marc@Apple.COM (Mark Dawson) (07/25/90)
I wrote a routine in assembly that I wanted to call from my C++ code, it
didn't end up working the way I expected--the C++ call kept passing a long,
when my assembly was expecting a short (2 bytes). I've of course modifed
my assembly to expect a long, but I'd like to know where my thinking went
wrong.
extern "C" void myProc(short x); // external declaration
....
short vRefNum = -2;
myProc(vRefNum); // I also tried myProc((short)vRefNum) and got the same res
...
--------
myProc PROC EXPORT
....
move.l (sp)+,A0 // pop return @ off stack
move.w (sp)+,D0 // get vRefNum off stack
....
ENDP
--------
When I did a debugger trap in myProc(), the stack always had an $FFFF FFFE,
not the $FFFE that I expected--it seemed to be passing 4 bytes instead of two.
Why?
Thanks,
Mark
--
---------------------------------
Mark Dawson Service Diagnostic Engineering
AppleLink: Dawson.M
Apple says what it says; I say what I say. We're different
---------------------------------chi@tybalt.caltech.edu (Curt Hagenlocher) (07/25/90)
In article <43315@apple.Apple.COM> marc@Apple.COM (Mark Dawson) writes: >I wrote a routine in assembly that I wanted to call from my C++ code, it >didn't end up working the way I expected--the C++ call kept passing a long, >when my assembly was expecting a short (2 bytes). I've of course modifed >my assembly to expect a long, but I'd like to know where my thinking went >wrong. > >(code deleted) > >When I did a debugger trap in myProc(), the stack always had an $FFFF FFFE, >not the $FFFE that I expected--it seemed to be passing 4 bytes instead of two. >Why? > MPW C always converts everything to 32 bits that's less than 32 bits in length. This includes characters and shorts. The information is put into the low (byte/word) and sign extended to make a long integer. I imagine this is done so that everything stays longword aligned. This information can be found in the chapter of the MPW C manual entitled ``Calling Conventions.'' I believe it is Appendix B. --- chi@tybalt.caltech.edu Curt Hagenlocher
beard@ux5.lbl.gov (Patrick C Beard) (07/26/90)
In article <1990Jul24.235007.20545@laguna.ccsf.caltech.edu> chi@tybalt.caltech.edu (Curt Hagenlocher) writes: #In article <43315@apple.Apple.COM> marc@Apple.COM (Mark Dawson) writes: #>I wrote a routine in assembly that I wanted to call from my C++ code, it #>didn't end up working the way I expected--the C++ call kept passing a long, #>when my assembly was expecting a short (2 bytes). I've of course modifed #>my assembly to expect a long, but I'd like to know where my thinking went #>wrong. #> # #MPW C always converts everything to 32 bits that's less than #32 bits in length. This includes characters and shorts. #The information is put into the low (byte/word) and sign extended #to make a long integer. This is correct. For all routines that use C calling conventions, all types are pushed as 32-bit quantities. One way to use smaller size parameters is to declare a routine with "pascal" calling conventions. So, if you really only want to push a word, declare your routine as: extern pascal void MyRoutine(short arg); And C++, or C will only push a 2-byte integer on the stack. -- ------------------------------------------------------------------------------- - Patrick Beard, Macintosh Programmer (beard@lbl.gov) - - Berkeley Systems, Inc. ".......<dead air>.......Good day!" - Paul Harvey - -------------------------------------------------------------------------------