2.2 Include Operation

The #include directive works by directing the C preprocessor to scan the specified file as input before continuing with the rest of the current file. The output from the preprocessor contains the output already generated, followed by the output resulting from the included file, followed by the output that comes from the text after the #include directive. For example, if you have a header file header.h as follows,

     char *test (void);

and a main program called program.c that uses the header file, like this,

     int x;
     #include "header.h"
     
     int
     main (void)
     {
       puts (test ());
     }

the compiler will see the same token stream as it would if program.c read

     int x;
     char *test (void);
     
     int
     main (void)
     {
       puts (test ());
     }

Included files are not limited to declarations and macro definitions; those are merely the typical uses. Any fragment of a C program can be included from another file. The include file could even contain the beginning of a statement that is concluded in the containing file, or the end of a statement that was started in the including file. However, an included file must consist of complete tokens. Comments and string literals which have not been closed by the end of an included file are invalid. For error recovery, they are considered to end at the end of the file.

To avoid confusion, it is best if header files contain only complete syntactic units—function declarations or definitions, type declarations, etc.

The line following the #include directive is always treated as a separate line by the C preprocessor, even if the included file lacks a final newline.