undef

#pragma

The #pragma directive is an implementation-defined directive that allows various instructions to be given to the compiler. For example, a compiler may have an option to support the tracing of program execution. A trace option would then be specified by a #pragma statement. You must check your compiler’s documentation for the pragmas it supports.

The C99 _Pragma Operator

C99 includes another way to specify a pragma in a program: the _Pragma operator. It has this general form:

_Pragma ( "directive")

Here, directive is the pragma being invoked. The _Pragma operator was added to allow pragmas to participate in macro replacement.

The C99 Built-In Pragmas

C99 defines the following built-in pragmas:

Pragma	Meaning
STDC FP_CONTRACT   ON/OFF/DEFAULT	When on, floating-point expressions are treated as indivisible units that are handled by hardware-based methods. The default state is implementation defined.
STDC FENV_ACCESS   ON/OFF/DEFAULT	Tells the compiler that the floating-point environment might be accessed. The default state is implementation defined.
STDC CX_LIMITED_RANGE   ON/OFF/DEFAULT	When on, tells the compiler that certain formulas involving complex values are safe. The default state is off.