C Pocket Reference

C Program Structure

Figure 1-1 illustrates the structure of a C program. The program shown consists of the functions main() and showPage() , and prints the beginning of a text file to be specified on the command line when the program is started.

Figure 1-1. A C program

The statements that make up the functions, together with the necessary declarations and preprocessing directives, form the source code of a C program. For small programs, the source code is written in a single source file . Larger C programs consist of several source files, which can be edited and compiled separately. Each such source file contains functions that belong to a logical unit, such as functions for output to a terminal, for example. Information that is needed in several source files, such as declarations, is placed in header files. These can then be included in each source file via the #include directive.

Source files have names ending in .c; header files have names ending in .h. A source file together with the header files included in it is called a translation unit .

There is no prescribed order in which functions must be defined. The function showPage() in Figure 1-1 could also be placed before the function main(). A function cannot be defined within another function, however.

The compiler processes each source file in sequence and decomposes its contents into tokens , such as function names and operators. ...