t.m < N) > (0); // Same meaning, but not treated as a template.
}

This warning can be disabled with -Wno-missing-template-keyword.

Do not warn if a multicharacter constant (‘‘FOOF’’) is used.
Usually they indicate a typo in the user’s code, as they have
implementation-defined values, and should not be used in portable code.

In ISO C and ISO C++, two identifiers are different if they are
different sequences of characters. However, sometimes when characters
outside the basic ASCII character set are used, you can have two
different character sequences that look the same. To avoid confusion,
the ISO 10646 standard sets out some normalization rules which
when applied ensure that two sequences that look the same are turned into
the same sequence. GCC can warn you if you are using identifiers that
have not been normalized; this option controls that warning.

There are four levels of warning supported by GCC. The default is
-Wnormalized=nfc, which warns about any identifier that is
not in the ISO 10646 “C” normalized form, NFC. NFC is the
recommended form for most uses. It is equivalent to
-Wnormalized.

Unfortunately, there are some characters allowed in identifiers by
ISO C and ISO C++ that, when turned into NFC, are not allowed in
identifiers. That is, there’s no way to use these symbols in portable
ISO C or C++ and have all your identifiers in NFC.
-Wnormalized=id suppresses the warning for these characters.
It is hoped that future versions of the standards involved will correct
this, which is why this option is not the default.

You can switch the warning off for all characters by writing
-Wnormalized=none or -Wno-normalized. You should
only do this if you are using some other normalization scheme (like
“D”), because otherwise you can easily create bugs that are
literally impossible to see.

Some characters in ISO 10646 have distinct meanings but look identical
in some fonts or display methodologies, especially once formatting has
been applied. For instance \u207F, “SUPERSCRIPT LATIN SMALL
LETTER N”, displays just like a regular n that has been
placed in a superscript. ISO 10646 defines the NFKC
normalization scheme to convert all these into a standard form as
well, and GCC warns if your code is not in NFKC if you use
-Wnormalized=nfkc. This warning is comparable to warning
about every identifier that contains the letter O because it might be
confused with the digit 0, and so is not the default, but may be
useful as a local coding convention if the programming environment
cannot be fixed to display these characters distinctly.

Do not warn about usage of functions (see Declaring Attributes of Functions)
declared with warning attribute. By default, this warning is
enabled. -Wno-attribute-warning can be used to disable the
warning or -Wno-error=attribute-warning can be used to
disable the error when compiled with -Werror flag.

Do not warn about usage of deprecated features. See Deprecated Features.

Do not warn about uses of functions (see Declaring Attributes of Functions),
variables (see Specifying Attributes of Variables), and types (see Specifying Attributes of Types) marked as deprecated by using the deprecated
attribute.

Do not warn about compile-time overflow in constant expressions.

Warn about One Definition Rule violations during link-time optimization.
Enabled by default.

Warn about potentially suboptimal choices related to OpenACC parallelism.

Warn if the vectorizer cost model overrides the OpenMP
simd directive set by user. The -fsimd-cost-model=unlimited
option can be used to relax the cost model.

Warn if an initialized field without side effects is overridden when
using designated initializers (see Designated
Initializers).

This warning is included in -Wextra. To get other
-Wextra warnings without this one, use -Wextra
-Wno-override-init.

Do not warn if an initialized field with side effects is overridden when
using designated initializers (see Designated
Initializers). This warning is enabled by default.

Warn if a structure is given the packed attribute, but the packed
attribute has no effect on the layout or size of the structure.
Such structures may be mis-aligned for little benefit. For
instance, in this code, the variable f.x in struct bar
is misaligned even though struct bar does not itself
have the packed attribute:

struct foo {
  int x;
  char a, b, c, d;
} __attribute__((packed));
struct bar {
  char z;
  struct foo f;
};

The 4.1, 4.2 and 4.3 series of GCC ignore the packed attribute
on bit-fields of type char. This was fixed in GCC 4.4 but
the change can lead to differences in the structure layout. GCC
informs you when the offset of such a field has changed in GCC 4.4.
For example there is no longer a 4-bit padding between field a
and b in this structure:

struct foo
{
  char a:4;
  char b:8;
} __attribute__ ((packed));

This warning is enabled by default. Use
-Wno-packed-bitfield-compat to disable this warning.

