HOWTO: Pass a short from C to MASM by Value & Returning short
ID: Q104617
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The information in this article applies to:
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Microsoft C for MS-DOS, versions 5.1, 5.1a, 6.0, 6.0a, 6.0ax
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Microsoft C for OS/2, versions 5.1, 6.0, 6.0a
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Microsoft C/C++ for MS-DOS, version 7.0
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Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5
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Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 4.0, 5.0, 6.0
SUMMARY
The sample code below demonstrates how to pass a short from a program
written in Microsoft C to a procedure written with the Microsoft Macro
Assembler (MASM). The MASM function also returns a short to the C
program.
Registers are used to return values of simple data types. For 16-bit
code, such as an MS-DOS program, use the following conventions for
returning data to a C program:
char AL
short, int, near * AX
long, far * DX: High order portion (segment)
AX: Low order portion (offset)
For 32-bit code, such as a Windows NT program, use the following
conventions for returning data to a C program:
char AL
short AX
long, int, * EAX
MORE INFORMATION
The samples below include one C file and two different assembly files.
The two assembly files demonstrate how to pass a variable in small
model for MS-DOS and in flat model for Windows NT. Link only the
appropriate assembly module to the C module.
Note that MASM 6.1 or later and the C/C++ 32-bit compiler that ships
with Visual C++, 32-bit Edition, are required to build the flat model
Windows NT version.
Sample Code
// Filename: CMAIN.C
// Compile options needed: /c
#include <stdio.h>
#ifdef __cplusplus
extern "C" {
#endif
short MasmSub(short);
#ifdef __cplusplus
}
#endif
main ()
{
short var = 1;
printf ("%d\n", var);
printf ("%d", MasmSub(var));
}
Sample Code for MS-DOS Small Model Version
; Filename: MASMSUB.ASM
; Assemble options needed for MASM: /MX
; Assemble options needed for ML: /c /Cx
.MODEL small, C
.286
.CODE
MasmSub PROC, \
sVar:WORD
mov ax, sVar ; Load the short into AX.
add ax, 32766 ; Because the function returns a short (a 2-byte
ret ; value), C will get the return value from AX.
MasmSub ENDP
END
Sample Code for Windows NT Flat Model Version
; Filename: MASMSUB.ASM
; Assemble options needed for ML: /c /Cx /coff
.386
.MODEL flat, C
.CODE
MasmSub PROC, \
sVar:WORD
mov ax, sVar ; Load the short into AX.
add ax, 32766 ; Because the function returns a short (a 2-byte
ret ; value), C will get the return value from AX.
MasmSub ENDP
END
The following is the output of the program:
1
32767
Additional query words:
mixed language
Keywords : kbcode kbLangC kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600 MASMLngIss
Version : MS-DOS:6.0,6.0a,6.0ax,7.0; OS/2:6.0,6.0a; WINDOWS:1.0,1.5; WINDOWS NT:1.0,2.0,4.0,5.0
Platform : MS-DOS NT OS/2 WINDOWS
Issue type : kbhowto