Each standard library function is declared in one or more of the standard headers . These headers also contain all the macro and type definitions that the C standard provides. This chapter describes the contents and use of the standard headers.
Each of the standard headers contains a set of related function declarations, macros, and type definitions. The standard headers are also called header files , as the contents of each header are usually stored in a file. Strictly speaking, however, the standard does not require the headers to be organized in files.
The C standard defines the following 24 headers. Those marked with an asterisk have been added in C99.
You can add the contents of a standard header to a source file
by inserting an
which must be placed outside all functions. You can include the
standard headers as many times as you want, and in any order. However,
#include directive for
any header, your program must not define any macro with the same name
as an identifier in that header. To make sure that your programs
respect this condition, always include the required standard headers
at the beginning of your source files, before any header files of your
C programs run in one ...