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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.
These options control the amount and kinds of warnings produced by GCC:
-fsyntax-only
-pedantic
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 section 5.39 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
-w
-Wno-import
-Wchar-subscripts
char. This is a common cause
of error, as programmers often forget that this type is signed on some
machines.
-Wcomment
-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 section 5.26 Declaring Attributes of Functions), 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 section 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
strftime
formats which may yield only a two-digit year.
-Wno-format-extra-args
printf or scanf format function. The C standard specifies
that such arguments are ignored.
-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=2
-Wimplicit-int
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
-Wimplicit
-Wmain
-Wmissing-braces
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
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-Wmultichar
-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 ();
}
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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, GNU C 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 ();
}
}
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-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.
There is some controversy over the precise meaning of the sequence point rules in subtle cases. Links to papers with alternative formal definitions and other related discussions may be found on our readings page 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 section 5.33 Specifying Attributes of Variables).
-Wunused-parameter
To suppress this warning use the `unused' attribute (see section 5.33 Specifying Attributes of Variables).
-Wunused-variable
To suppress this warning use the `unused' attribute (see section 5.33 Specifying Attributes of Variables).
-Wunused-value
To suppress this warning cast the expression to `void'.
-Wunused
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);
}
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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;
}
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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 section 5.26 Declaring Attributes of Functions.
-Wreorder (C++ only)
-Wunknown-pragmas
-Wall
-Wsystem-headers
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;
}
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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.)
x.h:
struct s { int f, g; };
struct t { struct s h; int i; };
struct t x = { 1, 2, 3 };
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x.h would be implicitly initialized to zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
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-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)
<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.
-Wundef
-Wshadow
-Wid-clash-len
-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. 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
-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
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').
-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;
};
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-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 behaviour which is selectable at compile-time.
-Winline
-Wlong-long
-Wdisabled-optimization
-Werror
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