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GNU C Manual

This manual explains the C language for use with the GNU Compiler Collection (GCC) on the GNU/Linux system and other systems. We refer to this dialect as GNU C. If you already know C, you can use this as a reference manual.

If you understand basic concepts of programming but know nothing about C, you can read this manual sequentially from the beginning to learn the C language.

If you are a beginner in programming, we recommend you first learn a language with automatic garbage collection and no explicit pointers, rather than starting with C. Good choices include Lisp, Scheme, Python and Java. C’s explicit pointers mean that programmers must be careful to avoid certain kinds of errors.

C is a venerable language; it was first used in 1973. The GNU C Compiler, which was subsequently extended into the GNU Compiler Collection, was first released in 1987. Other important languages were designed based on C: once you know C, it gives you a useful base for learning C++, C#, Java, Scala, D, Go, and more.

The special advantage of C is that it is fairly simple while allowing close access to the computer’s hardware, which previously required writing in assembler language to describe the individual machine instructions. Some have called C a “high-level assembler language” because of its explicit pointers and lack of automatic management of storage. As one wag put it, “C combines the power of assembler language with the convenience of assembler language.” However, C is far more portable, and much easier to read and write, than assembler language.

This manual describes the GNU C language supported by the GNU Compiler Collection, as of roughly 2017. Please inform us of any changes needed to match the current version of GNU C.

When a construct may be absent or work differently in other C compilers, we say so. When it is not part of ISO standard C, we say it is a “GNU C extension,” because it is useful to know that. However, standards and other dialects are secondary topics for this manual. For simplicity’s sake, we keep those notes short, unless it is vital to say more.

Likewise, we hardly mention C++ or other languages that the GNU Compiler Collection supports. We hope this manual will serve as a base for writing manuals for those languages, but languages so different can’t share one common manual.

Some aspects of the meaning of C programs depend on the target platform: which computer, and which operating system, the compiled code will run on. Where this is the case, we say so.

The C language provides no built-in facilities for performing such common operations as input/output, memory management, string manipulation, and the like. Instead, these facilities are provided by functions defined in the standard library, which is automatically available in every C program. See The GNU C Library in The GNU C Library Reference Manual.

GNU/Linux systems use the GNU C Library to do this job. It is itself a C program, so once you know C you can read its source code and see how its library functions do their jobs. Some fraction of the functions are implemented as system calls, which means they contain a special instruction that asks the system kernel (Linux) to do a specific task. To understand how those are implemented, you’d need to read Linux source code instead. Whether a library function is a system call is an internal implementation detail that makes no difference for how to call the function.

This manual incorporates the former GNU C Preprocessor Manual, which was among the earliest GNU manuals. It also uses some text from the earlier GNU C Manual that was written by Trevis Rothwell and James Youngman.

GNU C has many obscure features, each one either for historical compatibility or meant for very special situations. We have left them to a companion manual, the GNU C Obscurities Manual, which will be published digitally later.

Please report errors and suggestions to c-manual@gnu.org.