Q59438: Assembly Main Module Calling a C Function

Article: Q59438
Product(s): See article
Version(s): 5.10
Operating System(s): MS-DOS
Keyword(s): ENDUSER | S_QuickASM | mspl13_masm
Last Modified: 29-MAR-1990

It is possible to call C function from an Assembly main program. It is
not recommended because the functionality of the C code will be
limited. Since most C run-time functions call either the C startup
code or the C stack check routine, it usually is not possible to make
any C run-time calls.

When compiling the C source code, it is important to use the /Gs
switch to disable stack checking and add the following line to the C
source code to tell the compiler/linker to not link in the startup
code.

   int _acrtused = 0;

The following is sample code to demonstrate how to write an Assembly
main module calling a C function.

Code Example
------------

; Assembly switch: MASM /Mx

.MODEL MEDIUM,C

  EXTRN  ptrmsg:PROC

.STACK 100h

.DATA
  arg1 DB 65
  arg2 DW 5

.CODE

main   PROC
       mov   ax, @data           ; Set up DGROUP
       mov   ds, ax              ; DS pointing to DGROUP
       xor   ax, ax              ; Clear ax register
       mov   ax, OFFSET arg2
       push  ax                  ; Push second argument
       mov   ax, OFFSET arg1
       push  ax                  ; Push first argument
       call  ptrmsg              ; Call C function
       add   sp,4                ; Restore stack pointer
       mov   ax, 4C00h
       int   21h                 ; Terminate program
main   ENDP
       END

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

/* Compiler line: cl /Gs /c /AM      */

int _acrtused = 0;             /* Do not bring in the startup code */

void ptrmsg (char *a, int *b)
{
    *a += 1;
    *b += 1;
}

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