[TUHS] the sin of buffering [offshoot of excise process from a pipeline]

Noel Chiappa jnc at mercury.lcs.mit.edu
Thu Jul 17 07:46:32 AEST 2014

    > From: Doug McIlroy <doug at cs.dartmouth.edu>

    > Process A spawns process B, which reads stdin with buffering. B gets
    > all it deserves from stdin and exits. What's left in the buffer,
    > intehded for A, is lost. 

Ah. Got it.

The problem is not with buffering as a generic approach, the problem is that
you're trying to use a buffering package intended for simple,
straight-forward situations in one which doesn't fall into that category! :-)

Clearly, either B has to i) be able to put back data which was not for it
('ungets' as a system call), or ii) not read the data that's not for it - but
that may be incompatible with the concept of buffering the input (depending
on the syntax, and thus the ability to predict the approaching of the data B
wants, the only way to avoid the need for ungetc() might be to read a byte at
a time).

If B and its upstream (U) are written together, that could be another way to
deal with it: if U knows where B's syntatical boundaries are, it can give it
advance warning, and B could then use a non-trivial buffering package to do
the right thing. E.g. if U emits 'records' with a header giving the record
length X, B could tell its buffering package 'don't read ahead more than X
bytes until I tell you to go ahead with the next record'.

Of course, that's not a general solution; it only works with prepared U's.
Really, the only general, efficient way to deal with that situation that I can
see is to add 'ungets' to the operating system...


More information about the TUHS mailing list