demonstrating shared file description state across fork

fork_fd.c

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

int write_all(int fd, const void *buf, size_t count) {
  const char *cbuf = (const char *)buf;

  while (count > 0) {
    ssize_t rv = write(fd, cbuf, count);

    if (rv == (ssize_t)-1) {
      switch (errno) {
        case EAGAIN:
#if EAGAIN != EWOULDBLOCK
        case EWOULDBLOCK:
#endif
        case EINTR:
          continue;
        default:
          return errno;
      }
    }

    size_t written = (size_t)rv;
    cbuf += written;
    count -= written;
  }

  return 0;
}

int read_all(int fd, void *buf, size_t count) {
  char *cbuf = (char *)buf;

  while (count > 0) {
    ssize_t rv = read(fd, cbuf, count);

    if (rv == (ssize_t)-1) {
      switch (errno) {
        case EAGAIN:
#if EAGAIN != EWOULDBLOCK
        case EWOULDBLOCK:
#endif
        case EINTR:
          continue;
        default:
          return errno;
      }
    }

    size_t nread = (size_t)rv;
    cbuf += nread;
    count -= nread;
  }

  return 0;
}

int main(int argc, char **argv) {
  if (argc != 2) {
    fprintf(stderr, "Usage: %s file\n", argv[0] ? argv[0] : "fork_fd");
    return EXIT_FAILURE;
  }

  const char *filename = argv[1];
  int fd = open(filename, O_CREAT | O_TRUNC | O_RDWR, 0777);
  if (fd < 0) {
    fprintf(stderr, "open: %s: %s\n", strerror(errno), filename);
    return EXIT_FAILURE;
  }

  const char *data = "Hello, world!\n";
  int status = write_all(fd, data, strlen(data));
  if (status != 0) {
    fprintf(stderr, "write: %s: %s\n", strerror(status), filename);
    return EXIT_FAILURE;
  }

  off_t offset = lseek(fd, 0, SEEK_SET);
  if (offset == (off_t)-1) {
    fprintf(stderr, "lseek: %s: %s\n", strerror(errno), filename);
    return EXIT_FAILURE;
  }

  pid_t pid = fork();
  if (pid == (pid_t)-1) {
    fprintf(stderr, "fork: %s\n", strerror(errno));
    return EXIT_FAILURE;
  }

  if (pid == 0) {
    // Child
    char rbuf[8];
    status = read_all(fd, rbuf, 7);
    if (status != 0) {
      fprintf(stderr, "read: %s: %s\n", strerror(status), filename);
      return EXIT_FAILURE;
    }

    rbuf[7] = '\0';
    printf("Child process: read '%s'\n", rbuf);
  } else {
    // Parent
    int wstatus;
    pid_t wait_rv = waitpid(pid, &wstatus, 0);
    if (wait_rv == (pid_t)-1) {
      fprintf(stderr, "wait: %s\n", strerror(errno));
      return EXIT_FAILURE;
    }

    if (!WIFEXITED(wstatus)) {
      fprintf(stderr, "wait: child process exited abnormally\n");
      return EXIT_FAILURE;
    }

    if (WEXITSTATUS(wstatus) != 0) {
      return WEXITSTATUS(wstatus);
    }

    char rbuf[8];
    status = read_all(fd, rbuf, 7);
    if (status != 0) {
      fprintf(stderr, "read: %s: %s\n", strerror(status), filename);
      return EXIT_FAILURE;
    }

    rbuf[7] = '\0';
    printf("Parent process: read '%s'\n", rbuf);
  }

  return EXIT_SUCCESS;
}