Segmentation fault when wxThread::OnExit() is called

Question

Update 2: See the section after my code.

I'm using a thread for calculating PI using the GMP library, but somehow I'm now getting a segmentation fault when wxThread::OnExit() is internally in wxWidgets called.

Here is the line in wxWidgets source code: src/msw/thread.cpp#553

Here is the shortened code from my thread entry function:

while (i <= m_numIterations && !TestDestroy()) {
    mpf_div(result, perimeter, edgeCount);

    mpf_pow_ui(result, result, 2);
    mpf_ui_sub(result, 1, result);
    mpf_sqrt(result, result);
    mpf_div_ui(result, result, 2);

    mpf_sub(result, half, result);
    mpf_sqrt(result, result);
    mpf_mul_ui(result, result, 2);
    mpf_mul(result, result, edgeCount);

    mpf_set(perimeter, result);

    i++;
    mpf_mul_ui(edgeCount, edgeCount, 2);
}

// Free GMP variables we don't need anymore
mpf_clear(half);
mpf_clear(result);
mpf_clear(edgeCount);

// OUTPUT_DIGITS has a constant value, e.g. 12
char outputStr[OUTPUT_DIGITS];

mp_exp_t *expptr;

// If commented out, the error does not appear!
mpf_get_str(outputStr, expptr, 10, OUTPUT_DIGITS, perimeter);

Update 2: If I comment out the last line with mpf_get_str(), the error does not occur.
I also found a very old bug requests from 2003: http://gmplib.org/list-archives/gmp-discuss/2003-November/000888.html

Call stack from GCC Debugger:

#0 63AE80E9 wxThreadInternal::DoThreadOnExit(thread=0x2cfa978) (../../src/msw/thread.cpp:553)
#1 63B27ACF wxScopeGuardImpl1<void (*)(wxThread*) (../../include/wx/scopeguard.h:168)
#2 63B3F95B wxPrivate::OnScopeExit<wxScopeGuardImpl1<void (*)(wxThread*) (../../include/wx/scopeguard.h:67)
#3 63B27B36 wxScopeGuardImpl1<void (*)(wxThread*) (../../include/wx/scopeguard.h:166)
#4 63AE82FB wxThreadInternal::DoThreadStart(thread=0x2cfa978) (../../src/msw/thread.cpp:561)
#5 63AE83F2 wxThreadInternal::WinThreadStart(param=0x2cfa978) (../../src/msw/thread.cpp:602)
#6 75C4906A ui64tow() (C:\Windows\SysWOW64\msvcrt.dll:??)
#7 75C49147 msvcrt!iswalnum() (C:\Windows\SysWOW64\msvcrt.dll:??)
#8 76448543 UnregisterBadMemoryNotification() (C:\Windows\SysWOW64\kernel32.dll:??)
#9 00000000 0x02cfb178 in ??() (??:??)
#10 00000000    0x77e8ac69 in ??() (??:??)
#11 00000000    0x77e8ac3c in ??() (??:??)
#12 00000000    0x00000000 in ??() (??:??)

Answer 1

The exponent pointer (here expptr) has to be already initialized to an object.

This one-liner solves the problem:

expptr = new mp_exp_t();

// call mpf_get_str()

I also want to note that VZ. was right that GDB (GNU Debugger) can show the wrong line of source code in which the error should appear.

So do not only rely on that.