Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there may have been an error.
The following language-independent options do not enable specific warnings but control the kinds of diagnostics produced by GCC.
-fsyntax-only
-fmax-errors=
n
-w
-Werror
-Werror=
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
-W
foo
. However,
-Wno-error=
foo
does not imply anything.
-Wfatal-errors
You can request many specific warnings with options beginning with ‘ -W ’, for example -Wimplicit to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning ‘ -Wno- ’ to turn off warnings; for example, -Wno-implicit . This manual lists only one of the two forms, whichever is not the default. For further language-specific options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options .
Some options, such as -Wall and -Wextra , turn on other options, such as -Wunused , which may turn on further options, such as -Wunused-value . The combined effect of positive and negative forms is that more specific options have priority over less specific ones, independently of their position in the command-line. For options of the same specificity, the last one takes effect. Options enabled or disabled via pragmas (see Diagnostic Pragmas ) take effect as if they appeared at the end of the command-line.
When an unrecognized warning option is requested (e.g., -Wunknown-warning ), GCC emits a diagnostic stating that the option is not recognized. However, if the -Wno- form is used, the behavior is slightly different: no diagnostic is produced for -Wno-unknown-warning unless other diagnostics are being produced. This allows the use of new -Wno- options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present.
-Wpedantic
-pedantic
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 required version of ISO C). However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they are rejected.
-Wpedantic
does not cause warning messages for use of the alternate keywords whose names begin and end with ‘
__
’. 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 users try to use -Wpedantic to check programs for strict ISO C conformance. They soon find that it does not do quite what they want: it finds some non-ISO practices, but not all—only those for which ISO C requires a diagnostic, and some others for which diagnostics have been added.
A feature to report any failure to conform to ISO C might be useful in some instances, but would require considerable additional work and would be quite different from -Wpedantic . We don't have plans to support such a feature in the near future.
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.)
-pedantic-errors
-Wall
-Wall turns on the following warning flags:
-Waddress -Warray-bounds=1 (only with -O2 ) -Wbool-compare -Wbool-operation -Wc++11-compat -Wc++14-compat -Wcatch-value (C++ and Objective-C++ only) -Wchar-subscripts -Wcomment -Wduplicate-decl-specifier (C and Objective-C only) -Wenum-compare (in C/ObjC; this is on by default in C++) -Wformat -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++) -Wmissing-attributes -Wmissing-braces (only for C/ObjC) -Wmultistatement-macros -Wnarrowing (only for C++) -Wnonnull -Wnonnull-compare -Wopenmp-simd -Wparentheses -Wpointer-sign -Wreorder -Wrestrict -Wreturn-type -Wsequence-point -Wsign-compare (only in C++) -Wsizeof-pointer-div -Wsizeof-pointer-memaccess -Wstrict-aliasing -Wstrict-overflow=1 -Wstringop-truncation -Wswitch -Wtautological-compare -Wtrigraphs -Wuninitialized -Wunknown-pragmas -Wunused-function -Wunused-label -Wunused-value -Wunused-variable -Wvolatile-register-var
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.
-Wextra
-Wclobbered -Wcast-function-type -Wempty-body -Wignored-qualifiers -Wimplicit-fallthrough=3 -Wmissing-field-initializers -Wmissing-parameter-type (C only) -Wold-style-declaration (C only) -Woverride-init -Wsign-compare (C only) -Wtype-limits -Wuninitialized -Wshift-negative-value (in C++03 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:
<
, <=
, >
, or >=
.register
.register
.-Wchar-subscripts
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
.-Wchkp
-Wno-coverage-mismatch
-Wno-cpp
Suppress warning messages emitted by #warning
directives.
-Wdouble-promotion
(C, C++, Objective-C and Objective-C++ only)
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
.
-Wduplicate-decl-specifier
(C and Objective-C only)
const
, volatile
, restrict
or _Atomic
specifier. This warning is enabled by
-Wall
.-Wformat
-Wformat=
n
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 Function Attributes
), 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
-Wno-format-contains-nul
-Wno-format-extra-args
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.
-Wformat-overflow
-Wformat-overflow=
level
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
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
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); }
-Wno-format-zero-length
-Wformat=2
-Wformat-nonliteral
va_list
.-Wformat-security
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
.)-Wformat-signedness
-Wformat-truncation
-Wformat-truncation=
level
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
-Wformat-truncation=2
-Wformat-y2k
strftime
formats that may yield only a two-digit year.-Wnonnull
nonnull
function attribute.
