Return value of a C function to ASM
I'm trying to call a function from within ASM. I know how to call it, but i'm having trouble finding how to get the return val开发者_StackOverflowue of this function. An example follows:
C code:
int dummy() {
return 5;
}
(N)ASM code:
dummyFunction:
call dummy
;grab return into eax
inc eax ; eax should be 6 now
ret
Any ideas?
The return value is in eax
. If you've called a C function from asm, you can read the return value from eax
. If you're trying to return from an asm function to C, store the intended return value in eax
.
Things get a little bit more complicated for returning floating point values, long long
values, or structures, so ask if you need that and someone (maybe me) will help you.
Although the answers are sufficient to answer the OP's question, here's an extract covering most cases from DJGPP's manpage for completeness:
Return Value
- Integers (of any size up to 32 bits) and pointers are returned in the
%eax
register.- Floating point values are returned in the 387 top-of-stack register,
st(0)
.- Return values of type
long long int
are returned in%edx:%eax
(the most significant word in%edx
and the least significant in%eax
).- Returning a structure is complicated and rarely useful; try to avoid it. (Note that this is different from returning a pointer to a structure.)
If your function returns
void
(e.g. no value), the contents of these registers are not used.
It depends on the platform and the calling convention, but usually, the return value should already be returned in eax
if it's a primitive type or pointer and in the floating point register st(0)
if it's a floating point type, I think.
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