Warn if a structure field with explicitly specified alignment in a
packed struct or union is misaligned. For example, a warning will
be issued on struct S, like, warning: alignment 1 of
'struct S' is less than 8, in this code:

struct __attribute__ ((aligned (8))) S8 { char a[8]; };
struct __attribute__ ((packed)) S {
  struct S8 s8;
};

This warning is enabled by -Wall.

Warn if padding is included in a structure, either to align an element
of the structure or to align the whole structure. Sometimes when this
happens it is possible to rearrange the fields of the structure to
reduce the padding and so make the structure smaller.

Warn if anything is declared more than once in the same scope, even in
cases where multiple declaration is valid and changes nothing.

Warn when an object referenced by a restrict-qualified parameter
(or, in C++, a __restrict-qualified parameter) is aliased by another
argument, or when copies between such objects overlap. For example,
the call to the strcpy function below attempts to truncate the string
by replacing its initial characters with the last four. However, because
the call writes the terminating NUL into a[4], the copies overlap and
the call is diagnosed.

void foo (void)
{
  char a[] = "abcd1234";
  strcpy (a, a + 4);
  …
}

The -Wrestrict option detects some instances of simple overlap
even without optimization but works best at -O2 and above. It
is included in -Wall.

Warn if an extern declaration is encountered within a function.

Warn if a function that is declared as inline cannot be inlined.
Even with this option, the compiler does not warn about failures to
inline functions declared in system headers.

The compiler uses a variety of heuristics to determine whether or not
to inline a function. For example, the compiler takes into account
the size of the function being inlined and the amount of inlining
that has already been done in the current function. Therefore,
seemingly insignificant changes in the source program can cause the
warnings produced by -Winline to appear or disappear.

Warn about use of C++17 std::hardware_destructive_interference_size
without specifying its value with —param destructive-interference-size.
Also warn about questionable values for that option.

This variable is intended to be used for controlling class layout, to
avoid false sharing in concurrent code:

struct independent_fields {
  alignas(std::hardware_destructive_interference_size)
    std::atomic<int> one;
  alignas(std::hardware_destructive_interference_size)
    std::atomic<int> two;
};

Here ‘one’ and ‘two’ are intended to be far enough apart
that stores to one won’t require accesses to the other to reload the
cache line.

By default, —param destructive-interference-size and
—param constructive-interference-size are set based on the
current -mtune option, typically to the L1 cache line size
for the particular target CPU, sometimes to a range if tuning for a
generic target. So all translation units that depend on ABI
compatibility for the use of these variables must be compiled with
the same -mtune (or -mcpu).

If ABI stability is important, such as if the use is in a header for a
library, you should probably not use the hardware interference size
variables at all. Alternatively, you can force a particular value
with —param.

If you are confident that your use of the variable does not affect ABI
outside a single build of your project, you can turn off the warning
with -Wno-interference-size.

Warn for suspicious use of integer values where boolean values are expected,
such as conditional expressions (?:) using non-boolean integer constants in
boolean context, like if (a <= b ? 2 : 3). Or left shifting of signed
integers in boolean context, like for (a = 0; 1 << a; a++);. Likewise
for all kinds of multiplications regardless of the data type.
This warning is enabled by -Wall.

Suppress warnings from casts to pointer type of an integer of a
different size. In C++, casting to a pointer type of smaller size is
an error. Wint-to-pointer-cast is enabled by default.

Suppress warnings from casts from a pointer to an integer type of a
different size.

Warn if a precompiled header (see Using Precompiled Headers) is found in
the search path but cannot be used.

Warn if an invalid UTF-8 character is found.
This warning is on by default for C++23 if -finput-charset=UTF-8
is used and turned into error with -pedantic-errors.

Don’t diagnose invalid forms of delimited or named escape sequences which are
treated as separate tokens. Wunicode is enabled by default.

Warn if long long type is used. This is enabled by either
-Wpedantic or -Wtraditional in ISO C90 and C++98
modes. To inhibit the warning messages, use -Wno-long-long.

This warning is upgraded to an error by -pedantic-errors.

Warn if variadic macros are used in ISO C90 mode, or if the GNU
alternate syntax is used in ISO C99 mode. This is enabled by either
-Wpedantic or -Wtraditional. To inhibit the warning
messages, use -Wno-variadic-macros.

Do not warn upon questionable usage of the macros used to handle variable
arguments like va_start. These warnings are enabled by default.

Warn if vector operation is not implemented via SIMD capabilities of the
architecture. Mainly useful for the performance tuning.
Vector operation can be implemented piecewise, which means that the
scalar operation is performed on every vector element;
in parallel, which means that the vector operation is implemented
using scalars of wider type, which normally is more performance efficient;
and as a single scalar, which means that vector fits into a
scalar type.

