Re: Can't debug bash with gdb 8.2.1-1

[Jon Turney <jon dot turney at dronecode dot org dot uk>]

On 24/07/2019 15:42, Ken Brown wrote:
> On 7/24/2019 10:32 AM, Ken Brown wrote:
> > On 7/24/2019 8:33 AM, Ken Brown wrote:
> > > $ cat getpgrp_test.c
> > > #include <unistd.h>
> > > #include <stdio.h>
> > > #include <errno.h>
> > >
> > > int
> > > main ()
> > > {
> > >       pid_t pid = getpid ();
> > >       pid_t pgid = getpgid (0);
> > >       printf ("getpid () = %d, getpgid (0) = %d, errno = %d\n", pid, pgid, errno);
> > > }
> > >
> > > $ gcc -o getpgrp_test -Wall -g -O0 getpgrp_test.c
> > >
> > > $ ./getpgrp_test.exe
> > > getpid () = 1424, getpgid (0) = 1424, errno = 0
> > >
> > > $ gdb ./getpgrp_test.exe
> > > GNU gdb (GDB) (Cygwin 8.2.1-1) 8.2.1
> > > [...]
> > > (gdb) r
> > > Starting program: /home/kbrown/Documents/programming/misc_examples/getpgrp_test.exe
> > > [...]
> > > getpid () = 1427, getpgid (0) = -1, errno = 0
> > > [...]
> > > [Inferior 1 (process 23872) exited normally]
> > >
> > > So getpgid (0) returns -1 without setting errno.  I think this means that
> > > pinfo::init() didn't fail, but for some reason myself->pgid == -1.
> >
> > I just ran the above gdb session under strace and found several lines like this:
> >
> >       25 32828861 [main] gdb 1799 tcsetpgrp: 0 = tcsetpgrp(0, -1)
> >
> > I haven't looked at the gdb code, but doesn't this mean that gdb is trying to
> > set pgrp to -1?  If so, why?  And shouldn't tcsetpgrp return -1 with EINVAL?

Well spotted.

> I just looked at the gdb commit that Jon pointed to earlier in the thread.  It
> adds calls like this:
>
>     result = tcsetpgrp (0, getpgid (inf->pid));
>
> It then checks for errors and takes appropriate action.  But tcsetpgrp returns
> success.
>
> So this all seems to boil down to the fact that Cygwin's tcsetpgrp() doesn't
> check the validity of its second argument.

Great.  Thanks very much for looking into this.

I guess I need to look into gdb to see if it's trying to use -1 due to 
some other problem, or just expects that to be ignored.

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