Gdb hangs detaching from a threaded program

[Bruce Karsh]

When I attach to a threaded program, run it for a while, and then detach or
quit from it, gdb hangs.  I.e., it cannot be killed with ctl-c and it
doesn't offer a command prompt.

Here's an example session:

gdb testgdb 8974
GNU gdb 4.18
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welcome to change it and/or distribute copies of it under certain
conditions.
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There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "sparc-sun-solaris2.6"...
(no debugging symbols found)...

/home/bkarsh/TESTGDB/8974: No such file or directory.
Attaching to program `/home/bkarsh/TESTGDB/testgdb', process 8974
Reading symbols from /usr/lib/libpthread.so.1...(no debugging symbols
found)...
done.
Reading symbols from /usr/lib/libc.so.1...(no debugging symbols
found)...done.
Reading symbols from /usr/lib/libdl.so.1...(no debugging symbols
found)...done.
Reading symbols from /usr/platform/SUNW,Ultra-60/lib/libc_psr.so.1...
(no debugging symbols found)...done.
Reading symbols from /usr/lib/libthread.so.1...(no debugging symbols
found)...
done.
sol-thread active.
Symbols already loaded for /usr/lib/libpthread.so.1
Symbols already loaded for /usr/lib/libc.so.1
Symbols already loaded for /usr/lib/libdl.so.1
Symbols already loaded for /usr/platform/SUNW,Ultra-60/lib/libc_psr.so.1
Symbols already loaded for /usr/lib/libthread.so.1
0xff317088 in door_restart () from /usr/lib/libc.so.1
(gdb) c
Continuing.
^C[New Thread 4 (LWP 4)]
[Switching to Thread 4 (LWP 4)]

Program received signal SIGINT, Interrupt.
0xff31653c in _libc_nanosleep () from /usr/lib/libc.so.1
(gdb) detach

<At this point, it is hung>

This is on a Solaris 5.7 system:

SunOS ec007 5.7 Generic_106541-08 sun4u sparc SUNW,Ultra-60

Here's a test program that hangs when debugged:

#include <stdio.h>
#include <pthread.h>

#define NTIDS 7

void *start_func(void *arg);

main()
{
    pthread_attr_t attr;
    pthread_t      tid[NTIDS];
    void           *arg;
    int            i;

    setbuf(stdout,0);
    pthread_attr_init(&attr);
    pthread_attr_setscope(&attr, PTHREAD_SCOPE_SYSTEM);
    for(i=0;i<NTIDS;i++) {
        arg=(void *)(100+i);
        pthread_create(&tid[i], &attr, start_func, arg);
    }
    for(i=0;i<NTIDS;i++) {
        pthread_join(tid[i], &arg);
    }
}

void *
start_func(void *arg)
{
    int num;
    num = (int) arg;
    while(1) {
        printf("num=%d\n",num);
        sleep(1);
    }
    pthread_exit(arg);
}

Here's the Makefile:

testgdb:        testgdb.c
        cc -lpthread testgdb.c -o testgdb
clean:
        rm testgdb