QBasicAtomicInt::ref() - what the hell is this?

[Septi]

Hi, my app suddenly crashed inside the QBasicAtomicInt::ref() function. I see it's because of some foolishly uninitialized pointers, so I should be able to remedy the situation shortly. So what happened and how to debug it is out of question. Rather, I'm baffled by the function itself:

inline bool QBasicAtomicInt::ref()
{
    unsigned char ret;
    asm volatile("lock\n"
                 "incl %0\n"
                 "setne %1"
                 : "=m" (_q_value), "=qm" (ret)
                 : "m" (_q_value)
                 : "memory");
    return ret != 0;
}

inline bool QBasicAtomicInt::ref()
{
    unsigned char ret;
    asm volatile("lock\n"
                 "incl %0\n"
                 "setne %1"
                 : "=m" (_q_value), "=qm" (ret)
                 : "m" (_q_value)
                 : "memory");
    return ret != 0;
}

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What is it supposed to do? How does it do it? How does asm keyword combine with volatile? Why are asm commands quoted? What are the colons ( : ) doing here? What could '"m" (_q_value)' possibly mean? Please, someone, explain the magic to me

[high_flyer]

I am not an asm junkie, but as far as I can tell, most (all?) your questions are answered here:
http://www.ibiblio.org/gferg/ldp/GCC...WTO.html#ss5.4

[tbscope]

It's an atomic reference counter.

[Septi]

Thanks for the answer, that's some interesting reading material. I was only familiar with Intel syntax before

[bothorsen]

When this happens, it usually means an object has been double deleted. Use a memory checker like valgrind to go over your code to find the place.