Home » On what systems is //foo/bar different from /foo/bar?

On what systems is //foo/bar different from /foo/bar?


This is a compilation and index of the answers given so far. This post is community wiki, it can be edited by anybody with 100+ reputation and nobody gets reputation from it. Feel free to post your own answer and add a link to it in here (or wait for me to do it). Ideally, this answer should just be a summary (with short entries while individual other answers would have the details).

Currently actively maintained systems:

  • Cygwin. A POSIX layer for Microsoft Windows. Used for Windows UNC paths.
  • UWIN since 1.3. Another POSIX layer for Windows. Used at least for //host/file network file sharing paths.
  • @OlivierDulac IBM z/OS as mentioned in the POSIX bug tracker, z/OS resolves //pathname requests to MVS datasets, not to network files. Example.

Defunct systems

  • @BinaryZebra Apollo Domain/OS (confirmed). Also mentioned at Official Description UNC (Universal Naming Convention) as the possible origin of //host/path notations (see also, page 2-15).

    According to Donn Terry, it was HP (which acquired Apollo Computers) that pushed for inclusion of that provision in the POSIX spec for Domain/OS.

  • @jillagre Tektronix Utek (corroborated), where //host/path is a path on a distributed file system.

  • @gilles QNX 4 with the FLEET distributed processing system, where //123/path is a /path on node 123. (Mentioned in the QNX 6 documentation.)

  • @roaima AT&T SysV Release 3 (unverified). //host/path in (discontinued in SVR4) RFS Remote File Sharing system.

  • @Scott SEL/Gould UTX-32 (unverified). Used for //host/path.

Applications that treat //foo/bar specially for paths

  • @Prem Perforce where //depot/A/B/C/D refers to a path in a depot.
  • @WChargin Blender. In its configuration you use a // prefix for relative paths (to the blend associated with the data-block).
  • The Bazel build system uses a // prefix for labels of targets within the Bazel build graph.

Do some applications running on Unix-likes —if not the system’s API— treat //foo/bar Paths specially?

I am aware of Perforce which uses //depot/A/B/C/D Paths to refer to the Depot.
Perforce also supports //Client/C/D Paths, when the Client is pointing to //depot/A/B/. Here, local FileSystem may not have these Paths.

p4 filelog //depot/A/B/C/D will show the history of that file, even though there is no file /depot/A/B/C/D.

p4 filelog C/D will also show the history of that file, if executed from appropriate Directory.

Reference : https://www.perforce.com/perforce/r12.1/manuals/cmdref/o.fspecs.html

Several decades ago, Tektronix Utek (BSD 4.2 based Unix, first on National Semiconductors 32016 CPUs then Motorola 68020s) was providing something called DFS (distributed file system) under which //foo/bar was referring to the /bar file on the foo dfs server. It was later obsoleted by Sun’s NFS.

Unfortunately, I haven’t reference yet to back that but I might eventually find some Utek documentation in my cellar and update this reply.

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