The GNU C Reference Manual

This is a reference manual for the C programming language as implemented by the GNU Compiler Collection (GCC). Specifically, this manual aims to document:

The 1989 ANSI C standard, commonly known as “C89”
The 1999 ISO C standard, commonly known as “C99”, to the extent that C99 is implemented by GCC
The current state of GNU extensions to standard C
This manual describes C89 as its baseline. C99 features and GNU extensions are explicitly labeled as such.

By default, GCC will compile code as C89 plus GNU-specific extensions. Much of C99 is supported; once full support is available, the default compilation dialect will be C99 plus GNU-specific extensions. (Some of the GNU extensions to C89 ended up, sometimes slightly modified, as standard language features in C99.)

The C language includes a set of preprocessor directives, which are used for things such as macro text replacement, conditional compilation, and file inclusion. Although normally described in a C language manual, the GNU C preprocessor has been thoroughly documented in The C Preprocessor, a separate manual which covers preprocessing for C, C++, and Objective-C programs, so it is not included here.