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

• 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, versions 1.0, 1.5
• Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

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

Keywords          : CLngIss MASMLngIss kbcode kbfasttip
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