Mixed C and MASM with MASM Main Language, C Run-Time (86816)

SUMMARY

The following steps should be considered when doing mixed-language programming between the Microsoft Macro Assembler (MASM) and Microsoft C with MASM as the main language, but the C startup code is to be brought in to enable the assembly module to call a C run-time routine:

Include the statement
.MODEL <model>, c
in the assembly module to ensure that C naming and calling conventions are used and that the modules use the same default segments. The <model> will be small, medium, compact, or large.
Use the END directive in the assembly module, but do not specify the entry point.
The start of the assembly module should be labeled
main:

-or-

main PROC
and the label should be made public with the following statement:
PUBLIC main
Include the following statement to force a load of the C startup code:
EXTRN _acrtused:abs
Assemble the assembly module with /Mx to preserve the case of nonlocal names. If using MASM version 6.0 or later, use /Cx to preserve the case of nonlocal names.

Note that the stack declaration comes from the C startup code. The entry point is in the C startup code, so no entry point should be specified in the assembler module with the END directive. Doing so will cause only a part of the necessary initialization to be performed (that is, a stack will be created, but SS will not get its segment value).

MORE INFORMATION

The following is a mixed-language example. There is one C module and one assembly module that must be compiled and then linked together. No special link options are needed.

The example declares two words, arg1 and arg2, calls the C run-time routine printf to print out their values to the screen, and passes their addresses to a C routine. The C routine swaps the values of arg1 and arg2. The values of arg1 and arg2 are printed out a second time to show that they have been swapped.

Sample Code

/* Compile options needed: none
*/ 

void ptrswap( int *ptr1, int *ptr2 )
{
    int temp;

    temp = *ptr1;
    *ptr1 = *ptr2;
    *ptr2 = temp;
}

----------------------------------------------------------------------

; Assemble options needed: /Mx

        .MODEL small, c

        .DATA

arg1    DW     1234
arg2    DW     4321
format1 DB     "Arg1: %d", 10, 0      ; Format string for printf
format2 DB     "Arg2: %d", 10, 10, 0  ; Format string for printf

EXTRN  _acrtused:abs              ; Bring in C startup

        .CODE

EXTRN  ptrswap:proc               ; External C routine
EXTRN  printf:proc                ; External C run-time routine
PUBLIC main                       ; C startup requires the name _main

main:
        MOV    ax, arg1
        PUSH   ax                 ; Push 2nd argument (C convention)
        MOV    bx, offset format1
        PUSH   bx                 ; Push 1st argument (C convention)
        CALL   printf             ; Call C run-time routine

        MOV    ax, arg2
        PUSH   ax                 ; Push 2nd argument (C convention)
        MOV    bx, offset format2
        PUSH   bx                 ; Push 1st argument (C convention)
        CALL   printf             ; Call C run-time routine

        MOV    bx, offset arg2
        PUSH   bx                 ; Push 2nd argument (C convention)
        MOV    bx, offset arg1
        PUSH   bx                 ; Push 1st argument (C convention)
        CALL   ptrswap            ; Call C routine from module

        MOV    ax, arg1
        PUSH   ax                 ; Push 2nd argument (C convention)
        MOV    bx, offset format1
        PUSH   bx                 ; Push 1st argument (C convention)
        CALL   printf             ; Call C run-time routine

        MOV    ax, arg2
        PUSH   ax                 ; Push 2nd argument (C convention)
        MOV    bx, offset format2
        PUSH   bx                 ; Push 1st argument (C convention)
        CALL   printf             ; Call C run-time routine

        MOV    ah, 4ch            ; Terminate program
        int    21h

        END                       ; Entry point will be specified by
                                  ;    the C startup code