Simple file locking wrapper command in C

Question

A simple command that wraps another command, locking a file first. It is similar to flock, just simpler.

#include <stddef.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/file.h>
#include <err.h>
#include <stdlib.h>
void attemptLock(const char *lockFileName)
{
 int fd = open(lockFileName, O_CREAT | O_WRONLY, 0666);
 if (!fd || flock(fd, LOCK_EX))
 warn("Cannot lock %s", lockFileName);
}
int main(int argc, char *argv[])
{
 if (argc < 3)
 errx(EXIT_FAILURE, "Not enough arguments.\nUsage: %s LOCKFILENAME COMMAND...\n", argv[0]);
 attemptLock(argv[1]);
 argv += 2; // Skip our own command name and LOCKFILENAME.
 execvp(argv[0], argv);
 err(EXIT_FAILURE, "Cannot execute %s", argv[0]);
}

Anything fishy? Bogus? Wrong? Anything that can be improved (besides adding options like -h)?

Answer 1

Besides reinventing the wheel the things I'm concerned about are:

• the warn in attemptLock, because if either creating the lock file fails, or if you can't get the lock (or the call was interrupted), I would expect the wrapper to exit instead,
• the return value of open, which might very well be negative, so just check for that separately and give an error message for that,
• the return value of flock, where you should handle EINTR in a loop I believe?

• Christian Hujer: Once again shows how good it is to have code reviewed. The warn instead of err was intentional because of support for a specific version of Cygwin which has issues with successfully locking, but there should at least have been a comment for explaining the unexpected warn instead of err. And the other two issues that you've found are real bugs. So, thanks a lot!