Return status of execve

c, exec, process

Solution

You can verify that exec succeeded using `FD_CLOEXEC`:

create a pipe.

fork.

in the child, close the read end of the pipe and set the `FD_CLOEXEC` flag on the write end of the pipe. Then proceed with `exec`. If `exec` succeeds, the pipe will be automatically closed due to `FD_CLOEXEC`. If `exec` fails, write a byte to the pipe and exit.

in the parent, close the write end of the pipe and read from the read end. If you read 0 bytes (EOF), it means that `exec` succeeded. If you read a byte in the pipe, it means that `exec` failed.

The data written over the pipe in case of failure can be used to transmit error information, such as value of `errno` after `exec`.

The code, with error checking omitted for brevity, would look like this:

int pipe_fds[2];
pipe(pipe_fds);
if (!fork()) {
  close(pipe_fds[0]);
  fcntl(pipe_fds[1], F_SETFD, F_CLOEXEC);
  execve(...);
  write(pipe_fds[1], "", 1);
  _exit(1);
}
else {
  int n;
  char out;
  close(pipe_fds[1]);
  n = read(pipe_fds[0], &out, 1);
  if (n == 0)
    printf("exec successful\n");
  else
    printf("exec failed\n");
  close(pipe_fds[0]);
}

Note that this technique can be unsafe if other threads may exec their own processes in parallel. The issue is that there is a delay between when the pipe is created and when the close-on-exec flag gets set. If an unrelated thread forks and execs during that critical window, the child will inherit `pipe_fds[1]` and won't close it, causing the parent to hang. This can be fixed using the Linux-specific `pipe2` call which allows atomically creating the pipe with the close-on-exec flag set.

Problem

I want to send exit status 0 from the execve process to the process which started it. Because on Success execve never returns, So I am not able to do anything after that. But I want if execve ran successful.

Original source