Graceful termination of child threads when main thread ends

Question

On Solaris 10, I have a multi-threaded application, where I have concurrently running Posix threads. All child threads are created using pthread_create and as joinable using pthread_attr_setdetachstate(&attr,PTHREAD_CREATE_JOINABLE);. When main thread terminates, system appears to hang, as control in children threads want to resume but the threads may not be there. While running gdb, after main thread comes out it shows:

procfs: fetch_registers, get_gregs line 3497, /proc/16304/lwp/2: No such file or directory.

If I issue list command of gdb, it shows me some lines from the child threads, and info threads command is showing some thread is still there even when main thread seems to be exited. As reported in https://groups.google.com/forum/#!topic/comp.programming.threads/RKHLvBINYkE, could this be a bug within gdb or all is well? Or could it be because of joinable thread to do with this, may be it could be created as detached thread using pthread_attr_setdetachstate(&attr,PTHREAD_CREATE_DETACHED);? When I try pstack -F PID outside gdb to see who is doing what, I see the following:

-----------------  lwp# 1 / thread# 1  --------------------
001dfa70 main     (3c4170d0, 1, ffbff5f0, ffbff55c, 3c4170d0, 600c74) + ac8
0017f698 _start   (0, 0, 0, 0, 0, 0) + 5c
-----------------  lwp# 3 / thread# 3  --------------------
fcb4af80 lwp_park (0, 0, 0)
ff325208 sem_wait (3bcb8590, 3bcd4abc, 1, 0, fc2c0a40, 1) + 20
001de984 _Z19processForSRFReaderPv (0, fc0e0000, 0, 0, 1de968, 3bcd4a9c) + 1c
fcb4aee0 _lwp_start (0, 0, 0, 0, 0, 0)
-----------------  lwp# 4 / thread# 4  --------------------
fcb4af80 lwp_park (0, 0, 0)
ff325208 sem_wait (600a48, 0, fcbc7940, 0, fc2c1240, 19607c) + 20
00181d0c _Z14processForWorkPv (6009cc, fc0b0000, 0, 0, 181cc0, 3bcd4a9c) + 4c
fcb4aee0 _lwp_start (0, 0, 0, 0, 0, 0)
-----------------  lwp# 5 / thread# 5  --------------------
fcb4af80 lwp_park (0, 0, 0)
ff325208 sem_wait (600a78, 0, fcbc7940, 0, fc2c1a40, 0) + 20
00181d0c _Z14processForWorkPv (6009d8, fc080000, 0, 0, 181cc0, 3bcd4a9c) + 4c
fcb4aee0 _lwp_start (0, 0, 0, 0, 0, 0)
-----------------  lwp# 6 / thread# 6  --------------------
fcb4af80 lwp_park (0, 0, 0)
ff325208 sem_wait (600aa8, 0, fcbc7940, 0, fc2c2240, 19607c) + 20
00181d0c _Z14processForWorkPv (6009e4, fc050000, 0, 0, 181cc0, 3bcd4a9c) + 4c
fcb4aee0 _lwp_start (0, 0, 0, 0, 0, 0)