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    <p>There is no instruction to directly read the instruction pointer (EIP) on x86. You can get the address of the current instruction being assembled with a little inline assembly:</p> <pre><code>// GCC inline assembler; for MSVC, syntax is different uint32_t eip; __asm__ __volatile__("movl $., %0", : "=r"(eip)); </code></pre> <p>The <code>.</code> assembler directive gets replaced with the address of the current instruction by the assembler. Note that if you wrap the above snippet in a function call, you'll just get the same address (within that function) every time. If you want a more usable C function, you can instead use some non-inline assembly:</p> <pre><code>// In a C header file: uint32_t get_eip(void); // In a separate assembly (.S) file: .globl _get_eip _get_eip: mov 0(%esp), %eax ret </code></pre> <p>This means each time you want to get the instruction pointer, it's slightly less efficient since you need an extra function call. Note that doing it this way does not blow the return address stack (RAS). The return address stack is a separate stack of return addresses used internally by the processor to facilitate <a href="http://en.wikipedia.org/wiki/Branch_target_predictor" rel="nofollow noreferrer">branch target prediction</a> for RET instructions.</p> <p>Every time you have a CALL instruction, the current EIP gets pushed onto the RAS, and every time you have a RET instruction, the RAS is popped, and the top value is used as the branch target prediction for that instruction. If you mess up the RAS (such as by not matching each CALL with a RET, as in <a href="https://stackoverflow.com/questions/599968/reading-program-counter-directly/599982#599982">Cody's solution</a>), you're going to get a whole bunch of unnecessary branch mispredictions, slowing your program down. This method does not blow the RAS, since it has a matched pair of CALL and RET instructions.</p>
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    1. POReading program counter directly
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    1. USAdam Rosenfield
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    1. COThanks, you guys rock :)
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      1. USLiran Orevi
    2. COMany thanks for the info, I didn't know there were two stacks.. :)
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      1. USLiran Orevi
    3. COThe RAS is an internal stack used by the processor; it's not accessible to code in any way. It's only used for branch target prediction. Without it, code would still function correctly, just more slowly.
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