flock-playground

flock-playground.c

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

enum new_fd_method_t {
  flock_test_open,
  flock_test_dup,
};

int main() {

  const char *new_fd_method_env_key = "NEW_FD_METHOD";
  const char *path = "./mylock";
  enum new_fd_method_t new_fd_method;
  int fd, new_fd;

  const char *new_fd_method_env_val = getenv(new_fd_method_env_key);
  if (new_fd_method_env_val == NULL) {
    fprintf(stderr, "usage: Run with NEW_FD_METHOD set to `open` or `dup`\n");
    exit(EXIT_FAILURE);
  } else if (strcmp(new_fd_method_env_val, "open") == 0) {
    new_fd_method = flock_test_open;
  } else if (strcmp(new_fd_method_env_val, "dup") == 0) {
    new_fd_method = flock_test_dup;
  } else {
    fprintf(stderr, "usage: Run with NEW_FD_METHOD set to `open` or `dup`\n");
    exit(EXIT_FAILURE);
  }

  if ((fd = open(path, O_CREAT | O_TRUNC | O_RDWR)) < 0) {
    fprintf(stderr, "error creating lock file %s: %s\n", path, strerror(errno));
    exit(errno);
  }

  if ((flock(fd, LOCK_NB | LOCK_SH)) < 0) {
    fprintf(stderr, "error getting shared lock using fd %d: %s\n", fd, strerror(errno));
    exit(errno);
  }

  if ((flock(fd, LOCK_NB | LOCK_EX)) < 0) {
    fprintf(stderr, "error upgrading shared lock to exclusive lock using fd %d: %s\n", fd, strerror(errno));
    exit(errno);
  }

  if ((flock(fd, LOCK_UN)) < 0) {
    fprintf(stderr, "error unlockind lock using fd %d: %s\n", fd, strerror(errno));
    exit(errno);
  }

  if ((flock(fd, LOCK_NB | LOCK_SH)) < 0) {
    fprintf(stderr, "error reaquiring shared lock with original fd %d: %s\n", fd, strerror(errno));
    exit(errno);
  }

  switch (new_fd_method) {
  case flock_test_open:
    if ((new_fd = open(path, O_RDWR)) < 0) {
      fprintf(stderr, "error opening same lock file with new fd: %s\n", strerror(errno));
      exit(errno);
    }
    break;
  case flock_test_dup:
    if ((new_fd = dup(fd)) < 0) {
      fprintf(stderr, "error duplicating fd: %s\n", strerror(errno));
      exit(errno);
    }
    break;
  }

  if ((flock(new_fd, LOCK_NB | LOCK_EX)) < 0) {
    fprintf(stderr, "error obtaining exclusive lock with new fd %d: %s\n", new_fd, strerror(errno));
    exit(errno);
  }

  return 0;
}