Q172393: BUG: Can’t Export Symbols Differing Only by Leading Underscores
Article: Q172393
Product(s): Microsoft C Compiler
Version(s): 4.0,4.0a,4.1,4.2,4.2b,5.0,6.0
Operating System(s):
Keyword(s): kbLinker kbVC400 kbVC410 kbVC420 kbVC500 kbVC600
Last Modified: 13-JUN-2002
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The information in this article applies to:
- The Linker (LINK.EXE), used with:
- Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.0a, 4.1, 4.2b
- Microsoft Visual C++, 32-bit Enterprise Edition, versions 4.2, 5.0, 6.0
- Microsoft Visual C++, 32-bit Professional Edition, versions 4.2, 5.0, 6.0
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0
- Microsoft Visual C++.NET (2002)
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SYMPTOMS
========
When you are using the linker supplied with the Visual C++ compiler versions
mentioned above to build a DLL and the following applies:
1. You export multiple symbols from a DLL.
2. The symbols differ only by the number of leading underscores or by the number
of leading question marks.
3. The first letter in the symbol is uppercase.
4. The symbols are located in an assembly source file.
Then, the Visual C++ linker incorrectly matches some of the symbol names.
RESOLUTION
==========
There are several workarounds:
- Do not use leading underscores or leading question marks to differentiate
between different symbols.
-or-
- Use Visual C or Visual C++ instead of assembly.
-or-
- Use aliases in the EXPORTS section of the .def file as shown below. The
left-hand side of the "=" symbol is the name of the function that is exported
by the DLL and an external application/DLL can call. The right- hand side is
the symbol the linker will use to match during the link phase. Note the
missing underscore on the right hand side symbol.
EXPORTS
_symbol1=symbol1
Use the following link command to build the DLL:
link -dll -out:MYDLL.LIB -def:MYDLL.DEF MYDLL.OBJ
STATUS
======
This behavior is by design.
MORE INFORMATION
================
The following sample code is used to demonstrate the third (last listed)
workaround. The "-noentry" option is used for the linker in this sample because
no entry point function is defined for the DLL. The "-map" option is used for
comparison with the DUMPBIN output of exported symbols.
Assembler options needed: /nologo /coff /Cx /c
Linker options: -dll -noentry -map -out:MYDLL.LIB -def:MYDLL.DEF \ MYDLL.OBJ
TITLE mydll.asm
.386
.MODEL flat, SYSCALL
.CODE
Nosymbol PROC
ret
Nosymbol ENDP
_symbol PROC
ret
_symbol ENDP
__symbol PROC
ret
__symbol ENDP
___symbol PROC
ret
___symbol ENDP
____symbol PROC
ret
____symbol ENDP
_symbol1 PROC
ret
_symbol1 ENDP
__symbol2 PROC
ret
__symbol2 ENDP
___symbol3 PROC
ret
___symbol3 ENDP
____symbol4 PROC
ret
____symbol4 ENDP
END
Use the following module definition file to build the DLL:
; The module definition file MYDLL.DEF
EXPORTS
Nosymbol
_symbol=symbol
__symbol=_symbol
___symbol=__symbol
____symbol=___symbol
_symbol1=symbol1
__symbol2=_symbol2
___symbol3=__symbol3
____symbol4=___symbol4
Following are the contents of the mydll.map file:
mydll
Timestamp is 33e12fbb (Thu Jul 31 17:37:15 1997)
Preferred load address is 10000000
Start Length Name Class
0001:00000000 00000009H .text CODE
0002:00000000 000000e5H .edata DATA
Address Publics by Value Rva+Base Lib:Object
0001:00000000 Nosymbol 10001000 f mydll.obj
0001:00000001 _symbol 10001001 f mydll.obj
0001:00000002 __symbol 10001002 f mydll.obj
0001:00000003 ___symbol 10001003 f mydll.obj
0001:00000004 ____symbol 10001004 f mydll.obj
0001:00000005 _symbol1 10001005 f mydll.obj
0001:00000006 __symbol2 10001006 f mydll.obj
0001:00000007 ___symbol3 10001007 f mydll.obj
0001:00000008 ____symbol4 10001008 f mydll.obj
entry point at 0000:00000000
Static symbols
Compare the symbols of the above map file output with that of the "DUMPBIN
/exports mydll.dll" output below.
Microsoft (R) COFF Binary File Dumper Version 5.00.7022
Copyright (C) Microsoft Corp 1992-1997. All rights reserved.
<BR/><BR/>
Dump of file mydll.dll
<BR/><BR/>
File Type: DLL
Section contains the following Exports for mydll.dll
0 characteristics
33E12FBA time date stamp Thu Jul 31 17:37:14 1997
0.00 version
1 ordinal base
9 number of functions
9 number of names
ordinal hint name
1 0 Nosymbol (00001000)
2 1 ____symbol (00001004)
3 2 ____symbol4 (00001008)
4 3 ___symbol (00001003)
5 4 ___symbol3 (00001007)
6 5 __symbol (00001002)
7 6 __symbol2 (00001006)
8 7 _symbol (00001001)
9 8 _symbol1 (00001005)
Summary
1000 .rdata
1000 .reloc
1000 .text
Additional query words:
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Keywords : kbLinker kbVC400 kbVC410 kbVC420 kbVC500 kbVC600
Technology : kbVCsearch kbAudDeveloper kbLINKSearch kbLINK100 kbVCNET
Version : :4.0,4.0a,4.1,4.2,4.2b,5.0,6.0
Issue type : kbprb
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