-Wnonnull
is included in
-Wall
and
-Wformat
. It can be disabled with the
-Wno-nonnull
option.
-Wnonnull-compare
nonnull
function attribute against null inside the function.
-Wnonnull-compare
is included in
-Wall
. It can be disabled with the
-Wno-nonnull-compare
option.
-Wnull-dereference
-Winit-self
(C, C++, Objective-C and Objective-C++ only)
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++.
-Wimplicit-int
(C and Objective-C only)
-Wimplicit-function-declaration
(C and Objective-C only)
-Wimplicit
(C and Objective-C only)
-Wimplicit-fallthrough
-Wimplicit-fallthrough=
n
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:
.*
regular expression, any comment is used as fallthrough comment..*falls?[ \t-]*thr(ough|u).*
regular expression.-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]*)?
-fallthrough
@fallthrough@
lint -fallthrough[ \t]*
[ \t]*FALLTHR(OUGH|U)[ \t]*
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
.
-Wif-not-aligned
(C, C++, Objective-C and Objective-C++ only)
warn_if_not_aligned
attribute should be issued. This is is enabled by default. Use
-Wno-if-not-aligned
to disable it.-Wignored-qualifiers
(C and C++ only)
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
.
-Wignored-attributes
(C and C++ only)
-Wmain
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
.-Wmisleading-indentation
(C and C++ only)
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++.
-Wmissing-attributes
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 Function 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);
-Wmissing-braces
a
is not fully bracketed, but that for b
is fully bracketed. This warning is enabled by
-Wall
in C.
int a[2][2] = { 0, 1, 2, 3 }; int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by
-Wall
.
-Wmissing-include-dirs
(C, C++, Objective-C and Objective-C++ only)
-Wmultistatement-macros
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++.
-Wparentheses
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
.
-Wsequence-point
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 warning 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 http://gcc.gnu.org/readings.html .
This warning is enabled by
-Wall
for C and C++.
-Wno-return-local-addr
-Wreturn-type
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.
For C++, a function without return type always produces a diagnostic message, even when
-Wno-return-type
is specified. The only exceptions are main
and functions defined in system headers.
This warning is enabled by default for C++ and is enabled by
-Wall
.
-Wshift-count-negative
-Wshift-count-overflow
-Wshift-negative-value
-Wshift-overflow
-Wshift-overflow=
n
-Wshift-overflow=1
-Wshift-overflow=2
-Wswitch
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
.-Wswitch-default
switch
statement does not have a default
case.-Wswitch-enum
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.-Wswitch-bool
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.
-Wswitch-unreachable
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.
-Wsync-nand
(C and C++ only)
__sync_fetch_and_nand
and __sync_nand_and_fetch
built-in functions are used. These functions changed semantics in GCC 4.4.-Wunused-but-set-parameter
To suppress this warning use the unused
attribute (see Variable Attributes
).
This warning is also enabled by
-Wunused
together with
-Wextra
.
-Wunused-but-set-variable
To suppress this warning use the unused
attribute (see Variable Attributes
).
This warning is also enabled by
-Wunused
, which is enabled by
-Wall
.
-Wunused-function
-Wunused-label
To suppress this warning use the unused
attribute (see Variable Attributes
).
-Wunused-local-typedefs
(C, Objective-C, C++ and Objective-C++ only)
-Wunused-parameter
To suppress this warning use the unused
attribute (see Variable Attributes
).
-Wno-unused-result
warn_unused_result
(see Function Attributes
) does not use its return value. The default is
-Wunused-result
.-Wunused-variable
To suppress this warning use the unused
attribute (see Variable Attributes
).
-Wunused-const-variable
-Wunused-const-variable=
n
#define
s.
To suppress this warning use the unused
attribute (see Variable Attributes
).
-Wunused-const-variable=1
-Wunused-const-variable=2
-Wunused-value
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
.
-Wunused
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
.
-Wuninitialized
setjmp
call. In C++, warn if a non-static reference or non-static const
member appears in a class without constructors.
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 or clobbered elements of structure, union or array variables as well as for variables that are uninitialized or clobbered 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 depends 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.
-Winvalid-memory-model
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.
-Wmaybe-uninitialized
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 Function Attributes
.
This warning is enabled by
-Wall
or
-Wextra
.
-Wunknown-pragmas
#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.-Wno-pragmas
-Wstrict-aliasing
-Wstrict-aliasing=n
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.
