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 C11. Some of these extensions function similarly to their C11 cousins, but ISO C11 implemented other extensions rather differently. New code should use the standardized variants of these features. We won’t cover such extensions here; we’ll discuss only GCC-unique additions.


The flavor of C supported by GCC is often called GNU C. In the 1990s, GNU C filled in several gaps in the C language, providing features such as complex variables, zero-length arrays, inline functions, and named initializers. But after nearly a decade, C was finally upgraded, and with the standardization of ISO C99 and then ISO C11, GNU C extensions grew less relevant. Nonetheless, GNU C continues to provide useful features, and many Linux programmers still use a subset of GNU C—often just an extension or two—in their C99- or C11-compliant code.

One prominent ...

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