Warnings are diagnostic messages that report constructions which are not inherently erroneous but which 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
Check the code for syntax errors, but don't do anything beyond that.
-w
Inhibit all warning messages.
-Werror
Make all warnings into errors.
-Werror=
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. You can use the -fdiagnostics-show-option option to have each controllable warning amended with the option which controls it, to determine what to use with this option.
Note that specifying -Werror=
foo automatically implies
-W
foo. However, -Wno-error=
foo does not
imply anything.
-Wfatal-errors
This option causes the compiler to abort compilation on the first error occurred rather than trying to keep going and printing further error messages.
You can request many specific warnings with options beginning -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.
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C++. For ISO C, follows the version of the ISO 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 will 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.
-pedantic 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 -pedantic 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 -pedantic. 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 gnu89 or gnu99, there is a
corresponding base standard, the version of ISO C on which the GNU
extended dialect is based. Warnings from -pedantic are given
where they are required by the base standard. (It would 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
Like -pedantic, except that errors are produced rather than
warnings.
-Wall
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 C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.
-Wall turns on the following warning flags:
-Waddress
-Warray-bounds (only with
-O2
)
-Wc++0x-compat
-Wchar-subscripts
-Wimplicit-int
-Wimplicit-function-declaration
-Wcomment
-Wformat
-Wmain (only for C/ObjC and unless
-ffreestanding
)
-Wmissing-braces
-Wnonnull
-Wparentheses
-Wpointer-sign
-Wreorder
-Wreturn-type
-Wsequence-point
-Wsign-compare (only in C++)
-Wstrict-aliasing
-Wstrict-overflow=1
-Wswitch
-Wtrigraphs
-Wuninitialized (only with
-O1
and above)
-Wunknown-pragmas
-Wunused-function
-Wunused-label
-Wunused-value
-Wunused-variable
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
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
-Wempty-body
-Wignored-qualifiers
-Wmissing-field-initializers
-Wmissing-parameter-type (C only)
-Wold-style-declaration (C only)
-Woverride-init
-Wsign-compare
-Wtype-limits
-Wuninitialized (only with
-O1
and above)
-Wunused-parameter (only with
-Wunused
or
-Wall
)
The option -Wextra also prints warning messages for the following cases:
-Wno-import
Inhibit warning messages about the use of #import.
-Wchar-subscripts
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.
-Wcomment
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.
-Wformat
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 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 -pedantic is used
with -Wformat, warnings will be 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.
Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull.
-Wformat is included in -Wall. For more control over some
aspects of format checking, the options -Wformat-y2k,
-Wno-format-extra-args, -Wno-format-zero-length,
-Wformat-nonliteral, -Wformat-security, and
-Wformat=2 are available, but are not included in -Wall.
-Wformat-y2k
If -Wformat is specified, also warn about strftime
formats which may yield only a two-digit year.
-Wno-format-extra-args
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 will suppress the
warning if the unused arguments are all pointers, since the Single
Unix Specification says that such unused arguments are allowed.
-Wno-format-zero-length
(C and Objective-C only)
If -Wformat is specified, do not warn about zero-length formats.
The C standard specifies that zero-length formats are allowed.
-Wformat-nonliteral
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.
-Wformat-security
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.)
-Wformat=2
Enable -Wformat plus format checks not included in
-Wformat. Currently equivalent to -Wformat
-Wformat-nonliteral -Wformat-security -Wformat-y2k.
-Wnonnull
(C and Objective-C only)
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.
-Winit-self
(C, C++, Objective-C and Objective-C++ only)
Warn about uninitialized variables which are initialized with themselves. Note this option can only be used with the -Wuninitialized option, which in turn only works with -O1 and above.
For example, GCC will warn about i being uninitialized in the
following snippet only when -Winit-self has been specified:
int f()
{
int i = i;
return i;
}
-Wimplicit-int
(C and Objective-C only)
Warn when a declaration does not specify a type.
This warning is enabled by -Wall.
-Wimplicit-function-declaration
(C and Objective-C only)
Give a warning whenever a function is used before being declared. In
C99 mode (-std=c99 or -std=gnu99), this warning is
enabled by default and it is made into an error by
-pedantic-errors. This warning is also enabled by
-Wall.
