Dealing with error LNK2001: unresolved external symbol [email protected] in Python extensions

Simon Source

I am trying to build a C project. I think the source is fine, however I am getting this error:

error LNK2001: unresolved external symbol [email protected]

Or for the complete traceback:

C:\Users\Simon\Desktop\Learn>python setup.py build
running build
running build_ext
building 'sys_shutdown' extension
C:\Program Files (x86)\Microsoft Visual Studio\2017\WDExpress\VC\Tools\MSVC\14.14.26428\bin\HostX86\x86\cl.exe /c /nologo /Ox /W3 /GL /DNDEBUG /MT -IC:\Users\Simon\AppData\Local\Programs\Python\Python36-32\include -IC:\Users\Simon\AppData\Local\Programs\Python\Python36-32\include "-IC:\Program Files (x86)\Microsoft Visual Studio\2017\WDExpress\VC\Tools\MSVC\14.14.26428\include" "-IC:\Program Files (x86)\Windows Kits\NETFXSDK\4.6.1\include\um" "-IC:\Program Files (x86)\Windows Kits\10\include\10.0.17134.0\ucrt" "-IC:\Program Files (x86)\Windows Kits\10\include\10.0.17134.0\shared" "-IC:\Program Files (x86)\Windows Kits\10\include\10.0.17134.0\um" "-IC:\Program Files (x86)\Windows Kits\10\include\10.0.17134.0\winrt" "-IC:\Program Files (x86)\Windows Kits\10\include\10.0.17134.0\cppwinrt" /Tcmain.c /Fobuild\temp.win32-3.6\Release\main.obj
main.c
C:\Program Files (x86)\Microsoft Visual Studio\2017\WDExpress\VC\Tools\MSVC\14.14.26428\bin\HostX86\x86\link.exe /nologo /INCREMENTAL:NO /LTCG /nodefaultlib:libucrt.lib ucrt.lib /DLL /MANIFEST:EMBED,ID=2 /MANIFESTUAC:NO /LIBPATH:C:\Users\Simon\AppData\Local\Programs\Python\Python36-32\libs /LIBPATH:C:\Users\Simon\AppData\Local\Programs\Python\Python36-32\PCbuild\win32 "/LIBPATH:C:\Program Files (x86)\Microsoft Visual Studio\2017\WDExpress\VC\Tools\MSVC\14.14.26428\lib\x86" "/LIBPATH:C:\Program Files (x86)\Windows Kits\NETFXSDK\4.6.1\lib\um\x86" "/LIBPATH:C:\Program Files (x86)\Windows Kits\10\lib\10.0.17134.0\ucrt\x86" "/LIBPATH:C:\Program Files (x86)\Windows Kits\10\lib\10.0.17134.0\um\x86" /EXPORT:PyInit_sys_shutdown build\temp.win32-3.6\Release\main.obj /OUT:build\lib.win32-3.6\sys_shutdown.cp36-win32.pyd /IMPLIB:build\temp.win32-3.6\Release\sys_shutdown.cp36-win32.lib
   Creating library build\temp.win32-3.6\Release\sys_shutdown.cp36-win32.lib and object build\temp.win32-3.6\Release\sys_shutdown.cp36-win32.exp
main.obj : error LNK2001: unresolved external symbol [email protected]
build\lib.win32-3.6\sys_shutdown.cp36-win32.pyd : fatal error LNK1120: 1 unresolved externals
error: command 'C:\\Program Files (x86)\\Microsoft Visual Studio\\2017\\WDExpress\\VC\\Tools\\MSVC\\14.14.26428\\bin\\HostX86\\x86\\link.exe' failed with exit status 1120

I worked out from here that [email protected] is caused by the necessary runtime not being found.

Don't link using the #using but link using the linker command line. You can do that by adding the user32.lib to the linker command

I tried this in my setup script using extra_objects:

import setup, Extension

module = Extension(
    "sys_shutdown", 
    sources = ["main.c"],
        extra_objects = ["C:\\Program Files (x86)\\Windows Kits\\10\Lib\\10.0.17134.0\\um\\arm64\\User32.Lib"]
)

setup (
    name = "sys_shutdown",
    version = "1.0",
    ext_modules = [module])

My main.c file (so you know why and what I need to link to):

#include <Python.h>
#include <Windows.h>


/* The functions that need to be created */

static PyObject * sys_shutdown(PyObject *self) {
    ExitWindowsEx(EWX_POWEROFF, SHTDN_REASON_MINOR_OTHER); // Shutdown
    return Py_BuildValue("");
}

static PyObject * sys_restart(PyObject *self) {
    ExitWindowsEx(EWX_REBOOT, SHTDN_REASON_MINOR_OTHER); // Restart
    return Py_BuildValue("");
}

static PyObject * sys_log_out(PyObject *self) {
    ExitWindowsEx(EWX_LOGOFF, SHTDN_REASON_MINOR_OTHER); // Log out
    return Py_BuildValue("");
}

static PyMethodDef allMethods[] = {
    {"sys_shutdown", (PyCFunction)sys_shutdown, METH_NOARGS, "Shuts down the device"},
    {"sys_restart", (PyCFunction)sys_restart, METH_NOARGS, "Restarts the device"},
    {"sys_log_out", (PyCFunction)sys_log_out, METH_NOARGS, "Closes all processes and logs out the user from the device"},
    {NULL, NULL, 0, NULL}
};

static struct PyModuleDef allModule = {
    PyModuleDef_HEAD_INIT,
    "System Functions",
    "sys_shutdown",
    -1,
    allMethods
};

PyMODINIT_FUNC PyInit_sys_shutdown(void) {
    return PyModule_Create(&allModule);
}

This extension requires the ExitWindowsEx() function provided by the user32.dll

How can I link the user32.dll to my extension (or if I have missed the plot completely how can I make it compile correctly)?

pythoncpython-3.xpython-c-api

Answers

answered 3 months ago Simon #1

Right I found the solution. The linker path is already set for system runtimes. All I had to do was link the runtime itself. This can be done using the library attribute in the setup script:

from distutils.core import setup, Extension

module = Extension(
    "sys_shutdown", 
    sources = ["main.c"],
    libraries = ["user32"] # <-- Here it is
)

setup (
    name = "sys_shutdown",
    version = "1.0",
    ext_modules = [module])

comments powered by Disqus