Assembly code generates SEGMENTATION FAULT

Question

I have the following function:

int max(int num1, int num2)
{
    int retval;
    __asm__("maxii_start:;"
        "movl %1, %%eax;" 
        "subl $0, %%eax;" 
        "movl %2, %%edx;"
        "subl $0, %%edx;"
        "cmpl %%edx, %%eax;"
        "jle maxii0;"
        "addl $0, %%eax;"
        "jmp maxii1;"
        "maxii0:;"
        "movl %%edx, %%eax;"
        "maxii1:;"
        "movl %%eax, %0;"
        "ret;"
        :"=r"(retval)
        :"r"(num1), "r"(num2)
        );
    return retval;
}

When I use it, eg. like int m = max(5, 10); I see Segmentation fault (core dumped).

I think that there's sth incorrect with the way I use cmpl. What's wrong with this function?

Don't use inline assembly

Michael Petch
– Michael Petch

2019-03-05 13:15:03 +00:00
Commented Mar 5, 2019 at 13:15 — Michael Petch
– Michael Petch, Commented Mar 5, 2019 at 13:15

prl · Accepted Answer · 2019-03-05 12:50:20Z

2

You need to let the compiler know what registers you are changing in the assembly code (called “clobbers”).

Before the closing parenthesis of the asm, add

: “eax”, “edx”

Actually it would be better to let the compiler choose the registers to use, but that would require rewriting the whole thing.

Don’t put the “ret” in the asm statement. You need to let the compiler generate the function epilogue.

answered Mar 5, 2019 at 12:50

prl

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