4

I am currently writing an application in C, targetting BSD and Linux systems with a hope to being generall portable. This program a runtime dependency, in this case mplayer.

As it stands I am using execlp() to start mplayer. I am checking the error code of the execlp call and I am testing for EACCESS, so I know when I attempt to run mplayer if it exists or not.

Because of the way my program works, mplayer is a required dependency but may not be used for some time after my program starts. As a user experience it is poor for the program to have been running for some time before failing due to mplayer being missing. So I would like to test for mplayer existing as my program starts up. Probably delivering an error message if mplayer is not available.

Now I understand there is a race condition here so my current handling of an EACCESS error will have to stay. We could find a situation where a user starts my program running, then uninstalls mplayer. This is accepted.

My initial thought was to call execlp() early on in execution and however this results in mplayer visibly starting. To be honest I'd prefer not to be starting mplayer, just testing if I "could" start it (eg. does a file exist called mplayer somewhere in my path and is it executable).

A second thought was then to run those precise steps, looking through the path and testing if the matching file is executable. I've not yet coded this for two reasons. The first reason, to be sure execlp is finding the same thing I have found I would have to pass the discovered pathname to execlp, bypassing the builtin PATH searching mechanism. The other reason is simply I feel I'm missing an obvious trick.

Is there a function I should be using to do the search for an executable? Or do I really need to just get on and code it the long way.

Improve this question
6
  • Have you seen this related SO question? stackoverflow.com/questions/4629643/… Commented Nov 7, 2011 at 10:57
  • Is there something wrong with simply calling execlp when you want to start mplayer and emitting the error message when it fails? If you allow mplayer to be uninstalled while your program is running, presumably you allow it to be installed as well, so checking at startup (or at anytime other than the moment you actually want to start mplayer) is pointless. Commented Nov 7, 2011 at 11:00
  • I'll also point out that generally, dependencies are checked at install-time, not run-time. And missing dependencies at runtime usually result in "ugly" (as opposed to pretty) errors. Commented Nov 7, 2011 at 11:06
  • Jedwards, I had seen that question. The answer there and I would agree with it, is to just try and execute reporting on error. I'm already doing this. The error itself is already being caught, it would be nice to add additional notifications to the user before things go wrong. Commented Nov 7, 2011 at 11:12
  • Also, (agreeing with jedwards and supporting the idea that the dependency should be checked at install time), consider the naive user who writes "hello, world", calls the executable "mplayer", and prepends "." to PATH. No change that naive user to a malicious user. Doing the check at install time allows the person doing the install (or doing the packaging) to control which program is actually called. Commented Nov 7, 2011 at 11:13

3 Answers 3

Reset to default
3

Some systems (FreeBSD, Linux) support a which command that searches the user's path for a given command.

I suppose that begs the question in a sense... if your code might run on a variety of systems, you might feel the need to do which which just to determine if which is available. ;-) If that's a problem you might still have to consider building that functionality into your program, but the code could still be a helpful starting point.

Improve this answer
Sign up to request clarification or add additional context in comments.

1 Comment

This thought process crossed my mind. If it comes to coding it myself I'll certanly be looking at which.
1

with a hope to being generally portable

To POSIX platforms, I suppose? execlp is far from generally available.

There's no portable way to check for a command's availability except trying to execute it. What you could do is copy the path finding logic from BSD execlp (the userland part), or BSD's which command.

Improve this answer

Comments

0

There is no certain way in ANSI C. You may try fopen() and check return code. Try to use stat call (man 2 stat), it exists on Linux, but I'm not sure about BSD.

Improve this answer

Comments

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.