It's not hard, just a bit too long:

    #include <fcntl.h>
    #include <spawn.h>
    
    int
    main(void) {
      posix_spawn_file_actions_t file_actions;
      posix_spawn_file_actions_init(&file_actions);
      posix_spawn_file_actions_addopen(&file_actions, 0, "/dev/null", O_RDONLY, 0);
      posix_spawn_file_actions_addopen(&file_actions, 2, "/dev/null", O_WRONLY, 0);
      posix_spawnp(NULL, "ls", &file_actions, NULL, (const char *[]){"ls", "-l", "/proc/self/fd", NULL}, NULL);
      posix_spawn_file_actions_destroy(&file_actions);
    }