A common socket programming mistake: not handling short IO
With normal file IO, when you do do 'read(fd, buf, len)
' you'll almost
always get back len
bytes unless you hit EOF or a disk IO error. This
breeds a certain sloppyness when filling buffers; an awful lot of code
effectively ignores the return value of read()
except to check it for
errors.
This can and will bite you on the rear when writing socket code, because networks only give you so much data at once. Short reads are routine for socket IO; you can't assume that you can get all of what you want in a single read.
The mistake is especially pernicious because the mistaken code almost always works. Usually the lines or transactions you're reading from the network are small; usually you test on a fast local network. Speaking from personal experience, it's easy to forget this and then not notice.
(Today's case was some of my code that assumed it could read all of
a HTTP POST
body in one read()
.)
Whether or not a normal (blocking) write()
has similar issues is
probably system dependent. Linux seems to only return from a socket
write()
once all data has been pushed out, but I don't know what
other systems do in practice.
(According to the Single Unix Specification page for
write()
,
in theory you can count on this behavior on any SuS compliant system.)
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