WSABUF confusion - GameCreators Forum

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Michael P

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Location: London (UK)

Posted: 3rd Jul 2008 20:04 Edited at: 3rd Jul 2008 20:09

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I'm trying to create a function that will convert a string and store it in a WSABUF structure but I'm having trouble doing so. My failings are probably due to using pointers wrongly or not fully understanding the way in which WSABUF structures work.

In the code below "stringtowsa" is supposed to take parameter one (a string) and store it in parameter two (a pointer to a WSABUF structure). It does this, but when cTemp is deleted so is the buffer.

I think that the problem is with "Return->buf = cTemp;" as this may be setting the buf pointer to point to cTemp rather than setting it to equal what is in cTemp. If this is the case, how do I set it to equal what is in cTemp? I tried but kept getting errors.

This brings me onto my next question; why do I need to set the WSABUF to equal a character array before loading it with data from the string in order to avoid an error?

+ Code Snippet

#include <winsock2.h>
#include <ws2tcpip.h>
#include <iostream>

using namespace std;

// Converts string into WSA.
void stringtowsa(string String, WSABUF * Return)
{
	Return->len = String.length();
	char * cTemp = new char[String.length()];

	// **Problem is here**
	Return->buf = cTemp;

	for(unsigned int n = 0;n<String.length();n++)
	{
		Return->buf[n] = String[n];
	}

	// Note: If cTemp memory is deleted then Buf3.buf holds no data.
	//delete[] cTemp;
}

void main(void)
{
	WSABUF Buf3;
	stringtowsa("Hello",&Buf3);
	cout << "Buf3: "<<Buf3.buf<<"n";

	// Wait for input and then exit.
	int Blank;
	cin >> Blank;
	return;
}

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Lilith

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Location: Dallas, TX

Posted: 3rd Jul 2008 20:12 Edited at: 3rd Jul 2008 20:15

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Why not assign Return->buf directly rather than using cTemp

Lilith, Night Butterfly
I'm not a programmer but I play one in the office

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Michael P

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Location: London (UK)

Posted: 3rd Jul 2008 20:15

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I forgot to mention why I'm copying each character individually rather than doing it all at once using the = sign. I found that only the first character is copied (not sure why) in the below code. Comparing in debug mode I see 8 bytes for the string and only 1 for the WSABUF:

#include <winsock2.h>
#include <ws2tcpip.h>
#include <iostream>

using namespace std;

// Converts string into WSA.
void stringtowsa(string String, WSABUF * Return)
{
	Return->len = String.length();
	char * cTemp = new char[String.length()];

// **Problem is here**
	Return->buf = cTemp;

for(unsigned int n = 0;n<String.length();n++)
	{
		Return->buf[n] = String[n];
	}

// Note: If cTemp memory is deleted then Buf3.buf holds no data.
	//delete[] cTemp;
}

// Adds an integer to Buffer.
// Note: for use by system.
void _AddInt(int Add, string * Buffer)
{
	// Get the position to start adding. 
	int iStart = Buffer->length();

// Get the size in bytes of what we are adding.
	int iSize = sizeof(Add);

// Get the new end position after adding.
	int iEnd = iStart+iSize;

// Make space for 'Add'.
	Buffer->resize(iEnd);

// Add each byte of 'Add' to the buffer.
	*(int*)&Buffer->at(iStart) = Add;
}

void main(void)
{
	string String;
	_AddInt(1,&String);
	_AddInt(2,&String);

WSABUF Buf3;
	Buf3.len = String.length();
	Buf3.buf = (char*)String.c_str();
	
	cout << "Buf3: "<<Buf3.buf<<"\n";

// Wait for input and then exit.
	int Blank;
	cin >> Blank;
	return;
}

+ Code Snippet

#include <winsock2.h>
#include <ws2tcpip.h>
#include <iostream>

using namespace std;

// Converts string into WSA.
void stringtowsa(string String, WSABUF * Return)
{
	Return->len = String.length();
	char * cTemp = new char[String.length()];

	// **Problem is here**
	Return->buf = cTemp;

	for(unsigned int n = 0;n<String.length();n++)
	{
		Return->buf[n] = String[n];
	}

	// Note: If cTemp memory is deleted then Buf3.buf holds no data.
	//delete[] cTemp;
}

// Adds an integer to Buffer.
// Note: for use by system.
void _AddInt(int Add, string * Buffer)
{
	// Get the position to start adding. 
	int iStart = Buffer->length();

	// Get the size in bytes of what we are adding.
	int iSize = sizeof(Add);

	// Get the new end position after adding.
	int iEnd = iStart+iSize;

