I have a question on extern "C"

EnthusiasticNewbie · #1 November 2nd, 2009, 06:08 AM

--------------------------------------------------------------------------------

hello
I have a question..

I realized that when I compiled and made export functions in dll with 'extern "C" __declspec(dllexport)', function names didn't include underscores in front of their name.
but I heard that symbols of c functions should include underscores..
is it a just a feature of Visual C++ compiler?

Even in import and export sections(in EXE , DLL), the function name doesn't contain underscores in front of its name. but when I compiled function with __stdcall, function name was like _functionName@parametersize and that's exactly like what I learned before..

naming rule of __cdecl functions in C has been changed ? or is it a just matter of compiler ?

and one more question..

when we use standard API in kernel32.dll, user32.dll, gdi32.dll and so on
even if most functions are standard call functions, the function name is not like
_anyName@parametersize .. why ? were they not made by __declspec(dllexport) ?

I need help.. I've found no answers that meets my questions..

thanks

MrViggy · #2 November 3rd, 2009, 03:36 PM

Post your code. That should work (if I remember my syntax correctly).

Viggy

VijayDandur · #3 November 4th, 2009, 03:18 AM

Dear friend,
By adding this " __declspec(dllexport) " statement indicates that the function is to be exported , and __declspec is a standard calling convention. Thats all , it wont add and "Underscores" during function decoration.

What Extern "C" means , follow the "C" name decoration standards, Which doesn't add any extra characters in function decoration.

I Think you got it.
Thakns.

Igor Vartanov · #4 November 4th, 2009, 09:10 AM

Quote:

I heard that symbols of c functions should include underscores..
. . .
Even in import and export sections(in EXE , DLL), the function name doesn't contain underscores in front of its name.

You're missing the diffeence between function symbol and function export name. Symbol is a name used internally by linker. Export is a name used by loader for external dynamical linkage. Two absolutely different entities.

Let's see some details by example.

Code:

// 23.cpp
extern "C" __declspec(dllexport) void foo() {}

Code:

cl 23.cpp /link /dll /map:23.map /out:23.dll

Code:

; 23.map
 23

 Timestamp is 4af1883c (Wed Nov 04 16:57:16 2009)

 Preferred load address is 10000000

 Start         Length     Name                   Class
 0001:00000000 000063f4H .text                   CODE
 0002:00000000 000000f4H .idata$5                DATA
 . . .
 0003:00000000 00000cd0H .data                   DATA
 0003:00000ce0 00000bbcH .bss                    DATA

  Address         Publics by Value              Rva+Base       Lib:Object

 0000:00000000       __except_list              00000000     <absolute>
 0000:00000003       ___safe_se_handler_count   00000003     <absolute>
 0000:00000000       ___ImageBase               10000000     <linker-defined>
 0001:00000000       _foo                       10001000 f   23.obj       <-- symbol
 0001:00000005       __CRT_INIT@12              10001005 f   LIBCMT:dllcrt0.obj
 0001:000002d4       __DllMainCRTStartup@12     100012d4 f   LIBCMT:dllcrt0.obj

Code:

D:\Temp\23>exports 23.dll
ModuleType:
  PE executable (DLL)
Subsystem:
  Win32 GUI
ModuleName:
  23.dll
Exported Info: BaseOfOrdinals    = 1
               NumberOfNames     = 1
               NumberOfFunctions = 1

  RelVirtAddr  Ord  Hint   ExportedFuncName

  0x00001000     1     0   foo    <-- export name

Igor Vartanov · #5 November 4th, 2009, 09:23 AM

Let's see another case, a bit different comparing to the previous one.

Code:

// 23.cpp
extern "C" void foo() {}

Code:

; 23.def
LIBRARY 23.dll
EXPORTS
	oops = foo    ; here's the trick

Code:

cl 23.cpp 23.def /link /dll /map:23.map /out:23.dll

Code:

 23

 Timestamp is 4af18d73 (Wed Nov 04 17:19:31 2009)

 Preferred load address is 10000000

 Start         Length     Name                   Class
 0001:00000000 000063f4H .text                   CODE
 0002:00000000 000000f4H .idata$5                DATA
 0002:000000f4 00000004H .CRT$XCA                DATA
. . .
 0002:0000152c 000000f4H .idata$4                DATA
 0002:00001620 00000452H .idata$6                DATA
 0002:00001a80 0000003eH .edata                  DATA
 0003:00000000 00000cd0H .data                   DATA
 0003:00000ce0 00000bbcH .bss                    DATA

  Address         Publics by Value              Rva+Base       Lib:Object

 0000:00000000       __except_list              00000000     <absolute>
 0000:00000003       ___safe_se_handler_count   00000003     <absolute>
 0000:00000000       ___ImageBase               10000000     <linker-defined>
 0001:00000000       _foo                       10001000 f   23.obj
 0001:00000005       __CRT_INIT@12              10001005 f   LIBCMT:dllcrt0.obj
 0001:000002d4       __DllMainCRTStartup@12     100012d4 f   LIBCMT:dllcrt0.obj
 0001:000002f5       __amsg_exit                100012f5 f   LIBCMT:crt0dat.obj
 0001:00000319       ___crtCorExitProcess       10001319 f   LIBCMT:crt0dat.obj
. . .

Code:

D:\Temp\23>exports 23.dll
ModuleType:
  PE executable (DLL)
Subsystem:
  Win32 GUI
ModuleName:
  23.dll
Exported Info: BaseOfOrdinals    = 1
               NumberOfNames     = 1
               NumberOfFunctions = 1

  RelVirtAddr  Ord  Hint   ExportedFuncName

  0x00001000     1     0   oops

You see? Export name generally may have nothing common with the symbol it relates to.

EnthusiasticNewbie · #6 November 7th, 2009, 09:02 AM

Thanks for your kind reply

U seem to have understood what I wondered

But I have another question
Where's the mechanism that links symbol name (like _anyFunc) with exports name(like anyFunc) ?

That is, What can make loader find proper exports name when there's symbol like _anyfunc ?
Exports name can be various when there's only one symbol name .
The secret is in lib file ?

Igor Vartanov · #7 November 8th, 2009, 05:38 AM

Sure. This is exactly what import library is for. Just take a look inside some, and you find the symbol bound to export name. Though the second case will look more tricky.

EnthusiasticNewbie · #8 November 8th, 2009, 12:28 PM

really thanks for your great explaination
but I couldn't find any linkage in import library file..
there are only symbol names.. no exports name in it!
I wonder why.. but I understood that lib file is the only thing
that can link symbol and exports name even if I couldn't see what it is..
anyway thanks again

Igor Vartanov · #9 November 11th, 2009, 01:32 PM

Code:

0000000600: 20 20 20 20 20 20 31 32 │ 35 37 39 36 33 38 34 34        1257963844
0000000610: 20 20 20 20 20 20 20 20 │ 20 20 20 20 20 20 30 20                0
0000000620: 20 20 20 20 20 20 33 32 │ 20 20 20 20 20 20 20 20        32
0000000630: 60 0A 00 00 FF FF 00 00 │ 4C 01 44 01 FB 4A 0C 00  `◙  яя  L☺D☺ыJ♀
0000000640: 00 00 00 00 08 00 5F 66 │ 6F 6F 00 32 33 2E 64 6C      ◘ _foo 23.dl
0000000650: 6C 00                   │                          l

Well, this should be it. And seems like loader does some additional work and strips the underscore off... I'm embarrassed a bit...

EnthusiasticNewbie · #10 November 12th, 2009, 08:21 AM

Thanks

and thanks for my another question on ReadProcessMemory too !

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