Changing Data Structures

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#define SIZE 12
#pragma pack(1)
struct Data
{
    int x;
    long long y;
};

void readbyte(int f, void *p, unsigned int size) {
    int i, status;
    unsigned char* ptr = p;

    for(i = 0; i < size; i++) {
       status = -1;
       while(status != 1) 
           status = read(f, ptr, 1);
       ptr++;
    }

    return;
}

void rdstrct(int* fd, int* x, long long int* y) 
{
    readbyte(*fd, x, 4);
    readbyte(*fd, y, 8); 
    return;
}

void writebyte(int f, void *p, unsigned int size) {
    int i, status;
    unsigned char* ptr = p;

    for(i = 0; i < size; i++) {
       status = -1;
       while(status != 1) 
           status = write(f, ptr, 1);
       if(status == -1) return -1;   
       ptr++;
    }

    return 0;
}


int wrstrct(int* fd, int* x, long long int* y) 
{
    int n;
    writebyte(*fd, x, 4);
    writebyte(*fd, y, 8); 
    return 0;
}

int main(int argc, char* argv[])
{
    int A = atoi(argv[2]);
    int fd, fd_end, i;
    int x, x2, n;
    long long y, y2;

    if((fd = open(argv[1], O_RDWR)) == -1)
        return -1;
    // finding size of the file
    fd_end = fd;
    long long int n1 = lseek(fd, 0L, SEEK_SET);
    long long int n2 = lseek(fd_end, 0L, SEEK_END);
    long long int sz = n2 - n1;
    printf("%lld\n", sz); 
    for(i = 1; i <= sz / 24; i++ )
    {
        lseek(fd_end, sz - i * 12, SEEK_SET); 

        rdstrct(&fd_end, &x, &y);
        y += x * A;
 
        lseek(fd, (i - 1) * 12, SEEK_SET);
        rdstrct(&fd, &x2, &y2);
        y2 += x2 * A;

        lseek(fd, (i - 1) * 12, SEEK_SET);
        n = wrstrct(&fd, &x, &y);
        if(n == -1) return -1;        

        lseek(fd_end, sz - i * 12, SEEK_SET); 
        n = wrstrct(&fd_end, &x2, &y2);
        if(n == -1) return -1;
    }

    if(sz % 24 != 0)
    {
        lseek(fd, (sz / 24) * 12, SEEK_SET);
        rdstrct(&fd, &x2, &y2);
        y2 += x2 * A;
        lseek(fd, (sz / 24) * 12, SEEK_SET);
        n = wrstrct(&fd, &x2, &y2);
        if(n == -1) return -1;
    }

    close(fd);
    return 0;
}

Generating Binary File With Array Of Data Structures

#include <stdio.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdlib.h>
#pragma pack(1)
struct Data
{
    int x;
    long long y;
};

int main(int argc, char** argv)
{
    int fd, fp, i;
    struct Data arr[] = {
       1, 2,
       3, 4,
       5, 6,
    };

    struct Data *buf = (struct Data *)malloc(12);
    fd = creat(argv[1], 0777);

    for(i = 0; i < 3; i++) {
        printf("%d %lld\n", arr[i].x, arr[i].y);
	write(fd, &arr[i], 12);
    }
    close(fd);

    fp = open(argv[1], O_RDONLY);

    for(i = 0; i < 3; i++) {
        read(fp, buf, 12);
	printf("%d %lld\n", buf -> x, buf -> y);
    }
    
    close(fp);
    free(buf);
    return 0; 
}