Re: gdb and muti threads with recvfrom

[Michael Snyder <msnyder at vmware dot com>]

Kari Nalli wrote:
> Hi
> I wore little program that some times (not all times may be 1 in 3)
> behaves different when run in gdb.
>
> here is output when run from console
> $ ./Threads
> Thread=4 errno=11
> Thread=3 errno=11
> Thread=2 errno=11
> Thread=1 errno=11
> Thread=4 errno=11
> Thread=3 errno=11
> Thread=2 errno=11
> Thread=1 errno=11
>
>
> and from gdb
>
> (gdb) run
> Starting program: /home/nallkar/tmp/gdb_test/Threads
> [Thread debugging using libthread_db enabled]
> [New Thread 0xb7f836d0 (LWP 28416)]
> [New Thread 0xb7f82b90 (LWP 28417)]
> [New Thread 0xb7581b90 (LWP 28418)]
> [New Thread 0xb6b80b90 (LWP 28419)]
> [New Thread 0xb617fb90 (LWP 28420)]
> Thread=1 errno=4
> Thread=1 errno=11
> Thread=2 errno=11
> Thread=3 errno=11
> Thread=4 errno=11
> Thread=1 errno=11
> Thread=2 errno=11
> Thread=3 errno=11
> Thread=4 errno=11
>
> gdb information:
>
> (gdb) show version
> GNU gdb 6.8
> Copyright (C) 2008 Free Software Foundation, Inc.
> License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
> This is free software: you are free to change and redistribute it.
> There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
> and "show warranty" for details.
> This GDB was configured as "i686-pc-linux-gnu".
>
> And system information
> $ ldd Threads
>         linux-gate.so.1 =>  (0x007f6000)
>         libstdc++.so.6 => /usr/lib/libstdc++.so.6 (0x00101000)
>         libpthread.so.0 => /lib/libpthread.so.0 (0x00b45000)
>         libc.so.6 => /lib/libc.so.6 (0x009d4000)
>         libm.so.6 => /lib/libm.so.6 (0x00b16000)
>         libgcc_s.so.1 => /lib/libgcc_s.so.1 (0x00dc4000)
>         /lib/ld-linux.so.2 (0x009b2000)
>
> OS is CentOs 5.x compiled with premtive kernel.
> 2.6.18-92.1.6.el5.preemptive_kernel_local #1 SMP PREEMPT Mon Aug 4
> 09:08:42 EEST 2008 i686 i686 i386 GNU/Linux
>
> And the intressting function where the prints are comming is recvfrom
> (no data is send to sockets so they should timeout)
> According to IEEE Std 1003.1, 2004 Edition After timeout Recvfrom
> should return with errno set to [EAGAIN] or [EWOULDBLOCK]. In my
> system related errno defines are
> #define EINTR            4      /* Interrupted system call */
> #define EAGAIN  11      /* Try again */
>
> can any one tell what causes the different behaviour?
>
> Br, Kari

Something to do with signals and threads.
I think signals in one thread are not supposed to interrupt
other threads, but I'm not sure (and it may depend on the OS).

Could be that one thread was waiting on a blocking system call,
and another thread took a SIGTRAP from a gdb breakpoint or
something...