The Standard C Libraries

The definition of standard C specifies a library which must be present in any conforming implementation. The functions supplied in the library are declared in several header files, divided according to the type of services provided by the functions. Many of the functions defined by the standard are supplied even in older implementations and most of them are probably present on your system, even though the original C definition did not specify any of them. The header files declaring the library functions also specify macros and variables which may be used by the applications to accomplish various tasks as described below. Additionally, the headers may define macros with the same names as the corresponding library functions, provided that the macros can safely be used in any place and with any set of arguments with which the corresponding function can be called. In particular, it must be guaranteed that macro arguments are evaluated only once. The applications can always ensure that a true library function rather than a macro is called by using the #undef preprocessor directive. It can also be done locally by enclosing the function name in parentheses. For syntactic reasons, the closing parenthesis will prevent the interpretation of the identifier as a macro.