GNU Info
(gcc-295.info)Warning Options
Options to Request or Suppress Warnings
=======================================
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.
Note: 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 (Note: Variable Attributes.).
`-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'. Note: Function
Attributes.
`-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:
* 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.
* A comparison between signed and unsigned values could produce
an incorrect result when the signed value is converted to
unsigned. (But don't warn if `-Wno-sign-compare' is also
specified.)
* 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 };
* An aggregate has an initializer which does not initialize all
members. For example, the following code would cause such a
warning, because `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.
* 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'.
* A non-`static' function declaration follows a `static' one.
This construct is not accepted by some traditional C
compilers.
* The ANSI type of an integer constant has a different width or
signedness from its traditional type. This warning is only
issued if the base of the constant is ten. I.e. hexadecimal
or octal values, which typically represent bit patterns, are
not warned about.
`-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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