Article ID: 104618
Article Last Modified on 4/24/2006
APPLIES TO
- Microsoft Visual C++ 1.0 Professional Edition
- Microsoft Visual C++ 1.5 Professional Edition
- Microsoft Visual C++ 2.0 Professional Edition
- Microsoft Visual C++ 2.1
- 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 Q104618
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)
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
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
The following is the output of the program:
a
z
Keywords: kbhowtomaster kblangc kbcode KB104618
