mmap for darwin and linux(might work on some other platforms too)

mmap.c

#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <assert.h>
#include <fcntl.h>
#include <sys/mman.h>

#define SHMLEN sizeof(int) * 2
#define TIMES 30
#define SEC(x) ((int) (x * 1000000))

int* mem;

enum { VALUE, COUNTER };

/* Fork. */
void
x()
{
    while (mem[COUNTER] > 0) {
        mem[VALUE]++;
        mem[COUNTER]--;
        usleep(SEC(0.3));
    }
}

/* Original process. */
void
y()
{
    while (mem[COUNTER] > 0) {
        printf("*memory = %i\n", mem[VALUE]);
        usleep(SEC(1));
    }
}

void
fini()
{
    munmap(mem, SHMLEN);
}

int
main()
{
    int fd = -1;
    int flags = MAP_SHARED;

    /* http://lists.gnu.org/archive/html/bug-gnulib/2005-09/msg00124.html */
#ifdef MAP_ANON
    flags |= MAP_ANON;
#else
    assert((fd  = open("/dev/zero", O_RDWR)) != -1);
#endif

    assert((mem = mmap(0, SHMLEN, PROT_READ | PROT_WRITE,
                    flags, fd, 0)) != 0);
    mem[COUNTER] = TIMES;

    if (fork()) x(); else y();

    atexit(fini);

    return 0;
}