Warnings are diagnostic messages that report constructions that are not inherently erroneous but that 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 the compiler:
Check the code for syntax errors, but don't do anything beyond that.
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 2.35.
This option is not intended to be useful; it exists only to satisfy pedants who would otherwise claim that the compiler 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 the compiler.
Like “-pedantic”, except that errors are produced rather than warnings.
Inhibit all warning messages.
Inhibit warning messages about the use of “#import”.
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.
Warn whenever a comment-start sequence “/*” appears in a “/*” comment, or whenever a Backslash-Newline appears in a “//” comment.
Check calls to printf and scanf, and so on, to make sure that the arguments supplied have types appropriate to the format string specified.
Warn when a declaration does not specify a type.
Warn whenever a function is used before being declared.
Same as “-Wimplicit-int” “-Wimplicit-function-declaration”.
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.
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 ();
}
}
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.
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.
Warn if any trigraphs are encountered (assuming they are enabled).
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 (see Section 2.28.).
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 the compiler 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 the compiler 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 2.22.
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.
When using templates in a C++ program, warn if debugging is not yet fully available (C++ only).
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.
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 that 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 do not 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 };
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.
Warn if an undefined identifier is evaluated in an “#if” directive.
Warn whenever a local variable shadows another local variable.
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.
Warn whenever an object of larger than len bytes is defined.
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.
Warn whenever a function call is cast to a non-matching type. For example, warn if int malloc() is cast to anything *.
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 *.
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.
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.
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.
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”.
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.)
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 that specifies the argument types.)
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.
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.
Warn if anything is declared more than once in the same scope, even in cases where multiple declaration is valid and changes nothing.
Warn if an extern declaration is encountered within an function.
Warn if a function can not be inlined, and either it was declared as inline, or else the “-finline-functions” option was given.
Warn if an old-style (C-style) cast is used within a program.
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.
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 =”.
Make all warnings into errors.