I have a problem to translate a C program to QT GUI.

xmaze

I found what is wrong but i dont know why!!

If i try to allocate the memory with malloc in constructor everything its ok.
if i try to allocate memory with malloc in an other section of the program i receive NULL pointer and segmentation fault.

andreyc

It is hard to tell what is wrong without a code.
Could you post simplified example of not-working code.

xmaze

!http://i58.tinypic.com/s1pzdk.jpg!

!http://i62.tinypic.com/1053821.jpg!

This is the screen shots, the code is this one
@data = ((unsigned char *) malloc(data_len);@

If i put this line on Constructor i have the above results, it is successful,
if i put the code for example in a function like On_pushButton_clicked,
then i have the results in the second scree shot with a null pointer.

andreyc

Here is a simple example that works.
Could you try it and tell if it works in your environment.

@
#include <stdlib.h>
#include <iostream>

class Foo
{
public:
Foo()
{
data_len = 262620;
data = (unsigned char )malloc(data_len);
}
~Foo()
{
if (data) {
free((void)data);
}
}

    unsigned char* getData() { return data; }
    u_int32_t getDataLen() { return data_len; }

private:
    unsigned char* data;
    u_int32_t data_len;

};

int main(int, char **)
{
Foo a_foo;

std::cout << "Foo: data = " << std::hex << static_cast<void*>(a_foo.getData())
          << " data_len = " << std::dec << a_foo.getDataLen() << std::endl;

unsigned char* data;
u_int32_t data_len;
data_len = 262620;
data = (unsigned char *)malloc(data_len);
std::cout << "Local: data = " << std::hex << static_cast<void*>(a_foo.getData())
          << " data_len = " << std::dec << a_foo.getDataLen() << std::endl;
if (data) {
    free((void*)data);
}

return 0;

}
@

xmaze

yes its working

andreyc

It means that the problem is not in the location of this particular line but something else.

xmaze

ok, i worked around and i think it's ok.

Do you have a suggestion how can i use the while(1) {} with the GUI ?
without corruption ?

if i use in while() a function like addWidgetItem in a TreeWidget then will be ok ?

andreyc

If you have a code that requires to run its own loop then I suggest to put it into a separate thread and run the code using QThread::run().
Take a look on this "article":http://blog.debao.me/2013/08/how-to-use-qthread-in-the-right-way-part-1/ for an example.

Chrisw01

Hi, not that this would fix your problem, however, the most common mistake I've ever seen in ANSI C coding is uninitialized pointers before use. Make it a good habit to always delcare, then initialize, and then use. I use malloc all over the place in QT without problems, however, I always initalize it in the constructor. Example in your Foo constructor place data = NULL; also do not forget to test and free then in your destructor or you may get memory leakage, example if(data != NULL) free(data); (which I see you have done). May not be your problem but it is good practice.

JKSH

[quote author="xmaze" date="1423001212"]if i use in while() a function like addWidgetItem in a TreeWidget then will be ok ?[/quote]Your GUI will freeze until your while-loop exits.

So, if the loop runs for a very short time only, then it's ok. If the loop runs for a long time, then your GUI will stop responding.

xmaze

[quote author="JKSH" date="1423013834"][quote author="xmaze" date="1423001212"]if i use in while() a function like addWidgetItem in a TreeWidget then will be ok ?[/quote]Your GUI will freeze until your while-loop exits.

So, if the loop runs for a very short time only, then it's ok. If the loop runs for a long time, then your GUI will stop responding.[/quote]

QThread is it better or can i use also the pthread libs ?

JKSH

QThread is more portable.