gistfile1.txt

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/file.h>
#include <unistd.h>
#include <string.h>

struct record {
    int userid;
    char username[6];
};

char *usernames[] = {"userA", "userB", "userC", "userD"};

int main(int argc, char *argv[])
{
    int i, outfile;
    struct record eachrec;

    if ((outfile == open("recordfile", O_WRONLY | O_CREAT, 0664))<0) {
        perror("recordfile");
        exit(1);
    }

    for (i = 3; i>=0; i--) {
        eachrec.userid = i;
        strcpy(eachrec.username, usernames[i]);

        lseek(outfile, (long) i * sizeof(struct record), SEEK_SET);
        write(outfile, &eachrec, sizeof(struct record));
    }

    close(outfile);
    exit(0);
}