IPC via lseek

main.c

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

int main() {
    pid_t parent_pid = getpid();
    printf("parent pid is %d\n", parent_pid);
    int fd = open("file.tmp", O_CREAT | O_RDWR);
    if (fd < 0) {
        perror("open failure");
        exit(EXIT_FAILURE);
    }
    off_t offset = lseek(fd, 42, SEEK_SET); // set offset
    printf("offset is set %zu in parent\n", offset);
    pid_t pid = fork();
    if (pid < 0) {
        perror("fork failed");
        exit(EXIT_FAILURE);
    } else if (pid > 0) { //parent process
        printf("parent is waiting for child with pid %d\n", pid);
        int stat;
        wait(&stat);
        printf("child returned stat %d\n", WEXITSTATUS(stat));
        offset = lseek(fd, 0, SEEK_CUR); // get the current offset
        printf("offset in parent now is %zu\n", offset);
        close(fd);
    } else if (pid == 0) { //child process
        pid_t child_pid = getpid();
        printf("child pid is %d\n", child_pid);
        offset = lseek(fd, 64, SEEK_SET);
        printf("child set offset to %zu\n", offset);
        close(fd);
        printf("child exited\n");
        return EXIT_SUCCESS;
    }
    return 0;
}