HOWTO: Pass a char from C to MASM by Value & Returning a char

• Microsoft C for OS/2, versions 5.1, 6.0, 6.0a
• Microsoft C for MS-DOS, versions 5.1, 5.1a, 6.0, 6.0a, 6.0ax
• Microsoft C/C++ for MS-DOS, version 7.0
• Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5
• Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0, 5.0, 6.0

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) 

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

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 

  a
  z 

Additional query words:

Keywords : kbcode kbLangC kbVC100 kbVC150 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600 MASMLngIss
Version : MS-DOS:6.0,6.00a,6.00ax,7.0;WINDOWS:1.0,1.5;WINDOWS NT:1.0,2.0,2.1,4.0,5.0
Platform : MS-DOS NT WINDOWS
Issue type : kbhowto