-Wimplicit
Same as -Wimplicit-int and -Wimplicit-function-declaration.
This warning is enabled by -Wall.
-Wignored-qualifiers
(C and C++ only)
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.
-Wmain
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 -Wall.
-Wmissing-braces
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.
-Wmissing-include-dirs
(C, C++, Objective-C and Objective-C++ only)
Warn if a user-supplied include directory does not exist.
-Wparentheses
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 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 will issue 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
could belong to the enclosing if. The resulting code would
look like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
This warning is enabled by -Wall.
-Wsequence-point
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 defines 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 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++.
-Wreturn-type
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), and about a return statement with a
expression in a function whose return-type is void.
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 -Wall.
-Wswitch
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.
This warning is enabled by -Wall.
-Wswitch-default
Warn whenever a switch statement does not have a default
case.
-Wswitch-enum
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.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning of
the program (trigraphs within comments are not warned about).
This warning is enabled by -Wall.
-Wunused-function
Warn whenever a static function is declared but not defined or a
non-inline static function is unused.
This warning is enabled by -Wall.
-Wunused-label
Warn whenever a label is declared but not used. This warning is enabled by -Wall.
To suppress this warning use the unused attribute
(see Variable Attributes).
-Wunused-parameter
Warn whenever a function parameter is unused aside from its declaration.
To suppress this warning use the unused attribute
(see Variable Attributes).
-Wunused-variable
Warn whenever a local variable or non-constant static variable is unused aside from its declaration. This warning is enabled by -Wall.
To suppress this warning use the unused attribute
(see Variable Attributes).
-Wunused-value
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] will cause a warning, while x[(void)i,j] will not.
This warning is enabled by -Wall.
-Wunused
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.
-Wuninitialized
Warn if an automatic variable is used without first being initialized or
if a variable may be clobbered by a setjmp call.
These warnings are possible only in optimizing compilation, because they require data flow information that is computed only when optimizing. If you do not specify -O, you will not get these warnings. Instead, GCC will issue a warning about -Wuninitialized requiring -O.
If you want to warn about code which 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 which 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 will 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.
These warnings are made optional because GCC is not smart enough to see all the reasons why the code might be correct despite 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. Here is
another common case:
{
int save_y;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;
}
This has no bug because save_y is used only if it is set.
This option also warns when a non-volatile automatic variable might be
changed by a call to longjmp. These warnings as well are possible
only in optimizing compilation.
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 which 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 in
optimizing compilations (-O1 and above).
-Wunknown-pragmas
Warn when a #pragma directive is encountered which is not understood by
GCC. If this command line option is used, warnings will even be issued
for unknown pragmas in system header files. This is not the case if
the warnings were only enabled by the -Wall command line option.
-Wno-pragmas
Do not warn about misuses of pragmas, such as incorrect parameters,
invalid syntax, or conflicts between pragmas. See also
-Wunknown-pragmas.
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is active.
It warns about code which 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
-Wstrict-aliasing=n
This option is only active when -fstrict-aliasing is active. It warns about code which 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=n, with n=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 frontend 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 frontend 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 punn+dereference pattern in the frontend:
*(int*)&some_float.
If optimization is enabled, it also runs in the backend, 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
This option is only active when -fstrict-overflow is active. 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 which 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 which 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 will require, in particular when determining whether a loop will be executed at all.
-Wstrict-overflow=1
x + 1 > x; with -fstrict-overflow, the
compiler will simplify this 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 -fstrict-overflow is in effect, 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 will be simplified to x > 0.
-Wstrict-overflow=4
(x * 10) / 5 will be simplified to x * 2.
-Wstrict-overflow=5
x + 2 > y will
be 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 will give a very large number of
false positives.
-Warray-bounds
This option is only active when -ftree-vrp is active
(default for -O2 and above). It warns about subscripts to arrays
that are always out of bounds. This warning is enabled by -Wall.
-Wno-div-by-zero
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.
-Wsystem-headers
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 will not warn about unknown pragmas in system
headers—for that, -Wunknown-pragmas must also be used.
-Wfloat-equal
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
would 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)
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 which should be avoided.
