Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!spool.mu.edu!uunet!bria!mike
From: mike@bria
Newsgroups: comp.unix.programmer
Subject: Re: Detecting exec(2) failing after performing fork(2)?
Message-ID: <493@bria>
Date: 1 Mar 91 21:20:29 GMT
References: <shj.667793966@dkuugin>
Reply-To: uunet!bria!mike
Organization: MGI Group International, Los Angeles, CA
Lines: 70
Followup-To:
Keywords:

In an article, login.dkuug.dk!shj (Stig Jacobsen) writes:
|When my application desires to spawn off a background process,
|I use something roughly like this:

[ example deleted ]

|This is fine if the exec..() call goes well. However, if the exec()
|call fails, the error is not reported back to the parent. I get
|a SIGCLD of course, but what I'd really like is that my spawn()
|function just returns an error, if the exec() call fails. So far
|the best solution that I've come up with is,
|
|int spawn(char *path, ...)
|{
|
|   if (access(path, R_OK|X_OK) != 0)
|      return -1;
|   if (fork() == 0)
|      execlp(path, NULL);
|
|}
|
|But this is not an elegant solution, nor is it entirely safe:
|If someone unlinks 'path' between the access() and the exec()
|call (not _that_ unlikely if the system is heavily loaded),
|I won't detect the error anyways!
|
|So - what does everybody else do? Am I overseeing something
|totally obvious?

Some time ago, I wrote a spawn function (but then I got religion, and
only use fork/exec explicitly :-)

The only caveat is if the spawned program returns an exit status of 255,
it will look like the spawn failed (this was never a big issue for me,
so I left it).  Here goes:

#include <stdio.h>

main(argc,argv)
int argc;
char *argv[];
{
	printf("spawn returned %d\n",spawn(argv[1],&argv[1]));
	return(0);
}

spawn(path,argv)
char *path, *argv[];
{
int ret, xit;

	if ( (ret = fork()) > 0 ) {
		if ( wait(&xit) == -1 )
			return(-1);
		}
	else if ( ret == 0 ) {
		if ( execvp(path,argv) == -1 )
			exit(-1);
		}
	else
		return(-1);

	return(xit >> 8);
}
-- 
Michael Stefanik, MGI Inc., Los Angeles| Opinions stated are not even my own.
Title of the week: Systems Engineer    | UUCP: ...!uunet!bria!mike
-------------------------------------------------------------------------------
Remember folks: If you can't flame MS-DOS, then what _can_ you flame?