Spiking Thread

This is a Mac OS X / GDB counterpart to Spiking Thread pattern previously described for Windows platforms (Volume 1, page 305):

(gdb) info threads
4 0×00007fff85b542df in sqrt$fenv_access_off ()
3 0×00007fff8616ee42 in __semwait_signal ()
2 0×00007fff8616ee42 in __semwait_signal ()
* 1 0×00007fff8616ee42 in __semwait_signal ()

We notice a non-waiting thread and switch to it:

(gdb) thread 4
[Switching to thread 4 (core thread 3)]
0x00007fff85b542df in sqrt$fenv_access_off ()


(gdb) bt
#0 0x00007fff85b542df in sqrt$fenv_access_off ()
#1 0×000000010cc85dc9 in thread_three (arg=0×7fff6c884ac0)
#2 0×00007fff8fac68bf in _pthread_start ()
#3 0×00007fff8fac9b75 in thread_start ()

If we disassemble the return address for thread_three function called from SQRT call we see an infinite loop:

(gdb) disass 0x000000010cc85dc9
Dump of assembler code for function thread_three:
0x000000010cc85db0 <thread_three+0>:  push %rbp
0×000000010cc85db1 <thread_three+1>:  mov %rsp,%rbp
0×000000010cc85db4 <thread_three+4>:  sub $0×10,%rsp
0×000000010cc85db8 <thread_three+8>:  mov %rdi,-0×10(%rbp)
0×000000010cc85dbc <thread_three+12>: mov -0×10(%rbp),%ax
0×000000010cc85dc0 <thread_three+16>: movsd (%rax),%xmm0
0×000000010cc85dc4 <thread_three+20>: callq 0×10cc85eac <dyld_stub_sqrt>
0×000000010cc85dc9 <thread_three+25>: mov -0×10(%rbp),%rax
0×000000010cc85dcd <thread_three+29>: movsd %xmm0,(%rax)
0×000000010cc85dd1 <thread_three+33>: jmpq 0×10cc85dbc <thread_three+12>
End of assembler dump.

Here's the source code of the modeling application:

void * thread_one (void *arg)
{
        while (1)
        {
               sleep (1);
        }

        return 0;
}

void * thread_two (void *arg)
{
        while (1)
        {
               sleep (2);
        }

        return 0;
}

void * thread_three (void *arg)
{
        while (1)
        {
               *(double*)arg=sqrt(*(double *)arg);
        }

        return 0;
}

int main(int argc, const char * argv[])
{
        pthread_t threadID_one, threadID_two, threadID_three;
        double result = 0xffffffff;

        pthread_create (&threadID_one, NULL, thread_one, NULL);
        pthread_create (&threadID_two, NULL, thread_two, NULL);
        pthread_create (&threadID_three, NULL, thread_three,
        &result);
        pthread_join(threadID_three, NULL);

        return 0;
}