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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
Check the code for syntax errors, but don't do anything beyond that.
-pedantic
Issue all the warnings demanded by strict ANSI C and ISO C++; reject all programs that use forbidden extensions.
Valid ANSI C and ISO 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 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 Alternate Keywords.
This option is not intended to be useful; it exists only to satisfy pedants who would otherwise claim that GCC 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 don't have plans to support such a feature in the near future.
-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, or whenever a Backslash-Newline 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-int
Warn when a declaration does not specify a type.
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
Give a warning (or error) whenever a function is used before being declared.
-Wimplicit
Same as `-Wimplicit-int' and `-Wimplicit-function-'
`declaration'.
-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.
-Wmultichar
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.
-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 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, 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 (); } } |
-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.
In order to get a warning about an unused function parameter, you must specify both `-W' and `-Wunused'.
To suppress this warning for an expression, simply cast it to void. For unused variables, parameters and labels, 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 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.
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.
-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.
-Wall
All of the above `-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.
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
Print extra warning messages for these events:
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.
foo (a) { if (a > 0) return a; } |
static
are not the first things in
a declaration. According to the C Standard, this usage is obsolescent.
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 }; |
-Wtraditional
Warn about certain constructs that behave differently in traditional and ANSI C.
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.
-Wundef
Warn if an undefined identifier is evaluated in an `#if' directive.
-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
.
-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 `-W'; to get the other warnings of `-W' without this warning, use `-W -Wno-sign-compare'.
-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.
-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.
-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.
-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.
-Werror
Make all warnings into errors.
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