With pdns-2.9.22-3.el5, sending SIGSTOP and SIGCONT to a running pdns_server (for example, by launching it from the shell, pressing Control-Z, and typing fg) reliably causes it to immediately segfault:


# yum -y install pdns
# echo 'launch=bind' >> /etc/pdns/pdns.conf
# pdns_server
Nov 12 22:23:12 Reading random entropy from '/dev/urandom'
Nov 12 22:23:12 This is a standalone pdns
Nov 12 22:23:12 Listening on controlsocket in '/var/run/pdns.controlsocket'
Nov 12 22:23:12 It is advised to bind to explicit addresses with the --local-address option
Nov 12 22:23:12 UDP server bound to 0.0.0.0:53
Nov 12 22:23:12 TCP server bound to 0.0.0.0:53
Nov 12 22:23:12 PowerDNS 2.9.22 (C) 2001-2009 PowerDNS.COM BV (Jan 26 2009, 12:01:27, gcc 4.1.2 20080704 (Red Hat 4.1.2-44)) starting up
Nov 12 22:23:12 PowerDNS comes with ABSOLUTELY NO WARRANTY. This is free software, and you are welcome to redistribute it according to the terms of the GPL version 2.
Nov 12 22:23:12 Creating backend connection for TCP
Nov 12 22:23:12 About to create 3 backend threads for UDP
Nov 12 22:23:12 Done launching threads, ready to distribute questions
^Z
[1]+  Stopped                 pdns_server
# fg
pdns_server
Nov 12 22:23:13 Got a signal 11, attempting to print trace: 
Nov 12 22:23:13 pdns_server [0x485ce6]
Nov 12 22:23:13 /lib64/libc.so.6 [0x2b0ec242e2d0]
Nov 12 22:23:13 pdns_server(_ZN11DistributorI9DNSPacketS0_13PacketHandlerE10makeThreadEPv+0x12c) [0x4c788c]
Nov 12 22:23:13 /lib64/libpthread.so.0 [0x2b0ec21e873d]
Nov 12 22:23:13 /lib64/libc.so.6(clone+0x6d) [0x2b0ec24d1f6d]
Aborted


Attempting to run pdns_server under gdb also causes it to immediately segfault (probably because SIGSTOP is involved in the implementation of ptrace):


(gdb) run
Starting program: /usr/sbin/pdns_server 
[Thread debugging using libthread_db enabled]
Nov 12 22:43:14 Reading random entropy from '/dev/urandom'
Nov 12 22:43:14 This is a standalone pdns
Nov 12 22:43:14 Listening on controlsocket in '/var/run/pdns.controlsocket'
Nov 12 22:43:14 It is advised to bind to explicit addresses with the --local-address option
Nov 12 22:43:14 UDP server bound to 0.0.0.0:53
Nov 12 22:43:14 TCP server bound to 0.0.0.0:53
Nov 12 22:43:14 PowerDNS 2.9.22 (C) 2001-2009 PowerDNS.COM BV (Jan 26 2009, 12:01:27, gcc 4.1.2 20080704 (Red Hat 4.1.2-44)) starting up
Nov 12 22:43:14 PowerDNS comes with ABSOLUTELY NO WARRANTY. This is free software, and you are welcome to redistribute it according to the terms of the GPL version 2.
[New Thread 0x40a00940 (LWP 2335)]
Nov 12 22:43:14 Creating backend connection for TCP
[New Thread 0x41401940 (LWP 2336)]
Nov 12 22:43:14 About to create 3 backend threads for UDP
[New Thread 0x41e02940 (LWP 2337)]
[New Thread 0x42803940 (LWP 2338)]

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x41e02940 (LWP 2337)]
0x00002aaaab197672 in __gnu_cxx::__exchange_and_add (__mem=0xfffffffffffffff8, 
    __val=-1) at atomicity.cc:41
41				  : "0" (__val), "m" (*__mem));
(gdb) bt full
#0  0x00002aaaab197672 in __gnu_cxx::__exchange_and_add (
    __mem=0xfffffffffffffff8, __val=-1) at atomicity.cc:41
        __result = -1
#1  0x00000000004c7c1e in Distributor<DNSPacket, DNSPacket, PacketHandler>::makeThread (p=0x791830) at /usr/include/c++/4.1.2/bits/basic_string.h:232
        b = 0x792150
        qcount = 0
        queuetimeout = 1500
#2  0x00002aaaab87873d in start_thread (arg=<value optimized out>)
    at pthread_create.c:301
        __res = <value optimized out>
        pd = 0x41e02940
        unwind_buf = {cancel_jmp_buf = {{jmp_buf = {1105209664, 
                197107770085693007, 10485760, 3, 1105211392, 4096, 
                197107769030834047, 197154333293990220}, mask_was_saved = 0}}, 
          priv = {pad = {0x0, 0x0, 0x0, 0x0}, data = {prev = 0x0, 
              cleanup = 0x0, canceltype = 0}}}
        not_first_call = 0
        robust = <value optimized out>
#3  0x00002aaaabb61f6d in clone () from /lib64/libc.so.6
No symbol table info available.