问题
Context: I'm using Qt 5.9.3 on Windows, building for MinGW 32-bit. The Qt part is a side issue though - the problem seems to be with MinGW. The version of MinGW is 4.3.0, supplied prebuilt as part of the Qt installation.
I'm building a library which talks to a USB device over HID. Everything compiles fine, but it fails at the link stage with
./..\..\object\debug\usb_hid_device.o: In function `ZN8MyApp3USB5Win3213getDevicePathB5cxx11Ell':
<MYPATH>/../../source/win32/usb_hid_device.cpp:99: undefined reference to `HidD_GetAttributes(void*, _HIDD_ATTRIBUTES*)@8'
./..\..\object\debug\usb_hid_device.o: In function `ZN8MyApp3USB5Win3214CHIDDeviceImplC2EllRNS_15LogPerComponentE':
<MYPATH>/../../source/win32/usb_hid_device.cpp:200: undefined reference to `HidD_FlushQueue(void*)@4'
The linker command is
g++ -shared -mthreads -Wl,-subsystem,windows -Wl,--out-implib,<MYPATH>\bin\debug\libusb_hid_comms.a -o <MYPATH>\bin\debug\usb_hid_comms.dll object_script.usb_hid_comms.Debug -lhid -lsetupapi -LC:\Qt\Qt5.9.3\5.9.3\mingw53_32\lib C:\Qt\Qt5.9.3\5.9.3\mingw53_32\lib\libQt5Guid.a C:\Qt\Qt5.9.3\5.9.3\mingw53_32\lib\libQt5Cored.a
If I omit -lhid I get the same errors. I also get the same errors if I remove -lhid and explicitly set the path and filename to libhid.a. If I deliberately mistype the path and filename, it comes up with an error, so I know the command-line is getting parsed correctly. But for whatever reason, MinGW appears to not be linking with one of its own library files.
I've also tried removing -lsetupapi and I get the linker errors I'd expect for the functions defined in there. Likewise the Qt library files. But it seems that specifically for libhid.a, MinGW can see the library file but just isn't going to link with it.
Has anyone else seen this? Or can anyone else with the same (or similar) version of MinGW confirm or deny that they can link with libhid.a? Or is there something obviously wrong with what I'm doing?
回答1:
I've just found the answer. I'm posting an answer myself so that other people know in future, because I think this is still a valid question which people might want to know about.
The problem is the include file hidsdi.h. The majority of other header files which pull in Win32 API calls have extern "C" around the function declarations. However this one doesn't! The result is that we end up with C++ name mangling for linker symbols, instead of the C-style "_" in front of the linker symbols.
The solution is to use
extern "C"
{
#include <hidsdi.h>
}
and then everything works fine.
The version of hidsdi.h with the older version of MinGW (which I'm porting from) did have that protection around the function declarations. However it looks like it's gone in the newer version.
来源：https://stackoverflow.com/questions/49538390/problems-linking-with-mingw-v-4-3-0-and-libhid