Ctrl-C forwarded to command executed via popen on Unix

[Eric Brunel]

Hi all,

We've just noticed a very strange problem regarding the use of the Ctrl-C 
key in the Python interpreter when a command is being run through a pipe on 
Unix: the interpreter gets it, raising a KeyboardInterrupt exception as it 
should, but the running command also seems to get the interrupt!

Here is a small example involving a small interrupt handling C program:

--- handleCtrlC.c ----------------------------
#include <stdio.h>
#include <signal.h>
#include <unistd.h>

void interruptHandler(int ignored)
{
  printf("Ctrl-C caught!\n");
  fflush(stdout);
}

int main()
{
  signal(SIGINT, interruptHandler);
  while ( 1 )
  {
    printf("Tick\n");
    fflush(stdout);
    sleep(1);
  }
}
-----------------------------------------------

I compiled this program to handleCtrlC, then ran the Python interpreter and 
did:

>>> import os
>>> p = os.popen('./handleCtrlC')
>>> p.readline()
'Tick\n'

After a few readline's, I press Ctrl-C at the prompt; the interpreter 
answers as expected:

>>>
KeyboardInterrupt

Then I continue the readline's: they answer 'Tick\n' a few times, then 
'Ctrl-C caught!\n', which seems to indicate that the signal went along the 
pipe to the executed program?! [NB: if you try this, you may have to kill 
the interpreter, as the pipe won't close since there's always input on 
it...]

I tried this with Python 2.1 on Mandrake Linux 8.0 and Solaris 7, and the 
result is the same. Is it a feature or a bug? If it's a feature, what is 
the reason behind it? Or maybe it is the standard behaviour of popen?

TIA
-- 
- Eric Brunel <eric.brunel at pragmadev.com> -
PragmaDev : Real Time Software Development Tools - http://www.pragmadev.com