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Options to Request or Suppress Warnings
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Options
to request or suppress warnings
Warnings are diagnostic messages
that report constructions which are not inherently erroneous; they may
warn of risky constructions or constructions, actually, in error.
You can request many specific
warnings with options beginning with ‘-W’;
for instance, use ‘-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 instance, ‘-Wno-implicit’.
This documentation discusses only one of the two forms, whichever is not
the default.
The following 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 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 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, 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 <
ffb
/FONT>-Wimplicit-int
or -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, for instance, when there is an assignment
in a context where a truth value is expected, or when operators
are nested whose precedence may be confusing.
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.
The following input exemplifies such a case.
{
if (a)
if (b)
foo ();
else
bar ();
}
In C, every else branch
belongs to the innermost possible if
statement, which in the previous example is if
(b). This is often
not what the programmer expected, as illustrated in the 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 the following input.
{
if (a)
{
if (b)
foo ();
&n
ffb
bsp; 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 and parameters, use the unused
attribute (for explanation of specifications for this attribute, see 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:
There may be no warning
about a variable that is used only to compute a value that itself is never
used.
Such computations may be
deleted by data flow analysis before the warnings display.
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. The following
is one example of how this occurs.
{
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 make this determination. The
following 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 Declaring attributes
of functions.
-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, in the following example, the compiler will
warn that the member initializers for i
and j
will be rearranged to match the declaration order of the members.
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
-Wtemplate-debugging
When using templates in
a C++ program, warn if debugging is not yet fully available (C++ only).
-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 previous -W
options combined. This enables all the warnings about constructions that
some users consider objectionable, and that are easy to avoid (or to modify
in order to prevent the warning), even in conjunction with macros.
-
The remaining -W...
options are not implied by -Wall.
Some of them warn about constructions that users generally do not consider
questionable, but which, on occasion, need to be checked for; others warn
about constructions that are necessary or difficult to avoid in some cases,
and for which 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 instance, the following example shows how
a 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
ffb
the warning, cast the unused expression to void.
For example, an expression such as x[i,j]
will cause a warning, but one such as 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 obsolete.
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.
-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 code, at compile
time, 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 -Wall
doesn’t 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.
-
-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 a 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.
-
-Wold-style-cast
Warn if an old-style
(C-style) cast is used within a program.
-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 the g++
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&);’expression
while cfront
will use the user-defined ‘operator
=’ expression.
-
-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 the -pedantic
flag is used.
-
-Werror
Make all warnings into errors.
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