Thread: Dynamic Array Error

Hello, I am trying to export some code over from C# to C++. I am using visual studio 2008 pro. The error occurs at NewData = new char[PacketLength];. I was able to debug and make sure that PacketLength was indeed set to a reasonable number every time such as 9. When my code gets here it gets a segmentation fault every time. I am unfamiliar with using C++ to be honest and unsure what I am doing wrong...If i execute this code in main() it works fine.

Thanks for any input.

Code:

void DataStream::Parse(char * Buffer, int Length)
{
	char * NewData = NULL;
	short PacketLength = *(short *)Buffer;
	std::cout << "Length: " << Length << " PacketLength: " << PacketLength;

	while(PacketLength < Length)
	{
		NewData = new char[PacketLength];

		memcpy(NewData, Buffer, PacketLength);
		PacketLength = (short)(Buffer + PacketLength);
		std::cout << PacketLength;
		
	}
}

This is the smallest possible example...I'm not done porting it over. Im aware of the garbage collection issue. Right now I cant add the rest because of the problem that I originally posted. Im well aware that right now this function does nothing and the loop is incomplete. However, on the first iteration of the loop when say PacketLength = 9, the allocation of memory errors out with "bad pointer" in visual studio 2008 every time. Im not sure why, this is what I am asking.

NewData = new char[PacketLength]; <-----Error here.

From looking online and in books this syntax seems perfectly ok...but it still errors.

Originally Posted by Elysia

Well, can you post the smallest possible example that demonstrates the error?
And I hope you're aware that C++ does not have garbage collection, so your new char will result in a memory leak.
And NewData isn't really used anywhere (you just copy data into, but that's it), so it's kindof useless.

The example is in my original post :P.

*sigh * Thanks for the help guys I really appreciate it, but again the problem is on the first iteration of the loop, IE it never gets that far anon...yes I should have that line changed to:

PacketLength = *(short *)(Buffer + PacketLength);

But it never gets this far. It is erroring out where I have mentioned a couple times in this thread.

I get errors on the first allocation attempt of NewData = new char[PacketLength]; And I have verified that PacketLength at this location is what it should be(say 9 bytes). I get a Bad Pointer Error, and my program closes. Using vs 2008 pro.

Hmmm really? I wonder why...Im only allocating 9 bytes on this test. The strange thing is it doesn't occur in main() when I allocate say, 6k bytes...Only in an external class file when I call this method from main().

Hmm on further inspection you seem very right. It is now working 20&#37; of the time, and then throwing an error the rest of the time. What is traditionally the cause of this? I am on a modern system with plenty of memory available, and at the moment at most I only allocate 9 bytes to the heap.

Also an update to my code.

Code:

void DataStream::Parse(char * Buffer, int Length)
{
	char * NewData = NULL;
	short PacketLength = *(short *)Buffer;
	std::cout << "Length: " << Length << " PacketLength: " << PacketLength;

	while(PacketLength < Length)
	{
		NewData = new char[PacketLength];

		memcpy(NewData, Buffer, PacketLength);
		Packet(Buffer, PacketLength);
		Length -= PacketLength;
		PacketLength = *(short *)(Buffer + PacketLength);
		
	}
}

void DataStream::Packet(char * Buffer, int length)
{
	char PacketType = *(char *)(Buffer + 2);
	switch(PacketType)
	{
	case 1:
		std::cout << "Got a Chat Packet\n";
		break;
	}

	delete Buffer;
}

I call it by passing it a test array or some sort. I had built a packet parser that works perfectly in c#, was just trying to port it over to C++. So I makeup an array with the first two bytes of each packet being a short that describes the lemgth of the packet to be parsed out of the chunk. This length includes the length size itself, so an example would be.

Code:

char payload[] = {3, 0, 1, 3, 0, 2, 4, 0, 3, 1};
parse(payload, sizeof(payload));