Node: Warning Options, Next: Debugging Options, Previous: Language Independent Options, Up: Invoking GCC
Warnings are diagnostic messages that report constructions which are not inherently erroneous but which are risky or suggest there may have been an error.
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.
The following options control the amount and kinds of warnings produced by GCC; for further, language-specific options also refer to C++ Dialect Options and Objective-C Dialect Options.
-fsyntax-only
-pedantic
-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
-pedantic, except that errors are produced rather than
warnings.
-w
-Wno-import
#import.
-Wchar-subscripts
char. This is a common cause
of error, as programmers often forget that this type is signed on some
machines.
-Wcomment
/* appears in a /*
comment, or whenever a Backslash-Newline appears in a // comment.
-Wformat
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.
The formats are checked against the format features supported by GNU
libc version 2.2. These include all ISO C89 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.
-Wformat is included in -Wall. For more control over some
aspects of format checking, the options -Wno-format-y2k,
-Wno-format-extra-args, -Wformat-nonliteral,
-Wformat-security and -Wformat=2 are available, but are
not included in -Wall.
-Wno-format-y2k
-Wformat is specified, do not warn about strftime
formats which may yield only a two-digit year.
-Wno-format-extra-args
-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.
-Wformat-nonliteral
-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
-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
-Wformat plus format checks not included in
-Wformat. Currently equivalent to -Wformat
-Wformat-nonliteral -Wformat-security.
-Wimplicit-int
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
-Wimplicit
-Wimplicit-int and -Wimplicit-function-declaration.
-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.
-Wmissing-braces
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 } };
-Wparentheses
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, 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 ();
}
}
-Wsequence-point
The C standard defines the order in which expressions in a 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 standard specifies 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 present implementation of this option only works for C programs. A future implementation may also work for C++ programs.
The C 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 our readings page, at
http://gcc.gnu.org/readings.html.
-Wreturn-type
int. Also warn about any return statement with no
return-value in a function whose return-type is not 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.
-Wswitch
switch statement has an index of enumeral 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.
-Wtrigraphs
-Wunused-function
-Wunused-label
To suppress this warning use the unused attribute
(see Variable Attributes).
-Wunused-parameter
To suppress this warning use the unused attribute
(see Variable Attributes).
-Wunused-variable
To suppress this warning use the unused attribute
(see Variable Attributes).
-Wunused-value
To suppress this warning cast the expression to void.
-Wunused
-Wunused options combined.
In order to get a warning about an unused function parameter, you must
either specify -W -Wunused or separately specify
-Wunused-parameter.
-Wuninitialized
setjmp call.
These warnings are possible only in optimizing compilation,
because they require data flow information that is computed only
when optimizing. If you don't specify -O, you simply won't
get these warnings.
These warnings occur only for variables that are candidates for
register allocation. Therefore, they do not occur for a variable that
is declared volatile, or whose address is taken, or whose size
is other than 1, 2, 4 or 8 bytes. Also, they do not occur for
structures, unions or arrays, even when they are in registers.
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.
-Wreorder (C++ only)-Wunknown-pragmas
-Wall command line option.
-Wall
-W options combined. 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.
-Wdiv-by-zero
-Wno-div-by-zero. Floating
point division by zero is not warned about, as it can be a legitimate
way of obtaining infinities and NaNs.
-Wmultichar
'FOOF') is used. This is
default. To inhibit the warning messages, use -Wno-multichar.
Usually they indicate a typo in the user's code, as they have
implementation-defined values, and should not be used in portable code.
-Wsystem-headers
-Wall in conjunction with this
option will not warn about unknown pragmas in system
headers--for that, -Wunknown-pragmas must also be used.
The following -W... options 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.
-W
foo (a)
{
if (a > 0)
return a;
}
x[i,j] will cause a warning,
but x[(void)i,j] will not.
< or <=.
x<=y<=z appears; this is equivalent to
(x<=y ? 1 : 0) <= z, which is a different interpretation from
that of ordinary mathematical notation.
static are not the first things in
a declaration. According to the C Standard, this usage is obsolescent.
const.
Such a type qualifier has no effect, since the value returned by a
function is not an lvalue. (But don't warn about the GNU extension of
volatile void return types. That extension will be warned about
if -pedantic is specified.)
-Wall or -Wunused is also specified, warn about unused
arguments.
-Wno-sign-compare is also specified.)
x.h:
struct s { int f, g; };
struct t { struct s h; int i; };
struct t x = { 1, 2, 3 };
x.h would be implicitly initialized to zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
-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 analysing 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 only)# appeared in column 1 on the line. Therefore
-Wtraditional warns about directives that traditional C
understands but would ignore because the # does not appear as the
first character on the line. It also suggests you hide directives like
#pragma not understood by traditional C by indenting them. Some
traditional implementations would not recognize #elif, so it
suggests avoiding it altogether.
U integer constant suffix, or the F or L floating point
constant suffixes. (Traditional C does support the L suffix on integer
constants.) Note, these suffixes appear in macros defined in the system
headers of most modern systems, e.g. the _MIN/_MAX macros in <limits.h>.
Use of these macros in user code might normally lead to spurious
warnings, however gcc's integrated preprocessor has enough context to
avoid warning in these cases.
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.
-Wconversion.
-Wundef
#if directive.
-Wshadow
-Wlarger-than-len-Wpointer-arith
void. GNU C assigns these types a size of 1, for
convenience in calculations with void * pointers and pointers
to functions.
-Wbad-function-cast (C only)int malloc() is cast to anything *.
-Wcast-qual
const char * is cast
to an ordinary char *.
-Wcast-align
char * is cast to
an int * on machines where integers can only be accessed at
two- or four-byte boundaries.
-Wwrite-strings
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 constants to char *.
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.
-Wconversion
Also, warn if a negative integer constant expression is implicitly
converted to an unsigned type. For example, warn about the assignment
x = -1 if x is unsigned. But do not warn about explicit
casts like (unsigned) -1.
-Wsign-compare
-W; to get the other warnings
of -W without this warning, use -W -Wno-sign-compare.
-Waggregate-return
-Wstrict-prototypes (C only)-Wmissing-prototypes (C only)-Wmissing-declarations
-Wmissing-noreturn
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
-Wformat is enabled, also warn about functions which might be
candidates for format attributes. Note these are only possible
candidates, not absolute ones. 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. This option has no effect unless
-Wformat is enabled (possibly by -Wall).
-Wno-deprecated-declarations
deprecated attribute.
(see Function Attributes, see Variable Attributes,
see Type Attributes.)
-Wpacked
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
-Wredundant-decls
-Wnested-externs (C only)extern declaration is encountered within a function.
-Wunreachable-code
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
-Wlong-long
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.
-Wdisabled-optimization
-Werror