从 Linux 交叉编译到 Windows 时，MinGW 无法链接指定的库

Question

I am trying to compile some code with MinGW from Linux to Windows.

My code has two library includes: #include <curl/curl.h> and #include <fmt/core.h> .

When compiling with native g++ , the program compiles without warnings or errors:

g++ *.cpp -std=c++2a -O2 -DNDEBUG -Werror -lcurl -lfmt -o main

However, when I try to do the same with MinGW, I get the following error:

x86_64-w64-mingw32-g++ *.cpp -std=c++2a -O2 -DNDEBUG -Werror -lcurl -lfmt -o main

/usr/lib/gcc/x86_64-w64-mingw32/12.1.0/../../../../x86_64-w64-mingw32/bin/ld: cannot find -lcurl: No such file or directory
/usr/lib/gcc/x86_64-w64-mingw32/12.1.0/../../../../x86_64-w64-mingw32/bin/ld: cannot find -lfmt: No such file or directory
collect2: error: ld returned 1 exit status

When I specify the files in question directly by the following, the linker complains about undefined references.

x86_64-w64-mingw32-g++ *.cpp -std=c++2a -O2 -DNDEBUG -Werror /usr/x86_64-w64-mingw32/include/curl/curl.h /usr/x86_64-w64-mingw32/include/fmt/core.h -o main

/usr/lib/gcc/x86_64-w64-mingw32/12.1.0/../../../../x86_64-w64-mingw32/bin/ld: /tmp/ccoZVgs5.o:api.cpp:(.text+0x8bf): undefined reference to `__imp_curl_easy_init'
/usr/lib/gcc/x86_64-w64-mingw32/12.1.0/../../../../x86_64-w64-mingw32/bin/ld: /tmp/ccoZVgs5.o:api.cpp:(.text+0x8ce): undefined reference to `__imp_curl_easy_setopt'
/usr/lib/gcc/x86_64-w64-mingw32/12.1.0/../../../../x86_64-w64-mingw32/bin/ld: /tmp/ccoZVgs5.o:api.cpp:(.text+0x910): undefined reference to `__imp_curl_easy_perform'
/usr/lib/gcc/x86_64-w64-mingw32/12.1.0/../../../../x86_64-w64-mingw32/bin/ld: /tmp/ccoZVgs5.o:api.cpp:(.text+0x919): undefined reference to `__imp_curl_easy_cleanup'
/usr/lib/gcc/x86_64-w64-mingw32/12.1.0/../../../../x86_64-w64-mingw32/bin/ld: /tmp/ccXJhCsI.o:console.cpp:(.text+0x55d): undefined reference to `fmt::v8::vformat[abi:cxx11](fmt::v8::basic_string_view<char>, fmt::v8::basic_format_args<fmt::v8::basic_format_context<fmt::v8::appender, char> >)'
collect2: error: ld returned 1 exit status

When I use the -I flag, the code does not compile and aborts because of errors within the libraries.

x86_64-w64-mingw32-g++ *.cpp -std=c++2a -O2 -DNDEBUG -Werror -I /usr/x86_64-w64-mingw32/include/curl/ -I /usr/x86_64-w64-mingw32/include/fmt/ -o main

Specifying -L and then the directory produces the same output about undefined references.

I got the library code from MinGW, from here and here respectively. I put the contents of the include directory from the tar.zst s into /usr/x86_64-w64-mingw32/include/ and verified with my Linux libraries present inside /usr/include that the content is correct, albeit a bit different because of OS differences. I also did the same for the lib and bin contents of the libraries.

A minimally reproducible example where I use all of the used functionality in the actual project would be:

#include <fmt/core.h>
#include <curl/curl.h>
#include <iostream>
#include <string>

size_t _writeFunction(void* ptr, size_t size, size_t nmemb, std::string* data) {
    data->append((char*)ptr, size * nmemb);
    return size * nmemb;
}

int main() {
    std::cout << fmt::format("{:.1f}", 1.2345) << '\n';
    auto curl{curl_easy_init()};
    curl_easy_setopt(curl, CURLOPT_URL, "example.org");
    std::string response_string{};
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, _writeFunction);
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, &response_string);
    curl_easy_perform(curl);
    curl_easy_cleanup(curl);
    curl = NULL;
    std::cout << response_string;
    return 0;
}

Any help on how to proceed would be appreciated. Thanks.

Answer 1

To use libraries (in your case curl and fmt) when cross-building for a different platform you must have those libraries and their dependencies for that platform. You may also need to use -I compiler flags and -L linker flags to tell your cross-compiler where to find the header files and libraries of those libraries for the target platform.