From http://modelingwithdata.org/arch/00000128.htm

mmap for gigantic data sets.c

#include <stdio.h>
#include <unistd.h> //lseek, write, close
#include <stdlib.h> //exit
#include <fcntl.h>  //open
#include <sys/mman.h>

#define Mapmalloc(number, type, filename, fd) load_mmap(filename, &(fd), (number)*sizeof(type), 'y')
#define Mapload(number, type, filename, fd) load_mmap(filename, &(fd), (number)*sizeof(type), 'n')
#define Mapfree(number, type, fd, pointer) releasemmap(pointer, (number)*sizeof(type), fd)
#define Stopifnot(assertion, ...) if (!(assertion)){printf(__VA_ARGS__); exit(1);}

void *load_mmap(char *filename, int *fd, size_t size, char make_room) {
    *fd = open(filename, 
            make_room=='y' ?  O_RDWR | O_CREAT | O_TRUNC : O_RDWR,
            (mode_t)0600);
    Stopifnot(*fd != -1, "Error opening file");

    if (make_room=='y'){ // Stretch the file size to the size of the (mmapped) array
        int result = lseek(*fd, size-1, SEEK_SET);
        Stopifnot(result != -1, "Error calling lseek() to stretch the file");
        result = write(*fd, "", 1);
        Stopifnot (result == 1, "Error writing last byte of the file");
    }

    void *map = mmap(0, size, PROT_READ | PROT_WRITE, MAP_SHARED, *fd, 0);
    Stopifnot (map != MAP_FAILED, "Error mmapping the file");
    return map;
}

void releasemmap(void *map, size_t size, int fd){
    Stopifnot(munmap(map, size) != -1, "Error un-mmapping the file");
    close(fd);
}

int main(int argc, char *argv[]) {
    int fd;
    long int N=1e5+6;
    int *map = Mapmalloc(N, int, "mmapped.bin", fd);
    
    for (long int i = 0; i <N; ++i) map[i] = i; //you can't tell it's on disk.
    
    Mapfree(N, int, fd, map);

    //Now reopen and do some counting.
    int *readme = Mapload(N, int, "mmapped.bin", fd);

    long long int oddsum=0;
    for (long int i = 0; i <N; ++i) if (readme[i]%2) oddsum += i; 
    printf("The sum of odd numbers up to %li: %lli\n", N, oddsum);

    Mapfree(N, int, fd, readme);
}