Source programs

A C source program is a collection of one or more directives, declarations, and statements contained in one or more source files.

• Directives instruct the C preprocessor to act on the text of the program.
• Declarations establish names and define characteristics such as scope, data type and linkage.
• Statements specify the action to be performed.
• Definitions are declarations that allocate storage for data objects or define a body for functions. An object definition allocates storage and may optionally initialize the object.

A function definition precedes the function body. The function body is a compound statement that can contain declarations and statements that define what the function does. The function definition declares the function name, its parameters, and the data type of the value it returns.

The order and scope of declarations affect how you can use variables and functions in statements. In particular, an identifier can be used only after it is declared.

A program must contain at least one function definition. If the program contains only one function definition, the function must be called main. If the program contains more than one function definition, only one of the functions can be called main. The main function is the first function called when a program is run.

C source files

A C source file is the collection of text files that contains all of a C source program. It can include any of the functions that the program needs. To create an executable object module, you compile the source files individually and then link them as one program. With the #include directive, you can combine source files into larger source files.

A source file contains any combination of directives, declarations, and definitions. You can split items such as function definitions and large data structures between text files, but you cannot split them between compiled files. Before the source file is compiled, the preprocessor filters out preprocessor directives that may change the files. As a result of the preprocessing stage, preprocessor directives are completed, macros are expanded, and a source file is created containing C statements, completed directives, declarations, and definitions.

It is sometimes useful to place variable definitions in one source file and declare references to those variables in any source files that use them. This procedure makes definitions easy to find and change, if necessary. You can also organize constants and macros into separate files and include them into source files as required.

Directives in a source file apply to that source file and its included files only. Each directive applies only to the part of the file following the directive.

Data types

The void type

The void data type always represents an empty set of values. The keyword for this type is void. When a function does not return a value, you should use void as the type specifier in the function definition and declaration. The only object that can be declared with the type specifier void is a pointer.