26.1 Preprocessing Overview
GNU C performs preprocessing on each line of a C program as the first
stage of compilation. Preprocessing operates on a line only when it
contains a preprocessing directive or uses a macro—all
other lines pass through preprocessing unchanged.
Here are some jobs that preprocessing does. The rest of
this chapter gives the details.
- Inclusion of header files. These are files (usually containing
declarations and macro definitions) that can be substituted into your
program.
- Macro expansion. You can define macros, which are abbreviations
for arbitrary fragments of C code. Preprocessing replaces the macros
with their definitions. Some macros are automatically predefined.
- Conditional compilation. You can include or exclude parts of the
program according to various conditions.
- Line control. If you use a program to combine or rearrange source files
into an intermediate file that is then compiled, you can use line
control to inform the compiler where each source line originally came
from.
- Compilation control.
#pragma and _Pragma invoke
some special compiler features in how to handle certain constructs.
- Diagnostics. You can detect problems at compile time and issue errors
or warnings.
Except for expansion of predefined macros, all these operations happen
only if you use preprocessing directives to request them.