What is the higher-language compatible way to return a value from a lower level asm function. I've been pushing arguments onto the stack, then subtracting 4 from esp to leave room for a return value or pointer, then calling the subroutine and setting up a stack frame. Then, after the function cleans up returns, the first thing I pop off the stack is the return value... This has just been my convention for the longest time, but it occurs to me that that is probably not the standard. What is? Help appreciated,
Brodeur235
The convention is to return the function value in the EAX register. You should make sure your stack is balanced on procedure exit. With a MASM stack frame this is automatic, if you write your own proc without a stack frame with STDCALL, usually you use RET BYTECOUNT where BYTECOUNT = the number of bytes pushed by the caller. With C calling convention the caller adjusts the stack after the call has returned.
For the return value under 32-bit Windows the standard is basically to return 32-bit integers in EAX, 64-bit integers in the EDX:EAX register pair, floating-point values at the top of the FPU stack in ST(0), and a pointer in EAX for anything else. See calling_conventions.pdf, available here:
http://www.agner.org/optimize/
EAX for 32 bit and EDX:EAX for 64 bit, thanks you guys are always awesome help.
Quote
With C calling convention the caller adjusts the stack after the call has returned.
I prefer this way to allow for a variable number of arguments, like printf. If you use the ret command with some static integer that will always be added to the stack, you cannot adjust for a variable number of parameters.. I think, haha.
Thank you,
Brodeur235
You might look at VARARG in STDCALL mode (http://www.masm32.com/board/index.php?topic=13727.0)
brodeur235,
Your code also becomes larger if the function is called more than once. The cleanup code has to be repeated over and over instead of just once. Probably not a big deal but it adds up if the function is called many times.
QuoteI prefer this way to allow for a variable number of arguments, like printf.
you could also pass a pointer to a structure
structures can have a variable number of arguments, as well as variable-length data types (like strings)