error: invalid 'asm': operand number missing after %-letter when using inline assembly with GCC

Question

I\'m tring to convert a simple assembly code of MS to use with gcc, the MS assembly I try to convert is right below. I have two int variables, number

Answer 1

You can't do it like that because you're overwriting registers without telling the compiler about it. Also, the % is a special character, similar to printf.

It's also better to put all the instructions in one asm or else the compiler might do something unexpected in between.

Try this instead:

asm("movl %%eax, %1\n\t"
    "neg %%eax\n\t"
    "movl %0, %%eax" : "=g" ( _return )  : "g" ( number) : "eax");

There's probably a better way, though:

asm("neg %0": "=a" ( _return )  : "a" ( number));

I don't know why you can't just do (in C):

_return = -number;