	// Make space for 'Add'.
	Buffer->resize(iEnd);

	// Add each byte of 'Add' to the buffer.
	*(int*)&Buffer->at(iStart) = Add;
}

void main(void)
{
	string String;
	_AddInt(1,&String);
	_AddInt(2,&String);

	WSABUF Buf3;
	Buf3.len = String.length();
	Buf3.buf = (char*)String.c_str();
	
	cout << "Buf3: "<<Buf3.buf<<"\n";

	// Wait for input and then exit.
	int Blank;
	cin >> Blank;
	return;
}

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Michael P

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Joined: 6th Mar 2006

Location: London (UK)

Posted: 3rd Jul 2008 20:19

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Quote: "Why not assign Return->buf directly rather than using cTemp"

If I remove the cTemp code and assign directly I get access violations (I don't know why):

+ Code Snippet

#include <winsock2.h>
#include <ws2tcpip.h>
#include <iostream>

using namespace std;

// Converts string into WSA.
void stringtowsa(string String, WSABUF * Return)
{
	Return->len = String.length();
	//char * cTemp = new char[String.length()];

	// **Problem is here**
	//Return->buf = cTemp;

	for(unsigned int n = 0;n<String.length();n++)
	{
		Return->buf[n] = String[n];
	}

	// Note: If cTemp memory is deleted then Buf3.buf holds no data.
	//delete[] cTemp;
}

void main(void)
{
	WSABUF Buf3;
	stringtowsa("Hello",&Buf3);
	cout << "Buf3: "<<Buf3.buf<<"n";

	// Wait for input and then exit.
	int Blank;
	cin >> Blank;
	return;
}

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Benjamin

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Location: France

Posted: 3rd Jul 2008 20:19 Edited at: 3rd Jul 2008 20:20

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All you're doing is pointing buf to the same place as cTemp, so when you free that memory you can't use it anymore. Basically, don't free the memory at the end of the function. 'buf' is simply a pointer, you have to allocate memory for it yourself.

Multisync - TCP Server/Client Multiplayer Plugin (DBP/DBCe)

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Lilith

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Location: Dallas, TX

Posted: 3rd Jul 2008 20:29

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But if he assigns the memory using new he has to free it before exiting the function, otherwise the next time the function is called it will reallocate new space and create a memory leak.

As you said, you have to allocate the buffer space statically and pass the address of that buffer to Return->buf. That pretty much means either a global declaration or the address needs to be passed up the function chain.

Lilith, Night Butterfly
I'm not a programmer but I play one in the office

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Michael P

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Location: London (UK)

Posted: 3rd Jul 2008 20:38 Edited at: 3rd Jul 2008 20:41

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Quote: "buf' is simply a pointer, you have to allocate memory for it yourself."

I came to that conclusion too but just to be sure I tested it.

In the below code:
Surely both WSABUF should be equal to the Chars variable since they were allocated the same memory; both variables appear to have different memory unlinked to Chars.

+ Code Snippet

#include <winsock2.h>
#include <ws2tcpip.h>
#include <iostream>

using namespace std;

void main(void)
{
	char Chars[4];

	WSABUF Buf1;
	WSABUF Buf2;

	Buf1.len = 4;
	Buf2.len = 4;

	Buf1.buf = Chars;
	Buf2.buf = Chars;

	Buf1.buf = "Buf1";
	Buf2.buf = "Buf2";
	Chars[0] = 'C';

	cout << "Buf1: "<<Buf1.buf<<",   Buf2: "<<Buf2.buf<<",   Chars: "<<Chars<<".nn";

	// Wait for input and then exit.
	int Blank;
	cin >> Blank;
	return;
}

[edit]
Without these two lines:
Buf1.buf = "Buf1";
Buf2.buf = "Buf2";

they are linked with Chars but with them they are not, so this means that it is possible to get the WSABUF structure to allocate its own memory?

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Benjamin

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Posted: 3rd Jul 2008 20:44 Edited at: 3rd Jul 2008 20:53

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Quote: " Buf1.buf = Chars;
Buf2.buf = Chars;

Buf1.buf = "Buf1";
Buf2.buf = "Buf2";
"

You do realise you're just assigning both buffers twice? First you're pointing them to 'Chars', then you're pointing them to two different string constants.

It's not possible to get the WSABUF structure to allocate its own memory, no. You could write your own class that automatically handles memory allocation/deletion though.

Multisync - TCP Server/Client Multiplayer Plugin (DBP/DBCe)

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Michael P

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Posted: 3rd Jul 2008 21:07

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Quote: "You do realise you're just assigning both buffers twice?"