INFO: Passing a long from C to MASM by Value & Returning a long (104645)

SUMMARY

The sample code below demonstrates how to pass a long from a program written in Microsoft C to a procedure written with the Microsoft Macro Assembler (MASM). The MASM function also returns a long 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

long MasmSub (long, long);

#ifdef __cplusplus
}
#endif

main ()
   {
   long var1 = 98304, var2 = 147456;
   printf ("%ld + %ld = %ld", var1, var2, MasmSub (var1, var2));
   }

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, \ 
   lVar1:DWORD, \ 
   lVar2:DWORD

   mov ax, WORD PTR lVar1       ; Load the first long into DX:AX.
   mov dx, WORD PTR lVar1+2
   add ax, WORD PTR lVar2       ; Add the second long to DX:AX
   adc dx, WORD PTR lVar2+2
   ret              ; Because the function returns a long (a 4-byte
MasmSub ENDP        ; value), C will get the return value from DX:AX.
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, \ 
   lVar1:DWORD, \ 
   lVar2:DWORD

   mov eax, lVar1   ; Load the first long into EAX.
   add eax, lVar2   ; Add the second long to EAX.
   ret              ; Because the function returns a long (a 4-byte
MasmSub ENDP        ; value), C will get the return value from EAX.
END

The following is the output of the program:

   98304 + 147456 = 245760