How to pass a char between a program written in C/C++ and a function written with Microsoft Macro Assembler (MASM)

SUMMARY

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

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

char MasmSub (char);

#ifdef __cplusplus
}
#endif

main ()
{
   char var = 'a';
   printf ("%c\n", var);
   printf ("%c", 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, \ 
   cVar:BYTE

   mov al, cVar     ; Load the char into AL.
   add al, 25       ; Because the function returns a char (a 1-byte
   ret              ; value), C will get the return value from AL.
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, \ 
   cVar:BYTE

   mov al, cVar     ; Load the char into AL.
   add al, 25       ; Because the function returns a char (a 1-byte
   ret              ; value), C will get the return value from AL.
MasmSub ENDP
END
				

The following is the output of the program:

  a
  z