Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!uwvax!rocky.cs.wisc.edu!g-tookey
From: g-tookey@rocky.cs.wisc.edu. (Richard Schaut)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Calling C functions from assembly
Message-ID: <8096@spool.cs.wisc.edu>
Date: 3 Aug 89 18:34:50 GMT
References: <12324@s.ms.uky.edu> <216100115@trsvax>
Sender: news@spool.cs.wisc.edu
Reply-To: g-tookey@rocky.CS.WISC.EDU (Richard Schaut)
Organization: U of Wisconsin CS Dept
Lines: 42

In article <216100115@trsvax> don@trsvax.UUCP writes:
>
>>>I'm trying to call some C functions from assembly, using MSC 5.0 and either
>>>MASM or TASM.  I could write a dummy _main() in C, if this makes things
>>>easier, but I'm still not sure what needs to be setup to call a C subroutine.
>>>
>>>Can anyone clue me in on how to do this?
>
>>[...]
>>Note that the parameters are pushed onto the stack in reverse order.  As
>>for how to retrieve the functions return value, you'l have to consult
>>you compilers documentation.  There is no set convention, however most
>			       ^^^^^^^^^^^^^^^^^^^^^^^^^^
>>use one of the CPU registers. 
>
>Is this correct?  I thought it was a well-established convention to place
>the return value in the AX register.  Are there some commercial compilers
>which return it somewhere else?

Most reputable compilers use AX for 16 bit values and AX DX for 32 bit
values, but the off-brand compilers may use something else.  Also, once
the size of the return value gets larger than 32 bits, then all bets
are off (I've even seen some compilers use the stack, e.g. MIX C
under CP/M).  The safest thing is to not make any assumptions
whatsoever about how functions return values for a given compiler.
If the compiler's documentation is silent on the subject, then
it's best to drop the whole thing in /dev/nul and get a real
compiler.


Rick
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