Getting a return variable.. (mixing C and asm)

[earlz]

Ok, maybe this should go in general programming, but it is for my kernel.. lol

ok, how exactly do you get the return value of a C function from assembly?

yes.. I know it seems very noobish.. but I can't seem to find the answer.. I've only seen stuff about passing parameters, and nothing more... and I tried experimenting with pushing and popping different things and I can't seem to get it..

[JohnnyTheDon]

Usually RAX/EAX/AX/AL.

[earlz]

Usually RAX/EAX/AX/AL.

Thanks! that's the trick.. I always thought C intended to be more portable than that though...

[JohnnyTheDon]

Well the calling convention will always depend on the platform. For example, the C calling convention on x86-64 uses registers to hold parameters.

[quok]

Ok, maybe this should go in general programming, but it is for my kernel.. lol

Moved to General Programming.

[pcmattman]

I always thought C intended to be more portable than that though...

It is - the C compiler hides the machine-specific details from you.

[earlz]

I always thought C intended to be more portable than that though...

It is - the C compiler hides the machine-specific details from you.

Well, I always thought it an agreed upon unspoken standard that parameters are passed on the stack.. though I suppose a compiler could do just about anything it wants as long as it compiles C code and it works

[pcmattman]

Well, I always thought it an agreed upon unspoken standard that parameters are passed on the stack

That's not so on some RISC architectures.

That's the beauty of C - it still works on practically every architecture because the compiler actually creates the assembly code for the target architecture for you, following the relevant ABIs and other standards.

[AJ]

Hi,

You may be interested in Googling the System V ABI, particularly if you want to mix a lot of C/C++/ASM.

Cheers,
Adam

[Troy Martin]

(inappropriate remark removed - Combuster)