source: vendor/current/tests/shared_mmap.c

/* this tests whether we can use a shared writeable mmap on a file -
   as needed for the mmap variant of FAST_SHARE_MODES */

#if defined(HAVE_UNISTD_H)
#include <unistd.h>
#endif
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

#define DATA "conftest.mmap"

#ifndef MAP_FILE
#define MAP_FILE 0
#endif

main()
{
        int *buf;
        int i; 
        int fd = open(DATA,O_RDWR|O_CREAT|O_TRUNC,0666);
        int count=7;

        if (fd == -1) exit(1);

        for (i=0;i<10000;i++) {
                write(fd,&i,sizeof(i));
        }

        close(fd);

        if (fork() == 0) {
                fd = open(DATA,O_RDWR);
                if (fd == -1) exit(1);

                buf = (int *)mmap(NULL, 10000*sizeof(int), 
                                   (PROT_READ | PROT_WRITE), 
                                   MAP_FILE | MAP_SHARED, 
                                   fd, 0);

                if (buf == (int *)-1) exit(1);

                while (count-- && buf[9124] != 55732) sleep(1);

                if (count <= 0) exit(1);

                buf[1763] = 7268;
                exit(0);
        }

        fd = open(DATA,O_RDWR);
        if (fd == -1) exit(1);

        buf = (int *)mmap(NULL, 10000*sizeof(int), 
                           (PROT_READ | PROT_WRITE), 
                           MAP_FILE | MAP_SHARED, 
                           fd, 0);

        if (buf == (int *)-1) exit(1);

        buf[9124] = 55732;

        while (count-- && buf[1763] != 7268) sleep(1);

        unlink(DATA);
                
        if (count > 0) exit(0);
        exit(1);
}