PRB: Can't Export Symbols Differing Only by Leading Underscores

ID: Q172393

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, 4.2, 4.2b, 5.0, 6.0

SYMPTOMS

When you are using the linker supplied with the Visual C++ compiler versions mentioned above to build a DLL and the following applies:

You export multiple symbols from a DLL.

The symbols differ only by the number of leading underscores or by the number of leading question marks.

The first letter in the symbol is uppercase.

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.

Use Visual C or Visual C++ instead of assembly.

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:

Keywords : kbLinker kbVC400 kbVC410 kbVC420 kbVC500 kbVC600 LinkIss
Version : winnt:4.0,4.0a,4.1,4.2,4.2b,5.0,6.0
Platform : winnt
Issue type : kbprb