once

Option Type Default Value #pragma options C C++
-qoption noonce ONCE x x

Syntax 

    -qonce | -qnoonce 
    ONCE | NOONCE

Purpose
Avoids including a header file more than once even if it is specified in several of the files you are compiling.

Notes
The compiler uses the full path name to determine if a file has already been included. No attempt is made to resolve . or .. in the path name. #include statements that include . or .. in the path statements may cause the same file to be included more than once.

The #pragma options keyword ONCE may appear anywhere in your code. It can be turned on and off by specifying ONCE and NOONCE, respectively.

Important!
Do not use the -qonce option if both of the following conditions are true:
  1. You include both stdio.h and stdarg.h (in that order) in your source files, and,
  2. You are using the macro va_list.

va_list must be defined twice to have any effect, and -qonce defeats this purpose.

Example
The following example shows how the compiler resolves whether a file has already been included. 

#include <stdio.h>         /* Found in /usr/include/stdio.h */
#include <stdio.h>         /* Already included              */
#include </usr/include/stdio.h>    /* Already included      */
#include <./stdio.h>  /* Resolves to /usr/include/./stdio.h */
                      /* which is the same file, but this   */
                      /* file will be included again.       */


List of Compiler Options and Their Defaults