Warn if a variable-length array is used in the code.
-Wno-vla prevents the -Wpedantic warning of
the variable-length array.

This warning is upgraded to an error by -pedantic-errors.

If this option is used, the compiler warns for declarations of
variable-length arrays whose size is either unbounded, or bounded
by an argument that allows the array size to exceed byte-size
bytes. This is similar to how -Walloca-larger-than=byte-size
works, but with variable-length arrays.

Note that GCC may optimize small variable-length arrays of a known
value into plain arrays, so this warning may not get triggered for
such arrays.

-Wvla-larger-than=‘PTRDIFF_MAX’ is enabled by default but
is typically only effective when -ftree-vrp is active (default
for -O2 and above).

See also -Walloca-larger-than=byte-size.

Disable -Wvla-larger-than= warnings. The option is equivalent
to -Wvla-larger-than=‘SIZE_MAX’ or larger.

Warn about redeclarations of functions involving arguments of Variable
Length Array types of inconsistent kinds or forms, and enable the detection
of out-of-bounds accesses to such parameters by warnings such as
-Warray-bounds.

If the first function declaration uses the VLA form the bound specified
in the array is assumed to be the minimum number of elements expected to
be provided in calls to the function and the maximum number of elements
accessed by it. Failing to provide arguments of sufficient size or
accessing more than the maximum number of elements may be diagnosed.

For example, the warning triggers for the following redeclarations because
the first one allows an array of any size to be passed to f while
the second one specifies that the array argument must have at least n
elements. In addition, calling f with the associated VLA bound
parameter in excess of the actual VLA bound triggers a warning as well.

void f (int n, int[n]);
// warning: argument 2 previously declared as a VLA
void f (int, int[]);

void g (int n)
{
    if (n > 4)
      return;
    int a[n];
    // warning: access to a by f may be out of bounds
    f (sizeof a, a);
  …
}

-Wvla-parameter is included in -Wall. The
-Warray-parameter option triggers warnings for similar problems
involving ordinary array arguments.

Warn if a register variable is declared volatile. The volatile
modifier does not inhibit all optimizations that may eliminate reads
and/or writes to register variables. This warning is enabled by
-Wall.

Warn about uses of ^, the exclusive or operator, where it appears
the user meant exponentiation. Specifically, the warning occurs when the
left-hand side is the decimal constant 2 or 10 and the right-hand side
is also a decimal constant.

In C and C++, ^ means exclusive or, whereas in some other languages
(e.g. TeX and some versions of BASIC) it means exponentiation.

This warning is enabled by default. It can be silenced by converting one
of the operands to hexadecimal.

Warn if a requested optimization pass is disabled. This warning does
not generally indicate that there is anything wrong with your code; it
merely indicates that GCC’s optimizers are unable to handle the code
effectively. Often, the problem is that your code is too big or too
complex; GCC refuses to optimize programs when the optimization
itself is likely to take inordinate amounts of time.

Warn for pointer argument passing or assignment with different signedness.
This option is only supported for C and Objective-C. It is implied by
-Wall and by -Wpedantic, which can be disabled with
-Wno-pointer-sign.

This warning is upgraded to an error by -pedantic-errors.

This option is only active when -fstack-protector is active. It
warns about functions that are not protected against stack smashing.

Warn about string constants that are longer than the “minimum
maximum” length specified in the C standard. Modern compilers
generally allow string constants that are much longer than the
standard’s minimum limit, but very portable programs should avoid
using longer strings.

The limit applies after string constant concatenation, and does
not count the trailing NUL. In C90, the limit was 509 characters; in
C99, it was raised to 4095. C++98 does not specify a normative
minimum maximum, so we do not diagnose overlength strings in C++.

This option is implied by -Wpedantic, and can be disabled with
-Wno-overlength-strings.

Issue a warning for any floating constant that does not have
a suffix. When used together with -Wsystem-headers it
warns about such constants in system header files. This can be useful
when preparing code to use with the FLOAT_CONST_DECIMAL64 pragma
from the decimal floating-point extension to C99.

During the link-time optimization, do not warn about type mismatches in
global declarations from different compilation units.
Requires -flto to be enabled. Enabled by default.

Suppress warnings when a positional initializer is used to initialize
a structure that has been marked with the designated_init
attribute.