Linking error LNK2019 in MSVC, unresolved symbols

问题:

I'm running into linking problems in MSVC for a project that I wrote for g++. Here's the problem:

I build libssh as a static library as part of my application, adding the target in cmake with

add_library(ssh_static STATIC $libssh_SRCS)

Libssh is in C, so I have 'extern "C" {...}' wrapping the includes in my c++ sources. I then link the ssh_static target to my executable, sshconnectiontest, with

target_link_libraries(sshconnectiontest ... ssh_static ...)

This all works fine in linux with gcc, but now in MSVC I get

error LNK2019: unresolved external symbol __imp__[function names here] referenced in [filename]

for every libssh function I use.

Any ideas whats going wrong? I've read somewhere that the imp prefix means that the linker is expecting to link a .dll, but this should not be the case since ssh_static is declared a static library in the add_library call...

回答1:

From what I remember of my Windows days, in MinGW-built DLLs, the __imp__ symbol prefix is used for the trampoline function that calls into the DLL proper. This symbol is then provided by a small static library with the extension .dll.a.

When you include libssh headers, you need to set a #define to indicate that you're expecting to link statically. If you don't, the libssh functions in the header will be declared __declspec(dllimport) and so the __imp__ symbols will be expected at link time.

I had a look at the libssh source and found this at the top of libssh.h:

#ifdef LIBSSH_STATIC
  #define LIBSSH_API
#else
  #if defined _WIN32 || defined __CYGWIN__
    #ifdef LIBSSH_EXPORTS
      #ifdef __GNUC__
        #define LIBSSH_API __attribute__((dllexport))
      #else
        #define LIBSSH_API __declspec(dllexport)
      #endif
    #else
      #ifdef __GNUC__
        #define LIBSSH_API __attribute__((dllimport))
      #else
        #define LIBSSH_API __declspec(dllimport)
      #endif
    #endif
  #else
    #if __GNUC__ >= 4
      #define LIBSSH_API __attribute__((visibility("default")))
    #else
      #define LIBSSH_API
    #endif
  #endif
#endif

You need to define LIBSSH_STATIC, either through #define before the #include <libssh.h> line, or as a /D option. Since you're using CMake, you'll probably do this through add_definitions in CMakeLists.txt.

回答2:

Don't know if it is your case, but the imp prefix may mean that you are compiling a x64 library in a Win32 project.

回答3:

Somewhat late to the party, but I got the same error when mixing libraries with static and dynamic linkage to the CRT

回答4:

Using a .DEF File

If you choose to use __declspec(dllimport) along with a .DEF file, you should change the .DEF file to use DATA or CONSTANT to reduce the likelihood that incorrect coding will cause a problem:

// project.def
LIBRARY project
EXPORTS
   ulDataInDll   CONSTANT

The following table shows why:

Keyword      Emits in the import library   Exports
CONSTANT     _imp_ulDataInDll              _ulDataInDll
             _ulDataInDll                  

DATA         _imp_ulDataInDll              _ulDataInDll

http://msdn.microsoft.com/en-us/library/aa271769(v=vs.60).aspx

BUT CONSTANT is now deprecated

i found another way, in the .DEF file of exported .lib use :

 mainthreadid=_mainthreadid

and regenerate the lib with LIB.exe

in the import header file of the dll code...

extern "C" {
  extern const __declspec(dllexport) ulong mainthreadid;
}