__declspec(dllexport) is used to inform the compiler that this function or class is used as a public library, and __declspec(dllimport) is used to inform the compiler that this function or class is not defined in the current file and will be imported from other DLL files.

library.dll

__declspec(dllexport) void func()
{
    // ...
}

main.c

__declspec(dllimport) void func();

int main()
{
    func();
    reutrn 0;
}

Link this library at compile time.