From http://beej.us/guide/bgipc/output/html/multipage/mmap.html

beej-mmapdemo.c

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <errno.h>

int main(int argc, char *argv[])
{
    int fd, offset;
    char *data;
    struct stat sbuf;

    if (argc != 2) {
        fprintf(stderr, "usage: mmapdemo offset\n");
        exit(1);
    }

    if ((fd = open("mmapdemo.c", O_RDONLY)) == -1) {
        perror("open");
        exit(1);
    }

    if (stat("mmapdemo.c", &sbuf) == -1) {
        perror("stat");
        exit(1);
    }

    offset = atoi(argv[1]);
    if (offset < 0 || offset > sbuf.st_size-1) {
        fprintf(stderr, "mmapdemo: offset must be in the range 0-%d\n", \
                                                              sbuf.st_size-1);
        exit(1);
    }
    
    data = mmap((caddr_t)0, sbuf.st_size, PROT_READ, MAP_SHARED, fd, 0)) \
                                                           == (caddr_t)(-1)) {
    if (data == (caddr_t)(-1)) {
        perror("mmap");
        exit(1);
    }

    printf("byte at offset %d is '%c'\n", offset, data[offset]);

    return 0;
}