Appendix A. GCC Extensions to the C Language

The GNU Compiler Collection (GCC) provides many extensions to the C language, some of which have proven to be of particular value to system programmers. The majority of the additions to the C language that we'll cover in this appendix offer ways for programmers to provide additional information to the compiler about the behavior and intended use of their code. The compiler, in turn, utilizes this information to generate more efficient machine code. Other extensions fill in gaps in the C programming language, particularly at lower levels.

GCC provides several extensions now available in the latest C standard, ISO C99. Some of these extensions function similarly to their C99 cousins, but ISO C99 implemented other extensions rather differently. New code should use the ISO C99 variants of these features. We won't cover such extensions here; we'll discuss only GCC-unique additions.