Reading Unix manpages

July 25, 2006

One of the important skills for Unix programming is the ability to parse manpages carefully. This is not as easy as it looks, because manpages are traditionally written in a style where everything is important and you have to think carefully about the implications of the exact wording used.

(This can be useful for other things than Unix manpages, since a lot of specifications are written in the same style.)

For example, today I was emailed a comment on my Python socket module irritation entry pointing out the existence of the .makefile() method function, which:

Return a file object associated with the socket. [...] The file object references a dup()ped version of the socket file descriptor, so the file object and socket object may be closed or garbage-collected independently.

Thinking about how I would use this, one of the things I found myself wondering about was what would happen if you dup()ped a socket file descriptor and called shutdown() on only one of the file descriptors. (Bearing in mind that you have to close() all of the file descriptors for a socket before the socket goes away.)

So I consulted the manpage. The Linux shutdown(2) manpage contains the following description (emphasis mine):

The shutdown call causes all or part of a full-duplex connection on the socket associated with fd to be shut down.

(Similar wording appears in the Solaris and FreeBSD manual pages.)

Once I put on my spec reading hat, it was clear that saying 'the socket associated with fd' instead of something like 'the file descriptor fd' was important. Thus shutdown(2) is not like close() and has an immediate effect when called, no matter how many times the file descriptor has been dup()ped.

(And some quick Python later, I had confirmed this.)

Written on 25 July 2006.
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Last modified: Tue Jul 25 01:02:55 2006
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