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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 GNU
CC:
-fsyntax-only
- Check the code for syntax errors, but don't do anything beyond that.
-pedantic
- Issue all the warnings demanded by strict ANSI standard C; reject
all programs that use forbidden extensions.
Valid ANSI standard C programs should compile properly with or without
this option (though a rare few will require `-ansi'). However,
without this option, certain GNU extensions and traditional 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 Alternate Keywords.
This option is not intended to be useful; it exists only to satisfy
pedants who would otherwise claim that GNU CC fails to support the ANSI
standard.
Some users try to use `-pedantic' to check programs for strict ANSI
C conformance. They soon find that it does not do quite what they want:
it finds some non-ANSI practices, but not all--only those for which
ANSI C requires a diagnostic.
A feature to report any failure to conform to ANSI C might be useful in
some instances, but would require considerable additional work and would
be quite different from `-pedantic'. We recommend, rather, that
users take advantage of the extensions of GNU C and disregard the
limitations of other compilers. Aside from certain supercomputers and
obsolete small machines, there is less and less reason ever to use any
other C compiler other than for bootstrapping GNU CC.
-pedantic-errors
- Like `-pedantic', except that errors are produced rather than
warnings.
-w
- Inhibit all warning messages.
-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.
-Wcomment
- Warn whenever a comment-start sequence `/*' appears in a comment.
-Wformat
- Check calls to
printf
and scanf
, etc., to make sure that
the arguments supplied have types appropriate to the format string
specified.
-Wimplicit
- Warn whenever a function or parameter is implicitly declared.
-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.
-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
.
-Wswitch
- Warn whenever a
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
- Warn if any trigraphs are encountered (assuming they are enabled).
-Wunused
- Warn whenever a variable is unused aside from its declaration,
whenever a function is declared static but never defined, whenever a
label is declared but not used, and whenever a statement computes a
result that is explicitly not used.
To suppress this warning for an expression, simply cast it to void. For
unused variables and parameters, use the `unused' attribute
(see section Specifying Attributes of Variables).
-Wuninitialized
- An automatic variable is used without first being initialized.
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 GNU CC 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 GNU CC 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.
Some spurious warnings can be avoided if you declare all the functions
you use that never return as noreturn
. See section Declaring Attributes of Functions.
-Wenum-clash
- Warn about conversion between different enumeration types.
(C++ only).
-Wreorder (C++ only)
- Warn when the order of member initializers given in the code does not
match the order in which they must be executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
Here the compiler will warn that the member initializers for `i'
and `j' will be rearranged to match the declaration order of the
members.
-Wtemplate-debugging
- When using templates in a C++ program, warn if debugging is not yet
fully available (C++ only).
-Wall
- All of the above `-W' options combined. These are all the
options which pertain to usage that we recommend avoiding and that we
believe is easy to avoid, even in conjunction with macros.
The remaining `-W...' options are not implied by `-Wall'
because they warn about constructions that we consider reasonable to
use, on occasion, in clean programs.
-W
- Print extra warning messages for these events:
- A nonvolatile 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.
- A function can return either with or without a value. (Falling
off the end of the function body is considered returning without
a value.) For example, this function would evoke such a
warning:
foo (a)
{
if (a > 0)
return a;
}
- An expression-statement or the left-hand side of a comma expression
contains no side effects.
To suppress the warning, cast the unused expression to void.
For example, an expression such as `x[i,j]' will cause a warning,
but `x[(void)i,j]' will not.
- An unsigned value is compared against zero with `<' or `<='.
- 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.
- Storage-class specifiers like
static
are not the first things in
a declaration. According to the C Standard, this usage is obsolescent.
- If `-Wall' or `-Wunused' is also specified, warn about unused
arguments.
- An aggregate has a partly bracketed initializer.
For example, the following code would evoke such a warning,
because braces are missing around the initializer for
x.h
:
struct s { int f, g; };
struct t { struct s h; int i; };
struct t x = { 1, 2, 3 };
-Wtraditional
- Warn about certain constructs that behave differently in traditional and
ANSI C.
- Macro arguments occurring within string constants in the macro body.
These would substitute the argument in traditional C, but are part of
the constant in ANSI C.
- A function declared external in one block and then used after the end of
the block.
- A
switch
statement has an operand of type long
.
-Wshadow
- Warn whenever a local variable shadows another local variable.
-Wid-clash-len
- Warn whenever two distinct identifiers match in the first len
characters. This may help you prepare a program that will compile
with certain obsolete, brain-damaged compilers.
-Wlarger-than-len
- Warn whenever an object of larger than len bytes is defined.
-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.
-Wbad-function-cast
- Warn whenever a function call is cast to a non-matching type.
For example, warn if
int malloc()
is cast to anything *
.
-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
- 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. 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
- 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.
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
.
-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.)
-Wstrict-prototypes
- 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.)
-Wmissing-prototypes
- 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.
-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
- Warn if an
extern
declaration is encountered within an function.
-Winline
- Warn if a function can not be inlined, and either it was declared as inline,
or else the `-finline-functions' option was given.
-Woverloaded-virtual
- Warn when a derived class function declaration may be an error in
defining a virtual function (C++ only). In a derived class, the
definitions of virtual functions must match the type signature of a
virtual function declared in the base class. With this option, the
compiler warns when you define a function with the same name as a
virtual function, but with a type signature that does not match any
declarations from the base class.
-Wsynth (C++ only)
- Warn when g++'s synthesis behavior does not match that of cfront. For
instance:
struct A {
operator int ();
A& operator = (int);
};
main ()
{
A a,b;
a = b;
}
In this example, g++ will synthesize a default `A& operator =
(const A&);', while cfront will use the user-defined `operator ='.
-Werror
- Make all warnings into errors.
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