runInteractiveProcess and buffering

Adam Langley agl at
Fri Oct 12 23:52:46 EDT 2007

On 10/12/07, Henning Thielemann <lemming at> wrote:
> The data of 'out' seems to be buffered, because 'aseqdump' must produce
> many lines until even the first one is returned by 'hGetLine' and printed
> by 'putStrLn'. I tried to disable buffering for 'out' and indeed
> 'NoBuffering' is printed to the terminal, but the output from 'aseqdump'
> is still deferred.

[I suspect this is a job for haskell-cafe@, anyway]

Are you sure that you aseqdump isn't buffering itself? The standard C
libraries will buffer output at the line level at least, and are free
to buffer much more than that if they think that the output is not to
a terminal.

Try building this code:

#include <stdio.h>
#include <unistd.h>

int main()
  printf("line 1\n");
  write(1, "line 2\n", 7);
  printf("line 3\n");

  return 0;

For me (glibc 2.6), if I send stdout to a terminal then it's line
buffered and I get the lines in the correct order. However, if I
redirect to a file, line 3 comes first because the printf lines are
buffered until the process dies.


Adam Langley                                      agl at                       650-283-9641

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