1.2.1 Typical Problems - Sendmail¶

Because sendmail was unable to use the old flock() emulation, many sendmail installations use fcntl() instead of flock(). This is true of Slackware 3.0 for example. This gave rise to some other subtle problems if sendmail was configured to rebuild the alias file. Sendmail tried to lock the aliases.dir file with fcntl() at the same time as the GDBM routines tried to lock this file with flock(). With pre 1.3.96 kernels this could result in deadlocks that, over time, or under a very heavy mail load, would eventually cause the kernel to lock solid with deadlocked processes.

1.2.2 The Solution¶

The solution I have chosen, after much experimentation and discussion, is to make flock() and fcntl() locks oblivious to each other. Both can exists, and neither will have any effect on the other.

I wanted the two lock styles to be cooperative, but there were so many race and deadlock conditions that the current solution was the only practical one. It puts us in the same position as, for example, SunOS 4.1.x and several other commercial Unices. The only OS’s that support cooperative flock()/fcntl() are those that emulate flock() using fcntl(), with all the problems that implies.