-Wstrict-overflow
-Wstrict-overflow=
n
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
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
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
x + 1 > 1
is simplified to x > 0
.-Wstrict-overflow=4
(x * 10) / 5
is simplified to x * 2
.-Wstrict-overflow=5
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.-Wstringop-overflow
-Wstringop-overflow=
type
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 will issue 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
_FORTIFY_SOURCE
macro is defined to a non-zero value.-Wstringop-overflow=2
-Wstringop-overflow=3
-Wstringop-overflow=4
-Wstringop-truncation
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
.
Option
-Wstringop-truncation
is enabled by
-Wall
.
-Wsuggest-attribute=
[
pure
|
const
|
noreturn
|
format
|
cold
|
malloc
]
-Wsuggest-attribute=pure
-Wsuggest-attribute=const
-Wsuggest-attribute=noreturn
-Wsuggest-attribute=malloc
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
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
cold
attribute. This is based on static detection and generally will only warn 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
.-Wsuggest-final-types
final
specifier, or, if possible, declared in an anonymous namespace. This allows GCC to more aggressively devirtualize the polymorphic calls. This warning is more effective with link time optimization, where the information about the class hierarchy graph is more complete.-Wsuggest-final-methods
final
specifier, or, if possible, its type were declared in an anonymous namespace or with the final
specifier. This warning is more effective with link-time optimization, where the information about the class hierarchy graph is more complete. It is recommended to first consider suggestions of
-Wsuggest-final-types
and then rebuild with new annotations.-Wsuggest-override
-Walloc-zero
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.-Walloc-size-larger-than=
n
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 SIZE_MAX / 2
. The option argument n
may end in one of the standard suffixes designating a multiple of bytes such as kB
and KiB
for kilobyte and kibibyte, respectively, MB
and MiB
for megabyte and mebibyte, and so on.
-Walloc-size-larger-than=
PTRDIFF_MAX
is enabled by default. Warnings controlled by the option can be disabled by specifying n
of SIZE_MAX
or more. See Function Attributes
.-Walloca
alloca
in the source.-Walloca-larger-than=
n
alloca
that are not bounded by a controlling predicate limiting its argument of integer type to at most n
bytes, or calls to alloca
where the bound 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.
This warning is not enabled by -Wall , and is only active when -ftree-vrp is active (default for -O2 and above).
See also
-Wvla-larger-than=
n
.
-Warray-bounds
-Warray-bounds=
n
-Warray-bounds=1
-Warray-bounds=2
-Wattribute-alias
alias
and similar attributes whose target is incompatible with the type of the alias. See Declaring Attributes of Functions
.-Wbool-compare
true
/false
. For instance, the following comparison is always false:
int n = 5; ... if ((n > 1) == 2) { ... }
This warning is enabled by
-Wall
.
-Wbool-operation
This warning is enabled by
-Wall
.
-Wduplicated-branches
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:
int i = x ? *p : *p;
-Wduplicated-cond
if (p->q != NULL) { ... } else if (p->q != NULL) { ... }
-Wframe-address
-Wno-discarded-qualifiers
(C and Objective-C only)
const char *
variable is passed to a function that takes a char *
parameter. This option can be used to suppress such a warning.-Wno-discarded-array-qualifiers
(C and Objective-C only)
const int (*)[]
variable is passed to a function that takes a int (*)[]
parameter. This option can be used to suppress such a warning.-Wno-incompatible-pointer-types
(C and Objective-C only)
-Wno-int-conversion
(C and Objective-C only)
-Wno-div-by-zero
-Wsystem-headers
-Wtautological-compare
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
.
-Wtrampolines
-Wfloat-equal
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.
-Wtraditional
(C and Objective-C only)
#pragma
not understood by traditional C by indenting them. Some traditional implementations do not recognize #elif
, so this option suggests avoiding it altogether.<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.switch
statement has an operand of type long
.static
function declaration follows a static
one. This construct is not accepted by some traditional C compilers.__STDC__
to avoid missing initializer warnings and relies on default initialization to zero in the traditional C case.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.-Wtraditional-conversion
(C and Objective-C only)
-Wdeclaration-after-statement
(C and Objective-C only)
-Wshadow
-Wno-shadow-ivar
(Objective-C only)
-Wshadow=global
-Wshadow=local
-Wshadow=compatible-local
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
will not emit a warning. Because their types are incompatible, if a programmer accidentally uses one in place of the other, type checking will catch that and emit an error or warning. So not warning (about shadowing) in this case will not lead to undetected bugs. Use of this flag instead of
-Wshadow=local
can possibly reduce the number of warnings triggered by intentional shadowing.
