Programming in C

9.0 The Preprocessor 465

In either format, a previously defined name can be supplied and expansion occurs. So

the following sequence works:

#define DATABASE_DEFS </usr/data/database.h>

...

#include DATABASE_DEFS

9.2.7 The#lineDirective

The general format for declaring the #linedirective is as follows:

#line constant "fileName"

This directive causes the compiler to treat subsequent lines in the program as if the name

of the source file were fileName, and as if the line number of all subsequent lines began

at constant. If fileNameis not specified, the filename specified by the last #linedirec-

tive or the name of the source file (if no filename was previously specified) is used.

The #linedirective is primarily used to control the filename and line number that

are displayed whenever an error message is issued by the compiler.

9.2.8 The #pragmaDirective

The general format for declaring the #pragmadirective is as follows:

#pragma text

This causes the preprocessor to perform some implementation-defined action. For

example,

#pragma loop_opt(on)

might cause special loop optimization to be performed on a particular compiler. If this

pragma is encountered by a compiler that doesn’t recognize the loop_optpragma, it is

ignored.

The special keyword STDCis used after the #pragmafor special meaning. Current sup-

ported “switches” that can follow a #pragma STDCare FP_CONTRACT,FENV_ACCESS, and

CX_LIMITED_RANGE.

9.2.9 The #undefDirective

The general format for declaring the #undefdirective is as follows:

#undef identifier

The specified identifierbecomes undefined to the preprocessor. Subsequent #ifdef

or #ifndefdirectives behave as if the identifier were never defined.

9.2.10 The#Directive

This is a null directive and is ignored by the preprocessor.

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