<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)
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.
-Wdeclaration-after-statement
(C and Objective-C only)
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 and was not supported by
GCC versions before GCC 3.0. See Mixed Declarations.
-Wundef
Warn if an undefined identifier is evaluated in an #if directive.
-Wno-endif-labels
Do not warn whenever an #else or an #endif are followed by text.
-Wshadow
Warn whenever a local variable shadows another local variable, parameter or
global variable or whenever a built-in function is shadowed.
-Wlarger-than-
len
Warn whenever an object of larger than len bytes is defined.
-Wunsafe-loop-optimizations
Warn if the loop cannot be optimized because the compiler could not
assume anything on the bounds of the loop indices. With
-funsafe-loop-optimizations warn if the compiler made
such assumptions.
-Wpointer-arith
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 -pedantic.
-Wtype-limits
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.
-Wbad-function-cast
(C and Objective-C only)
Warn whenever a function call is cast to a non-matching type.
For example, warn if int malloc() is cast to anything *.
-Wc++-compat
(C and Objective-C only)
void * to a pointer to non-void type.
-Wc++0x-compat
(C++ and Objective-C++ only)
-Wcast-qual
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 *.
-Wcast-align
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.
-Wwrite-strings
When compiling C, give string constants the type const
char[
length
] so that
copying the address of one into a non-const
char *
pointer will get a warning; when compiling C++, warn about the
deprecated conversion from string literals to char *. This
warning, by default, is enabled for C++ programs.
These warnings will 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 will just be a nuisance;
this is why we did not make -Wall request these warnings.
-Wclobbered
Warn for variables that might be changed by longjmp or
vfork. This warning is also enabled by -Wextra.
-Wconversion
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 conversions between NULL and non-pointer
types; confusing overload resolution for user-defined conversions; and
conversions that will 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.
-Wempty-body
Warn if an empty body occurs in an if, else or do
while statement. Additionally, in C++, warn when an empty body occurs
in a while or for statement with no whitespacing before
the semicolon. This warning is also enabled by -Wextra.
-Wsign-compare
Warn when a comparison between signed and unsigned values could produce
an incorrect result when the signed value is converted to unsigned.
This warning is also enabled by -Wextra; to get the other warnings
of -Wextra without this warning, use -Wextra -Wno-sign-compare.
-Wsign-conversion
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.
-Waddress
Warn about suspicious uses of memory addresses. These include using
the address of a function in a conditional expression, such as
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
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.
-Waggregate-return
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.)
-Wno-attributes
Do not warn if an unexpected __attribute__ is used, such as
unrecognized attributes, function attributes applied to variables,
etc. This will not stop errors for incorrect use of supported
attributes.
-Wstrict-prototypes
(C and Objective-C only)
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 which specifies the argument
types.)
-Wold-style-declaration
(C and Objective-C only)
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.
-Wold-style-definition
(C and Objective-C only)
Warn if an old-style function definition is used. A warning is given
even if there is a previous prototype.
-Wmissing-parameter-type
(C and Objective-C only)
A function parameter is declared without a type specifier in K&R-style functions:
void foo(bar) { }
This warning is also enabled by -Wextra.
-Wmissing-prototypes
(C and Objective-C only)
Warn if a global function is defined without a previous prototype
declaration. This warning is issued even if the definition itself
provides a prototype. The aim is to detect global functions that fail
to be declared in header files.
-Wmissing-declarations
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 function templates,
or for inline functions, or for functions in anonymous namespaces.
-Wmissing-field-initializers
Warn if a structure's initializer has some fields missing. For
example, the following code would cause such a warning, because
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 would not trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
This warning is included in -Wextra. To get other -Wextra
warnings without this one, use -Wextra -Wno-missing-field-initializers.
-Wmissing-noreturn
Warn about functions which might be candidates for attribute noreturn.
Note these are only possible candidates, not absolute ones. Care should
be taken to manually verify functions actually do not ever return before
adding the noreturn attribute, otherwise subtle code generation
bugs could be introduced. You will not get a warning for main in
hosted C environments.
-Wmissing-format-attribute
Warn about function pointers which might be candidates for format
attributes. Note these are only possible candidates, not absolute ones.
