The directives #if and #endif can be used to surround a portion of text that should not always be compiled. For example:

  div_t result
#if __STDC_VERSION__ > 199901
    = { .quot = 10, .rem = .4 }
#endif
    ;
#if __STDC_VERSION__ <= 199901
  result.quot = 10;
  result.rem = 4;
#endif

#if is followed by text which is subject to macro expansion and then treated as an integer expression, which is the directive's condition. Names in the expression that are not macros are replaced with 0. If the condition is zero, the enclosed text will be skipped. In our example, the text becomes either this:

  div_t result
    = { .quot = 10, .rem = .4 }
    ;

…or this:

  div_t result
    ;
  result.quot = 10;
  result.rem = 4;

…both of which will compile in C99 or later, but the first of which will not compile in earlier versions.

Several alternative chunks of code can be selected from using the directives #else and #elif:

#if condition
  . . .
#elif condition
  . . .
#elif condition
  . . .
#else
  . . .
#endif

The section of code immediately after first condition evaluating to non-zero (true) is selected for further processing, and the others are discarded. The optional #else section is used only if all conditions are false.

The code inside an unselected sectionis not completely ignored, as nested conditional sections are permitted. Further #if. . .#endif constructs may appear in the section, and the closing #endif of such a construct does not prematurely terminate the enclosing construct.

Preprocessor conditions can only be based on integer constants, integer operators, logical operators, and macros that expand to them. The operator defined can also be used to test whether a macro is currently defined:

#if defined __STDC_NO_ATOMICS__
  // We cannot use atomics.
#else
  // We can use atomics.
#endif

It can also be used with brackets around the macro name in question:

#if defined(__STDC_NO_ATOMICS__)

Finally, this is such a common construct, that two more directives exist, #ifdef and #ifndef:

#ifdef __STDC_NO_ATOMICS__
  // We cannot use atomics.
#else
  // We can use atomics.
#endif

#ifndef __STDC_NO_ATOMICS__
  // We can use atomics.
#else
  // We cannot use atomics.
#endif

It is not possible to use constructs such as sizeof, that only have meaning in later translation phases.

It's possible that a program requires some optional feature or capability, and there's no point in trying to build it if the feature isn't available. For this, you can use the #error directive, which causes an error message to be printed during translation, and ultimately causes translation to fail. For example, if atomics are absolutely required:

#if __STDC_VERSION__ < 201112L && defined __STDC_NO_ATOMICS__
#error "This program needs atomics."
#endif

#error can also be useful for debugging. If you think a section of source code is not actually being compiled, temporarily insert an #error directive, and re-compile. If you were wrong, you'll get an error while compiling.

Standard predefined macros

Name	Description	Notes				Source	Availability
__DATE__	Compilation date	L		M		Predefined	C89	C90	C95	C99	C11
__FILE__	Compilation file name	L		M		Predefined	C89	C90	C95	C99	C11
__LINE__	Compilation line number	L		M		Predefined	C89	C90	C95	C99	C11
__STDC__	Indicator of conforming implementation	L		M		Predefined	C89	C90	C95	C99	C11
__STDC_ANALYZABLE__	Indicator of analyzability	L	?	M		Predefined					C11
__STDC_HOSTED__	Indicator of hosted implementation	L		M		Predefined			C95	C99	C11
__STDC_IEC_559__	Indicator of support for IEC 60559-compatible floating-point arithmetic	L	?	M		Predefined				C99	C11
__STDC_IEC_559_COMPLEX__	Indicator of support for IEC 60559-compatible complex arithmetic	L	?	M		Predefined				C99	C11
__STDC_ISO_10646__	Unicode version	L	?	M		Predefined				C99	C11
__STDC_LIB_EXT1__	Identifier of version of bounds-checking conformance	L	?	M		Predefined					C11
__STDC_MB_MIGHT_NEQ_WC__	Indicator for lack of guaranteed equivalence of multibyte and wide characters in the basic character set	L	?	M		Predefined					C11
__STDC_NO_ATOMICS__	Indicator of lack of standard atomics	L	?	M		Predefined					C11
__STDC_NO_COMPLEX__	Indicator for lack of complex arithmetic	L	?	M		Predefined				C99	C11
__STDC_NO_THREADS__	Indicator of lack of standard threading	L	?	M		Predefined					C11
__STDC_NO_VLA__	Indicator of lack of support for variable-length arrays	L	?	M		Predefined				C99	C11
__STDC_UTF_16__	Indicator of equivalence of char16_t and UTF-16	L	?	M		Predefined					C11
__STDC_UTF_32__	Indicator of equivalence of char32_t and UTF-32	L	?	M		Predefined					C11
__STDC_VERSION__	Standard version identifier	L		M		Predefined			C95	C99	C11
__TIME__	Compilation time	L		M		Predefined	C89	C90	C95	C99	C11

control-line

# error pp-tokensopt new-line

group-part

if-section

if-section

if-group elif-groupsopt else-groupopt else-line

if-group

# if constant-expression new-line group

# ifdef identifier new-line group

# ifndef identifier new-line group

elif-groups

elif-group

elif-groups elif-group

elif-group

# elif constant-expression new-line group

else-line

# endif new-line