UNIX domain sockets

nospam@example.com (Daniel Fischer) — Wed, 19 Dec 2007 12:05:00 +0100

Unlike sockets in other domains, sockets in the UNIX domain are visible in the file system. Because of this, they're sometimes confused with regular files. When they're not being confused with regular files, their specific restrictions that don't apply to other files are still easily forgotten. One such restriction of a UNIX domain socket is the length of its name.

Path names can be rather long on UNIX-like systems these days. Gone are the days of file names limited to 14 characters and for path names, POSIX-compliant operating systems generally support up to 256 characters. On many platforms, path names can be even longer than that. For example, PATH_MAX is 1024 on Mac OS X and 4096 on GNU/Linux.

In contrast, the full path name of a UNIX domain socket must fit into a struct sockaddr_un. Its component sun_path generally has much less room for the socket's name than 1024 characters. Typical numbers are 108 (Linux, Solaris, Cygwin), 104 (AIX, BSDs, Mac OS X), and 92 (HP-UX). At some point, it was only 14 in Interix 5.2.

This means that, while UNIX domain sockets do appear as if they were files, they can't be placed in an arbitrary location in the file system. Now, imagine an automated testing process that can test multiple instances of the software at a time, and keeps all files relevant to one test run within one directory. Path names can easily become longer than 92 characters in a scenario like this. In one case, this happened in a system where the name of one such instance's directory didn't have a constant length and occasionally brought the complete path to more than 92 characters, causing random total failures on HP-UX.

Portability Blog (Entries tagged as ipc)

UNIX domain sockets