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| _Wimplicit | Warn whenever a function or parameter is implicitly declared. |
| _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. |
| _Wunused | Warn whenever a local variable is unused aside from its declaration, whenever a function is declared static but never defined, and whenever a statement computes a result that is explicitly not used. |
| _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. |
| _Wcomment | Warn whenever a comment-start sequence / appears in a comment. |
| _Wtrigraphs | Warn if any trigraphs are encountered (assuming they are enabled). |
| _Wformat | Check calls to printf and scanf, and so on, to make sure that the arguments supplied have types appropriate to the format string specified. |
| _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. |
| _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 GNU CC 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);
}
{
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 as volatile all the functions you use that never return. |
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| _Wparentheses | Warn if parentheses are omitted in certain contexts. |
| _Wtemplate_debugging | When using templates in a C++ program, warn if debugging is not yet fully available (C++ only). |
| _Wall | All of the preceding _W options combined. These are all the options that pertain to usage that we recommend avoiding and that we believe is easy to avoid, even in conjunction with macros. |
The remaining _W... options are not implied by _Wall because they warn about constructions that we consider reasonable to use, on occasion, in clean programs.
| _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. |
| _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. |
| _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. |
| _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. |
| _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 an function. |
| _Wenum_clash | Warn about conversion between different enumeration types (C++ only). |
| _Woverloaded_virtual | (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. Use this option to request warnings when a derived class declares a function that may be an erroneous attempt to define a virtual function; |