Re: SIGTSTP and select

[Bob Rossi <bob_rossi at cox dot net>]

On Fri, Apr 13, 2007 at 03:25:01PM +0200, Corinna Vinschen wrote:
> On Apr  3 10:13, Bob Rossi wrote:
> > On Mon, Apr 02, 2007 at 08:37:53PM -0400, Christopher Faylor wrote:
> > > On Mon, Apr 02, 2007 at 08:07:23PM -0400, Bob Rossi wrote:
> > > >When ctrl-z is typed, CGDB receives a SIGTSTP on both linux and cygwin.
> > > >When CGDB is at the select loop and this happens on linux select returns
> > > >-1 and errno is set to EINTR. My code simple does a 'continue' when this
> > > >happens and the select loop is reentered. All works well. On cygwin,
> > > >select does not return with -1. (I didn't check the return value but I
> > > >can, I just compare to -1 in an if statement). In fact, select also
> > > >detects that input is ready on stdin. This causes CGDB to get to a read
> > > >system call (which is non blocking) and the read system call fails with
> > > >errno set to EAGAIN. This causes CGDB to exit.
> > > >
> > > >The main loop looks something like this,
> > > >  if (select (max + 1, &rset, NULL, NULL, NULL) == -1)
> > > >    { 
> > > >      if (errno == EINTR)
> > > >        continue;
> > > >      ..
> > > >    }
> > > >
> > > >  if (FD_ISSET (STDIN_FILENO, &rset)) {
> > > >    handle_stdin
> > > >  }
> > > >
> > > >So, my question is, is there a bug with select on cygwin? Is select
> > > >working properly and I should handle the read call differently? Why does
> > > >it act differently than linux?
> > > 
> > > You say that something changed between different releases but you don't
> > > mention what those releases are.  Is this releases of Cygwin?  If so,
> > > what releases?
> > > 
> > > If this is as easy to demonstrate as you say, then a simple test case
> > > is definitely called for.
> > 
> > Hi Christopher,
> > 
> > Attached is the test case. If I run it under linux, and then type ctrl-z, I 
> > never get into the user_input_loop call. If I run it in cygwin, I do.
> > 
> > Hopefully I'm doing something wrong here. Please advise!
> 
> I'm a bit puzzled.  I don't see any difference in behaviour on Linux and
> Cygwin related to your testcase.  I have no problems to trigger the
> user_input_loop call on Linux and Cygwin.  After I press ctrl-z, I don't
> get into it on both systems.  After unsuspending the process, I get into
> user_input_loop on both systems again.  Either your testcase is wrong,
> or you should exactly specify what has to be typed to trigger the
> problem.  I tested this with Cygwin 1.5.24 and Linux 2.6.20.5, btw.

Hi Corinna,

Thanks for testing this! I definately do not get the same results as
you. On ubuntu linux,
  $ uname -a
  Linux black 2.6.17-11-386 #2 Thu Feb 1 19:50:13 UTC 2007 i686
  GNU/Linux

I'm running cygwin version 1.5.24 and have attached cygcheck.out.
I've modified the main program slightly to better show the problem.

On both platforms I do,
  gcc -g main.c -o main

On cygwin when I type './main' and then I type 'ctrl-z', I see this,
  $ ./main.exe
  Select return value:1
  In user_input_loop
On linux when I type './main' and then I type 'ctrl-z', I see this,
  $ ./main
  [1]+  Stopped                 ./main
  $ fg
  ./main

In fact, I never see the user_input_loop on linux.

What's interesting and annoying is that when I tested this last time on
linux, I was sure that after the SIGTSTP was sent, the select loop
returned. The value of val was -1 and errno was EINTR and I did a
continue to the loop again. However, with the example I just posted, it
appears that linux never breaks free of the select loop. I am still
seeing a difference between linux and cygwin as shown above.

I'm curious to know if this is a programming error on my part or if it
is a bug in the select call on cygwin.

Thanks!
Bob Rossi

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/select.h>
#include <sys/ioctl.h>
#include <errno.h>
#include <termios.h>

static struct termios term_attributes;

int 
tty_cbreak (int fd, struct termios *orig)
{
  struct termios buf;
   
  if (tcgetattr (fd, &buf) < 0)
    return -1;

  /* Save the original state, for resetting later */
  *orig = buf;
      
  buf.c_lflag &= ~(ECHO | ICANON);
  buf.c_iflag &= ~(ICRNL | INLCR);
  buf.c_cc[VMIN] = 1;
  buf.c_cc[VTIME] = 0;

#if defined (VLNEXT) && defined (_POSIX_VDISABLE)
  buf.c_cc[VLNEXT] = _POSIX_VDISABLE;
#endif

#if defined (VDSUSP) && defined (_POSIX_VDISABLE)
  buf.c_cc[VDSUSP] = _POSIX_VDISABLE;
#endif

  if (tcsetattr (fd, TCSAFLUSH, &buf) < 0)
    return -1;

  return 0;   
}

int 
tty_set_attributes (int fd, struct termios *buf)
{
  if (tcsetattr (fd, TCSAFLUSH, buf) < 0)
    return -1;
      
  return 0;   
}

int
user_input_loop ()
{
   fprintf (stderr, "In user_input_loop\r\n");
   return 0;
}

static int
main_loop (void)
{
  fd_set rset;
  int max = STDIN_FILENO;
  int val;

  for (;;)
  {
     /* Reset the fd_set, and watch for input from GDB or stdin */
     FD_ZERO (&rset);

     FD_SET (STDIN_FILENO, &rset);

     /* Wait for input */
     val =select (max + 1, &rset, NULL, NULL, NULL);
     fprintf (stderr, "Select return value:%d\r\n", val);
     if (val == -1)
     {
        if (errno == EINTR)
           continue;
        else
        {
           fprintf (stderr, __FILE__, __LINE__,
                 "select failed: %s", strerror (errno));
           return -1;
        }
     }

     /* Input received:  Handle it */
     if (FD_ISSET (STDIN_FILENO, &rset)) {
        if (user_input_loop () == -1)
           return -1;
        return 0;
     }
  }

  return 0;
}

int
main (int argc, char *argv[])
{
  if (tty_cbreak (STDIN_FILENO, &term_attributes) == -1)
    {
      fprintf (stderr, __FILE__, __LINE__, "tty_cbreak error");
      exit (-1);
    }

  /* Enter main loop */
  main_loop ();

  if (tty_set_attributes (STDIN_FILENO, &term_attributes) == -1)
    fprintf (stderr, __FILE__, __LINE__, "tty_reset error");

  return 0;
}

Attachment: cygcheck.out
Description: Text document

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