Cython编译调用厂商DLL时extern语句相关报错问题求助

编写Cython代码调用厂商提供的DLL文件时，运行构建脚本出现无法自行解决的错误，执行命令python setup.py build_ext -i的输出如下：

PS C:\Users\Ben\Documents\UCLA\Research\Hardware\fw_bell_magnetic_field_probe\usb5100-x64\x64-dist\cython_attempt_2> python setup.py build_ext -i
running build_ext
cythoning FWB5180_pythonWrapper.pyx to FWB5180_pythonWrapper.c
C:\Users\Ben\AppData\Local\Programs\Python\Python39\lib\site-packages\Cython\Compiler\Main.py:369: FutureWarning: Cython directive 'language_level' not set, using 2 for now (Py2). This will change in a later release! File: C:\Users\Ben\Documents\UCLA\Research\Hardware\fw_bell_magnetic_field_probe\usb5100-x64\x64-dist\cython_attempt_2\FWB5180_pythonWrapper.pyx
  tree = Parsing.p_module(s, pxd, full_module_name)
building 'fwb' extension
creating build
creating build\temp.win-amd64-3.9
creating build\temp.win-amd64-3.9\Release
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.32.31326\bin\HostX86\x64\cl.exe /c /nologo /Ox /W3 /GL /DNDEBUG /MD -IC:\Users\Ben\AppData\Local\Programs\Python\Python39\include -IC:\Users\Ben\AppData\Local\Programs\Python\Python39\include -IC:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.32.31326\ATLMFC\include -IC:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.32.31326\include -IC:\Program Files (x86)\Windows Kits\NETFXSDK\4.8\include\um -IC:\Program Files (x86)\Windows Kits\10\include\10.0.19041.0\ucrt -IC:\Program Files (x86)\Windows Kits\10\include\10.0.19041.0\shared -IC:\Program Files (x86)\Windows Kits\10\include\10.0.19041.0\um -IC:\Program Files (x86)\Windows Kits\10\include\10.0.19041.0\winrt -IC:\Program Files (x86)\Windows Kits\10\include\10.0.19041.0\cppwinrt /TcFWB5180_pythonWrapper.c /Fobuild\temp.win-amd64-3.9\Release\FWB5180_pythonWrapper.obj
FWB5180_pythonWrapper.c
C:\Users\Ben\Documents\UCLA\Research\Hardware\fw_bell_magnetic_field_probe\usb5100-x64\x64-dist\cython_attempt_2\FWB5180.h(15): error C2059: syntax error: 'string'
C:\Users\Ben\Documents\UCLA\Research\Hardware\fw_bell_magnetic_field_probe\usb5100-x64\x64-dist\cython_attempt_2\FWB5180.h(16): error C2059: syntax error: 'string'
C:\Users\Ben\Documents\UCLA\Research\Hardware\fw_bell_magnetic_field_probe\usb5100-x64\x64-dist\cython_attempt_2\FWB5180.h(17): error C2059: syntax error: 'string'
FWB5180_pythonWrapper.c(1112): warning C4013: 'openUSB5100' undefined; assuming extern returning int
error: command 'C:\\Program Files\\Microsoft Visual Studio\\2022\\Community\\VC\\Tools\\MSVC\\14.32.31326\\bin\\HostX86\\x64\\cl.exe' failed with exit code 2

其中字符串语法错误指向extern声明中的"C"字段，同时伴随openUSB5100未定义的告警，已按规则编写相关函数声明。
所有相关文件、依赖的usb5100.dll、libusb0.dll均存放在同一目录下，使用Microsoft Visual Studio 2022作为编译器，此前已成功构建其他示例项目，可排除基础环境配置问题。

• 头文件FWB5180.h

#ifdef USB5100_EXPORTS
#define USB5100_API __declspec(dllexport)
#else
#define USB5100_API __declspec(dllimport)
#endif

extern "C" USB5100_API unsigned int openUSB5100(void);
extern "C" USB5100_API void closeUSB5100(unsigned int fwb5000ID);
extern "C" USB5100_API int scpiCommand(unsigned int usbID,char* cmd, char* result, int len);

• Cython适配声明文件c_FWB5180.pxd

cdef extern from "FWB5180.h":

    int openUSB5100()
    void closeUSB5100(int devID)
    int scpiCommand(int devID,char* cmd, char* result, int length)

• Python包装实现文件FWB5180_pythonWrapper.pyx

cimport c_FWB5180

def open_connection():

    devID = c_FWB5180.openUSB5100()
    print(devID)
    return devID

• 构建脚本setup.py

from distutils.core import setup
from distutils.extension import Extension
from Cython.Distutils import build_ext

ext_modules = [
    Extension('fwb',
              ['FWB5180_pythonWrapper.pyx'],
              language="c",  
              libraries=['usb5100.dll','libusb0.dll'],
              library_dirs=['.'])
    ]

setup(
    name = 'FWB python wrapper',
    cmdclass = {'build_ext': build_ext},
    ext_modules = ext_modules
)

核心报错原因 #

setup.py中明确指定了language="c"，MSVC编译器会按照纯C标准编译生成的C文件，但厂商提供的头文件里使用了C++专属的extern "C"语法，纯C编译器无法识别该写法，直接把"C"判定为非法语法抛出C2059错误。头文件解析失败后，内部声明的函数自然无法被编译器识别，进而触发后续openUSB5100未定义的告警。

修复步骤 #

1. 修改头文件兼容C编译环境
   给extern "C"添加C++环境判断宏，纯C编译时自动忽略该段语法，修改后的FWB5180.h代码如下：

#ifdef USB5100_EXPORTS
#define USB5100_API __declspec(dllexport)
#else
#define USB5100_API __declspec(dllimport)
#endif

#ifdef __cplusplus
extern "C" {
#endif

USB5100_API unsigned int openUSB5100(void);
USB5100_API void closeUSB5100(unsigned int fwb5000ID);
USB5100_API int scpiCommand(unsigned int usbID,char* cmd, char* result, int len);

#ifdef __cplusplus
}
#endif

2. 修正函数类型不匹配问题
   厂商头文件中openUSB5100返回值、closeUSB5100入参均为unsigned int，pxd中声明为int会引发类型不匹配问题，极端情况会导致运行时异常，修改c_FWB5180.pxd为：

cdef extern from "FWB5180.h":
    ctypedef unsigned int uint
    uint openUSB5100()
    void closeUSB5100(uint devID)
    int scpiCommand(uint devID, char* cmd, char* result, int length)

同步调整FWB5180_pythonWrapper.pyx中的变量类型适配即可。
3. 消除版本兼容警告
在FWB5180_pythonWrapper.pyx第一行添加编译指令，指定Python3语法规则，避免默认使用Py2兼容模式触发的FutureWarning：

# cython: language_level=3
cimport c_FWB5180

def open_connection():
    devID = c_FWB5180.openUSB5100()
    print(devID)
    return devID

4. 修正链接库配置
   Windows下MSVC链接动态库不需要带.dll后缀，直接填写库名即可，将setup.py中libraries参数修改为libraries=['usb5100','libusb0']，否则会触发找不到链接库的错误。

完成上述修改后重新执行python setup.py build_ext -i即可正常完成编译。

内容的提问来源于stack exchange，提问作者bpound