This warning is enabled by
-Wshadow=local
.
-Wlarger-than=
len
-Wframe-larger-than=
len
alloca
, variable-length arrays, or related constructs is not included by the compiler when determining whether or not to issue a warning.-Wno-free-nonheap-object
-Wstack-usage=
len
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 .
warning: stack usage is 1120 bytes
warning: stack usage might be 1648 bytes
warning: stack usage might be unbounded
-Wno-pedantic-ms-format
(MinGW targets only)
printf
/ scanf
format width specifiers I32
, I64
, and I
used on Windows targets, which depend on the MS runtime.-Waligned-new
alignof(std::max_align_t)
but uses an allocation function without an explicit alignment parameter. This option is enabled by
-Wall
.
Normally this only warns about global allocation functions, but
-Waligned-new=all
also warns about class member allocation functions.
-Wplacement-new
-Wplacement-new=
n
char buf [64]; new (buf) int[64];
This warning is enabled by default.
-Wplacement-new=1
new
expression is not diagnosed at this level even though it has undefined behavior according to the C++ standard because it writes past the end of the one-element array.
struct S { int n, a[1]; }; S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]); new (s->a)int [32]();
-Wplacement-new=2
struct S { int n, a[]; }; S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]); new (s->a)int [32]();
-Wpointer-arith
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
.-Wpointer-compare
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.
-Wtype-limits
<
or >=
. This warning is also enabled by
-Wextra
.
-Wcomment
-Wcomments
-Wtrigraphs
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
’.
-Wundef
#if
directive. Such identifiers are replaced with zero.-Wexpansion-to-defined
-Wunused-macros
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
-Wno-endif-labels
#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.
-Wbad-function-cast
(C and Objective-C only)
-Wc90-c99-compat
(C and Objective-C only)
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__
.-Wc99-c11-compat
(C and Objective-C only)
_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__
.-Wc++-compat
(C and Objective-C only)
void *
to a pointer to non-void
type.-Wc++11-compat
(C++ and Objective-C++ only)
-Wc++14-compat
(C++ and Objective-C++ only)
-Wc++17-compat
(C++ and Objective-C++ only)
-Wcast-qual
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';
-Wcast-align
char *
is cast to an int *
on machines where integers can only be accessed at two- or four-byte boundaries.-Wcast-align=strict
char *
is cast to an int *
regardless of the target machine.-Wcast-function-type
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
.-Wwrite-strings
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.
-Wcatch-value
-Wcatch-value=
n
(C++ and Objective-C++ only)
-Wclobbered
longjmp
or vfork
. This warning is also enabled by
-Wextra
.-Wconditionally-supported
(C++ and Objective-C++ only)
-Wconversion
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.
-Wno-conversion-null
(C++ and Objective-C++ only)
NULL
and non-pointer types.
-Wconversion-null
is enabled by default.-Wzero-as-null-pointer-constant
(C++ and Objective-C++ only)
nullptr
in C++11.-Wsubobject-linkage
(C++ and Objective-C++ only)
-Wdangling-else
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
.
-Wdate-time
__TIME__
, __DATE__
or __TIMESTAMP__
are encountered as they might prevent bit-wise-identical reproducible compilations.-Wdelete-incomplete
(C++ and Objective-C++ only)
-Wuseless-cast
(C++ and Objective-C++ only)
-Wempty-body
if
, else
or do while
statement. This warning is also enabled by
-Wextra
.-Wenum-compare
-Wextra-semi
(C++, Objective-C++ only)
-Wjump-misses-init
(C, Objective-C only)
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.
-Wsign-compare
-Wsign-conversion
-Wfloat-conversion
-Wno-scalar-storage-order
-Wsized-deallocation
(C++ and Objective-C++ only)
void operator delete (void *) noexcept; void operator delete[] (void *) noexcept;
without a definition of the corresponding sized deallocation function
void operator delete (void *, std::size_t) noexcept; void operator delete[] (void *, std::size_t) noexcept;
or vice versa. Enabled by
-Wextra
along with
-fsized-deallocation
.
-Wsizeof-pointer-div
sizeof (ptr) / sizeof (ptr[0])
if ptr
is not an array, but a pointer. This warning is enabled by
-Wall
.-Wsizeof-pointer-memaccess
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
.
