1.2.1 Typical Problems - Sendmail¶

Because sendmail was unable to use the oldflock() emulation, many sendmailinstallations use fcntl() instead offlock(). This is true of Slackware 3.0for example. This gave rise to some other subtle problems if sendmail wasconfigured to rebuild the alias file. Sendmail tried to lock the aliases.dirfile with fcntl() at the same time as the GDBM routines tried to lock thisfile withflock(). 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 kernelto lock solid with deadlocked processes.

1.2.2 The Solution¶

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

I wanted the two lock styles to be cooperative, but there were so manyrace and deadlock conditions that the current solution was the onlypractical one. It puts us in the same position as, for example, SunOS4.1.x and several other commercial Unices. The only OS’s that supportcooperativeflock()/fcntl() are those that emulateflock() usingfcntl(), with all the problems that implies.