On 20 October 2010 04:17, Ken Brown <kbr...@cornell.edu> wrote:
> Emacs creates a subprocess that runs an interactive bash shell. Emacs wants
> to get the PGID of the foreground process group associated to the tty of
> this shell, and it does this on Linux via TIOCGPGRP (or equally well
> tcgetpgrp). I think it uses the file descriptor of the master of the pty
> for this purpose. If you (or some other programmer reading this) could give
> me the code for setting all this up, I could play with it and try to figure
> out why I'm seeing a difference between Linux and Cygwin here. I just don't
> know how to create a subprocess, give it a terminal, etc.
Here's a test along those lines that does show a difference between
Linux and Cygwin:
#include <stdio.h>
#include <pty.h>
int main(void)
{
int pid, fd;
pid = forkpty(&fd, 0, 0, 0);
if (!pid)
sleep(2);
else {
sleep(1);
printf("pid=%i fd=%i pgrp=%i\n", pid, fd, tcgetpgrp(fd));
}
}
On Linux, where it requires -lutil to link, this gives:
pid=13308 fd=3 tcgetpgrp(fd)=13308
On Cygwin:
pid=268 fd=3 tcgetpgrp(fd)=0
Neither of those looks POSIX-compliant to me, because tcgetpgrp should
return -1 since fd 3 isn't the controlling terminal of the calling
process, but the Linux behaviour is rather useful. Perhaps they
decided to apply that restriction only to the slave side?
Andy
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