GCC will guess 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 will also warn about function definitions which might be
candidates for format attributes. Again, these are only
possible candidates. GCC will guess 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.
-Wno-multichar
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.
-Wnormalized=<none|id|nfc|nfkc>
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 which have not been normalized; this option controls that warning.
There are four levels of warning that GCC supports. The default is -Wnormalized=nfc, which warns about any identifier which is not in the ISO 10646 “C” normalized form, NFC. NFC is the recommended form for most uses.
Unfortunately, there are some characters which ISO C and ISO C++ allow in identifiers that when turned into NFC aren't allowable as 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. You would only want to do this if you were 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”, will display just like a regular n which 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 will warn 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 is
unable to be fixed to display these characters distinctly.
-Wno-deprecated-declarations
Do not warn about uses of functions (see Function Attributes),
variables (see Variable Attributes), and types (see Type Attributes) marked as deprecated by using the deprecated
attribute.
-Wno-overflow
Do not warn about compile-time overflow in constant expressions.
-Woverride-init
(C and Objective-C only)
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.
-Wpacked
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
will be 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;
};
-Wpadded
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.
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even in
cases where multiple declaration is valid and changes nothing.
-Wnested-externs
(C and Objective-C only)
Warn if an extern declaration is encountered within a function.
-Wunreachable-code
Warn if the compiler detects that code will never be executed.
This option is intended to warn when the compiler detects that at least a whole line of source code will never be executed, because some condition is never satisfied or because it is after a procedure that never returns.
It is possible for this option to produce a warning even though there are circumstances under which part of the affected line can be executed, so care should be taken when removing apparently-unreachable code.
For instance, when a function is inlined, a warning may mean that the line is unreachable in only one inlined copy of the function.
This option is not made part of -Wall because in a debugging
version of a program there is often substantial code which checks
correct functioning of the program and is, hopefully, unreachable
because the program does work. Another common use of unreachable
code is to provide behavior which is selectable at compile-time.
-Winline
Warn if a function can not be inlined and it was declared as inline. Even with this option, the compiler will 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.
-Wno-invalid-offsetof
(C++ and Objective-C++ only)
Suppress warnings from applying the offsetof macro to a non-POD type. According to the 1998 ISO C++ standard, applying offsetof to a non-POD type is undefined. In existing C++ implementations, however, offsetof typically gives meaningful results even when applied to certain kinds of non-POD types. (Such as a simple struct that fails to be a POD type only by virtue of having a constructor.) 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.
-Wno-int-to-pointer-cast
(C and Objective-C only)
Suppress warnings from casts to pointer type of an integer of a
different size.
-Wno-pointer-to-int-cast
(C and Objective-C only)
Suppress warnings from casts from a pointer to an integer type of a
different size.
-Winvalid-pch
Warn if a precompiled header (see Precompiled Headers) is found in
the search path but can't be used.
-Wlong-long
Warn if long long type is used. This is default. To inhibit
the warning messages, use -Wno-long-long. Flags
-Wlong-long and -Wno-long-long are taken into account
only when -pedantic flag is used.
-Wvariadic-macros
Warn if variadic macros are used in pedantic ISO C90 mode, or the GNU
alternate syntax when in pedantic ISO C99 mode. This is default.
To inhibit the warning messages, use -Wno-variadic-macros.
-Wvla
Warn if variable length array is used in the code.
-Wno-vla will prevent the -pedantic warning of
the variable length array.
-Wvolatile-register-var
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.
-Wdisabled-optimization
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 were unable to handle the code
effectively. Often, the problem is that your code is too big or too
complex; GCC will refuse to optimize programs when the optimization
itself is likely to take inordinate amounts of time.
-Wpointer-sign
(C and Objective-C only)
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 -pedantic, which can be disabled with
-Wno-pointer-sign.
-Wstack-protector
This option is only active when -fstack-protector is active. It
warns about functions that will not be protected against stack smashing.
-Woverlength-strings
Warn about string constants which are longer than the “minimum maximum” length specified in the C standard. Modern compilers generally allow string constants which 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 C89, 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 -pedantic, and can be disabled with -Wno-overlength-strings.