Question

I need to be able to:

1. fork a process and make it execvp (I did that)
2. check if the child process execvp was successful (don’t know how)
3. check if the child process finished (having problems)

I’m forking a process and I don’t have any way to check if the childs’s execvp worked or failed. If it failed I need to be able to know that it failed. Currently I’m using

-1 != waitpid( pid, &status, WNOHANG )

But it seems that if the execv of the pid process fails the waitpid does not return -1.

How could I check that? I read the waitpid man page, but it isn’t clear to me; maybe my English isn’t good enough.

EDIT: in order to explain more:
I’m building my own terminal for a Home Work. I need to get as an input a command string, lets say “ls” and then I have to execute the command.
After the child forks, the child calls execvp in order to execute the command ( after I parse the string ), and the parent need to check whether there was a ‘&’ at the end of the command or not.
if the sign ‘&’ does not exist at the end of the command then the parent need to wait for the child to execute.

so I need to know if the execvp failed. If it didn’t failed then the parent use waitpid to wait for the child to finish it execution. If it failed then the parent will not wait for the child.

Answer 1

A common solution to #2 is to open a pipe prior to the fork(), then write to it in the child following the exec. In the parent, a successful read means the exec failed; an unsuccessful read means the exec succeeded and the write never took place.

// ignoring all errors except from execvp...
int execpipe[2];
pipe(execpipe);
fcntl(execpipe[1], F_SETFD, fcntl(execpipe[1], F_GETFD) | FD_CLOEXEC);
if(fork() == 0)
{
    close(execpipe[0]);
    execvp(...); // on success, never returns
    write(execpipe[1], &errno, sizeof(errno));
    // doesn't matter what you exit with
    _exit(0);
}
else
{
    close(execpipe[1]);
    int childErrno;
    if(read(execpipe[0], &childErrno, sizeof(childErrno)) == sizeof(childErrno))
    {
        // exec failed, now we have the child's errno value
        // e.g. ENOENT
    }
}

This lets the parent unambiguously know whether the exec was successful, and as a byproduct what the errno value was if unsuccessful.

If the exec was successful, the child process may still fail with an exit code, and examining the status with the WEXITSTATUS macro give you that condition as well.

NOTE: Calling waitpid with the WNOHANG flag is nonblocking, and you may need to poll the process until a valid pid is returned.