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# INFO: Passing a long from C to MASM by Value & Returning a long

Article ID: 104645

Article Last Modified on 7/5/2005

APPLIES TO

- Microsoft C Professional Development System 6.0a
- Microsoft C Professional Development System 6.0a
- Microsoft Visual C++ 1.0 Professional Edition
- Microsoft Visual C++ 1.5 Professional Edition
- Microsoft Visual C++ 2.0 Professional Edition
- Microsoft Visual C++ 4.0 Standard Edition
- Microsoft Visual C++ 5.0 Enterprise Edition
- Microsoft Visual C++ 6.0 Enterprise Edition
- Microsoft Visual C++ 5.0 Professional Edition
- Microsoft Visual C++ 6.0 Professional Edition
- Microsoft Visual C++ 6.0 Standard Edition

This article was previously published under Q104645

## SUMMARY

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

## 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

    long MasmSub (long, long);

    #ifdef __cplusplus
    }
    #endif

    main ()
       {
       long var1 = 98304, var2 = 147456;
       printf ("%ld + %ld = %ld", var1, var2, MasmSub (var1, var2));
       }

### 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, \
       lVar1:DWORD, \
       lVar2:DWORD

       mov ax, WORD PTR lVar1       ; Load the first long into DX:AX.
       mov dx, WORD PTR lVar1+2
       add ax, WORD PTR lVar2       ; Add the second long to DX:AX
       adc dx, WORD PTR lVar2+2
       ret              ; Because the function returns a long (a 4-byte
    MasmSub ENDP        ; value), C will get the return value from DX:AX.
    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, \
       lVar1:DWORD, \
       lVar2:DWORD

       mov eax, lVar1   ; Load the first long into EAX.
       add eax, lVar2   ; Add the second long to EAX.
       ret              ; Because the function returns a long (a 4-byte
    MasmSub ENDP        ; value), C will get the return value from EAX.
    END

The following is the output of the program:

       98304 + 147456 = 245760

Additional query words: mixed language

Keywords: kbinfo kblangc KB104645