#include <stdio.h>
#include <stdlib.h>

void main()
  {
    char *path;
    path = getenv( "INCLUDE" );
    if( path != NULL )
        printf( "INCLUDE=%s\n", path );
    if( putenv( "INCLUDE=mylib;yourlib" ) != 0 )
        printf( "putenv failed" );
    path = getenv( "INCLUDE" );
    if( path != NULL )
        printf( "INCLUDE=%s\n", path );
    if( putenv( "INCLUDE=" ) != 0 )
        printf( "putenv failed" );
  }