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  and ISO 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 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 the -Wimplicit-int or -Wimplicit-function-declaration flags.
-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 ();
       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 (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.

 
-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 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 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; }; 
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.
-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.

-Wmissing-noreturn

Warn about functions which might be candidates for the noreturn attribute. Note these are only possible candidates, not absolute ones. Care should be taken to verify manually which 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 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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