Article: Q104618
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,2.0,2.1,4.0,5.0,6.0
Operating System(s):
Keyword(s): kbcode kbLangC kbVC100 kbVC150 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600 kbHOWTOmaster
Last Modified: 14-NOV-2001
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The information in this article applies to:
- Microsoft C for OS/2
- Microsoft C for MS-DOS
- Microsoft C/C++ for MS-DOS
- Microsoft Visual C++, versions 1.0, 1.5, 2.0, 2.1, 4.0
- Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0
- Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0
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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
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// 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
Additional query words:
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Keywords : kbcode kbLangC kbVC100 kbVC150 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600 kbHOWTOmaster
Technology : kbVCsearch kbVC400 kbAudDeveloper kbZNotKeyword8 kbvc150 kbvc100 kbCCompSearch kbZNotKeyword3 kbVC500 kbVC600 kbVC200 kbVC210 kbVC32bitSearch kbVC500Search
Version : :1.0,1.5,2.0,2.1,4.0,5.0,6.0
Issue type : kbhowto
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