QLibrary functions work slow on first call - c++

Question

I'm using QLibrary to load functions from one .dll file.
I succesfully load it, succesfully resolve functions.
But when i use some function from that .dll for the first time, this function works very slow(even if it is very simple one). Next time i use it again - and the speed is just fine (immediately, as it should be).
What is the reason for such behaviour? I suspect some caсhing somewhere.
Edit 1: Code:
typedef int(*my_type)(char *t_id);
QLibrary my_lib("Path_to_lib.dll");
my_lib.load();
if(my_lib.isLoaded){
my_type func = (my_type)my_lib.resolve("_func_from_dll");
if(func){
char buf[50] = {0};
char buf2[50] = {0};
//Next line works slow
qint32 resultSlow = func(buf);
//Next line works fast
qint32 resultFast = func(buf2);
}
}

Answer 1

I wouldn't blame QLibrary: func simply takes long the first time it's invoked. I bet that you'll have identical results if you resolve its address using platform-specific code, e.g. dlopen and dlsym on Linux. QLibrary doesn't really do much besides wrapping the platform API. There's nothing specific to it that would make the first call slow.
There is some code smell of doing file I/O in constructors of presumably generic classes: do the users of the class know that the constructor may block on disk I/O and thus ideally shouldn't be invoked from the GUI thread? Qt makes the doing this task asynchronously fairly easy, so I'd at least try to be nice that way:
class MyClass {
QLibrary m_lib;
enum { my_func = 0, other_func = 1 };
QFuture<QVector<FunctionPointer>> m_functions;
my_type my_func() {
static my_type value;
if (Q_UNLIKELY(!value) && m_functions.size() > my_func)
value = reinterpret_cast<my_type>(m_functions.result().at(my_func));
return value;
}
public:
MyClass() {
m_lib.setFileName("Path_to_lib.dll");
m_functions = QtConcurrent::run{
m_lib.load();
if (m_lib.isLoaded()) {
QVector<QFunctionPointer> funs;
funs.push_back(m_lib.resolve("_func_from_dll"));
funs.push_back(m_lib.resolve("_func2_from_dll"));
return funs;
}
return QVector<QFunctionPointer>();
}
}
void use() {
if (my_func()) {
char buf1[50] = {0}, buf2[50] = {0};
QElapsedTimer timer;
timer.start();
auto result1 = my_func()(buf1);
qDebug() << "first call took" << timer.restart() << "ms";
auto result2 = my_func()(buf2);
qDebug() << "second call took" << timer.elapsed() << "ms";
}
}
};