-Wsizeof-array-argument
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.-Wmemset-elt-size
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
.-Wmemset-transposed-args
memset
built-in function, if the second argument is not zero and the third argument is zero. This warns e.g. about memset (buf, sizeof buf, 0)
where most probably memset (buf, 0, sizeof buf)
was meant instead. The diagnostics is only emitted if the third argument is literal zero. If it is some expression that is folded to zero, a cast of zero to some type, etc., it is far less likely that the user has mistakenly exchanged the arguments and no warning is emitted. This warning is enabled by
-Wall
.-Waddress
void func(void); if (func)
, and comparisons against the memory address of a string literal, such as if (x == "abc")
. Such uses typically indicate a programmer error: the address of a function always evaluates to true, so their use in a conditional usually indicate that the programmer forgot the parentheses in a function call; and comparisons against string literals result in unspecified behavior and are not portable in C, so they usually indicate that the programmer intended to use strcmp
. This warning is enabled by
-Wall
.-Wlogical-op
extern int a; if (a < 0 && a < 0) { ... }
-Wlogical-not-parentheses
int a; ... if (!a > 1) { ... }
It is possible to suppress the warning by wrapping the LHS into parentheses:
if ((!a) > 1) { ... }
This warning is enabled by
-Wall
.
-Waggregate-return
-Wno-aggressive-loop-optimizations
-Wno-attributes
__attribute__
is used, such as unrecognized attributes, function attributes applied to variables, etc. This does not stop errors for incorrect use of supported attributes.-Wno-builtin-declaration-mismatch
-Wno-builtin-macro-redefined
__TIMESTAMP__
, __TIME__
, __DATE__
, __FILE__
, and __BASE_FILE__
.-Wstrict-prototypes
(C and Objective-C only)
-Wold-style-declaration
(C and Objective-C only)
static
are not the first things in a declaration. This warning is also enabled by
-Wextra
.-Wold-style-definition
(C and Objective-C only)
-Wmissing-parameter-type
(C and Objective-C only)
void foo(bar) { }
This warning is also enabled by
-Wextra
.
-Wmissing-prototypes
(C and Objective-C only)
-Wmissing-declarations
-Wmissing-field-initializers
x.h
is implicitly zero:
struct s { int f, g, h; }; struct s x = { 3, 4 };
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
.
-Wno-multichar
-Wnormalized=
[
none
|
id
|
nfc
|
nfkc
]
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.
-Wno-deprecated
-Wno-deprecated-declarations
deprecated
attribute.-Wno-overflow
-Wno-odr
-Wopenmp-simd
-Woverride-init
(C and Objective-C only)
This warning is included in
-Wextra
. To get other
-Wextra
warnings without this one, use
-Wextra -Wno-override-init
.
-Woverride-init-side-effects
(C and Objective-C only)
-Wpacked
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; };
-Wpacked-bitfield-compat
packed
attribute on bit-fields of type char
. This has been 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.
-Wpacked-not-aligned
(C, C++, Objective-C and Objective-C++ only)
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
.
-Wpadded
-Wredundant-decls
-Wno-restrict
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
.
-Wnested-externs
(C and Objective-C only)
extern
declaration is encountered within a function.-Wno-inherited-variadic-ctor
-Winline
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.
-Wno-invalid-offsetof
(C++ and Objective-C++ only)
offsetof
macro to a non-POD type. According to the 2014 ISO C++ standard, applying offsetof
to a non-standard-layout type is undefined. In existing C++ implementations, however, offsetof
typically gives meaningful results. This flag is for users who are aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about it.
The restrictions on offsetof
may be relaxed in a future version of the C++ standard.
-Wint-in-bool-context
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
.-Wno-int-to-pointer-cast
-Wno-pointer-to-int-cast
(C and Objective-C only)
-Winvalid-pch
-Wlong-long
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
.-Wvariadic-macros
-Wvarargs
va_start
. This is default. To inhibit the warning messages, use
-Wno-varargs
.-Wvector-operation-performance
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.-Wno-virtual-move-assign
-Wvla
-Wvla-larger-than=
n
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.
This warning is not enabled by -Wall , and is only active when -ftree-vrp is active (default for -O2 and above).
See also
-Walloca-larger-than=
n
.
-Wvolatile-register-var
-Wdisabled-optimization
-Wpointer-sign
(C and Objective-C only)
-Wstack-protector
-Woverlength-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
.
-Wunsuffixed-float-constants
(C and Objective-C only)
FLOAT_CONST_DECIMAL64
pragma from the decimal floating-point extension to C99.-Wno-designated-init
(C and Objective-C only)
designated_init
attribute.-Whsa