#include <sys/types.h>
#include <dirent.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
#include <fcntl.h>
#include <sys/mman.h>

int main(void)
{
    int fd;
    size_t length;
    int i;

    struct A {
        double f;
        double f2;
    } *ptr, *tmpptr;

    fd = open("mapped.dat", O_RDWR, 0666);

    if( fd == -1){
        printf("File open failed!\n");
        exit(1);
    }

    length = 100000*sizeof(struct A);

    ptr = (struct A *)mmap(NULL, length, PROT_WRITE | PROT_READ, MAP_SHARED, fd, 0);

    close(fd);

    if( ptr == MAP_FAILED ){
        printf("Mapping failed!\n");
        exit(2);
    }

    tmpptr = ptr;

    double sum = 0;

    for(i = 1; i <=100000; i++){
        sum += tmpptr->f2;
        tmpptr++;
    }

    printf("Сумма = %f\n", sum);

    munmap((void *)ptr, length);
    return 0;
}