Two or more consecutive slashes in a path element following a directory d is replaced by all subdirectories of d: first those subdirectories directly under d, then the subsubdirectories under those, and so on. At each level, the order in which the directories are searched is unspecified. (It's "directory order", and definitely not alphabetical.)
If you specify any filename components after the `//', only subdirectories which match those components are included. For example, `/a//b' would expand into directories `/a/1/b', `/a/2/b', `/a/1/1/b', and so on, but not `/a/b/c' or `/a/1'.
You can include multiple `//' constructs in the path.
`//' at the beginning of a path is ignored; you didn't really want to search every directory on the system, did you?
I should mention one related implementation trick, which I took from GNU find. Matthew Farwell suggested it, and David MacKenzie implemented it.
The trick is that in every real Unix implementation (as opposed to the
POSIX specification), a directory which contains no subdirectories will
have exactly two links (namely, one for `.' and one for `..').
That is to say, the st_nlink field in the `stat' structure
will be two. Thus, we don't have to stat everything in the bottom-level
(leaf) directories--we can just check st_nlink, notice it's two,
and do no more work.
But if you have a directory that contains a single subdirectory and 500
regular files, st_nlink will be 3, and Kpathsea has to stat every
one of those 501 entries. Therein lies slowness.
You can disable the trick by undefining UNIX_ST_LINK in
`kpathsea/config.h'. (It is undefined by default except under Unix.)
Unfortunately, in some cases files in leaf directories are
stat'd: if the path specification is, say,
`$TEXMF/fonts//pk//', then files in a subdirectory
`.../pk', even if it is a leaf, are checked. The reason cannot
be explained without reference to the implementation, so read
`kpathsea/elt-dirs.c' (search for `may descend') if you are
curious. And if you can find a way to solve the problem, please
let me know.
Subdirectory expansion is implemented in the source file `kpathsea/elt-dirs.c'.
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