Microsoft KB Archive/104645

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

Article ID: 104645

Article Last Modified on 7/5/2005

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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

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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


 * 1) include 

extern "C" {
 * 1) ifdef __cplusplus
 * 1) endif

long MasmSub (long, long);

}
 * 1) ifdef __cplusplus
 * 1) 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

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