The C language has very few reserved words or built-in operators. Rather, it relies upon the existence of function LIBRARIES to empower the programmer. The use of function libraries allows the C language to be customized for particular environments and application areas, contributing to its efficiency and flexibility.
ANSI C compilers include a standard function library which provides functions for mathematical operations, input/output, string handling, memory allocation, etc. (Pre-ANSI C compilers often used separate libraries for these collections of functions.) These functions may be accessed directly from any of the programmer's own functions, provided an appropriate declaration for the library function precedes its use in the source file.
The programmer should not declare the standard functions himself/herself. Rather, the appropriate standard header files should be referenced, as discussed further in Chapters 7 and 8. These header files themselves contain the declarations for the corresponding library functions. In this way, the program is less likely to be affected by future changes to the function libraries.
