Hi all.

I'm trying to write a C function that returns a dynamic list of servers. My pointer memory is a bit rusty so I just wanted to check if the code below is correct. Although the output is as expected, i.e. 3 servers with IP & name, and there appears to be no memory leaks (used Visual Leak Detector) I just want to triple check. Note, I'm just interested in knowing if the pointer usage is correct rather than actual logic or error checking. I'm using VS 2013 express. Thanks in advance.

#include "stdafx.h"
#include <stdio.h>
#include <string.h>
#include <stdlib.h> 

struct serverInfo
{
	char serverName[50];
	char serverIP[20];
};

int getServerListTest(serverInfo** pServers);

int _tmain(int argc, _TCHAR* argv[])
{
	serverInfo *serverList = (serverInfo*)calloc(1, sizeof(serverInfo));
	serverInfo **serverListPtr = &serverList;
	int serverCount = getServerListTest(serverListPtr);
	
	for (int i = 0; i < serverCount; i++)
	{
		printf("ip:%s name:%s\n", (*serverListPtr+i)->serverIP, (*serverListPtr+i)->serverName);
	}
	
	free(serverList);
	serverListPtr = NULL;

	getchar();
	return 0;
}

int getServerListTest(serverInfo** pServers)
{
	serverInfo* current = NULL;
	int serverCount= 0;

	char serverNo[5];
	for (int i = 0; i < 3; i++)
	{
		if (serverCount > 0)
		{
			current = (serverInfo*)realloc((*pServers), sizeof(serverInfo) * (serverCount+1));
			if (current)
			{
				pServers = &current;
			}
			else
			{
				return -1;
			}
		}
		_itoa(serverCount+1, serverNo, 10);
		strcpy((*pServers + serverCount)->serverIP, "10.10.100.10");
		strcat((*pServers + serverCount)->serverIP, serverNo);
		strcpy((*pServers + serverCount)->serverName, "Server");
		strcat((*pServers + serverCount)->serverName, serverNo);
		serverCount++;
	}
	
	return serverCount;
}

It's very wrong, there is potential for leak, double free, reading from freed memory and maybe more, because what realloc returns is never written through pServer (just to it's local copy, this is a substantial difference).

There is much much more wrong with this code all around too and it looks as if it's obfuscated on purpose and it's not even proper C...

I voted your answer down FRex because without any concrete examples it is unnecessarily negative.

Saying that it's "obfuscated on purpose" and that "it's not even proper C" is net helpful. Unless you demonstrate how to do it 'properly'.

Just my thoughts.

I am not really an expert in C so I can't offer any advice on how sound/unsound the code is.

However, the code doesn't look awful at all. ;)

I will not demonstrate how to do it properly, because this is not what is being asked here, the question asked if the pointer usage is okay in this sample, and the answer (which I gave already, which you proudly downvoted, while contributing nothing, saying this broken code is okay and admitting you don't even know C well enough to help) is: No, there is potential for many bugs because what realloc returns is never written through pServer pointer, just to its' local copy.

The OP also specified to ignore actual logic and error checking, so I did. But it didn't ask to ignore other issues, like the fact that it's written in a (possibly) very confusing way, or that it's not proper C, so I pointed these two out.

This code may read freed memory.

This code may double free.

This code may leak memory.

This code is not proper standard C - it must be compiled with a C++ compiler (or a C compiler that accepts non standard code), at which point you might as well use safer C++ alternatives instead of juggling pointers.

although those aren't the pointer problems the OP requested comments on.

These aren't the issues we were asked to ignore either, and it's only fair to assume someone who asks for help with pointers is not aware about much more intricate issues with this code like the fact that it won't compile with more strict C compilers (GCC and clang couldn't do it), or with the fact that it's very hard to read at a glance to even semi experienced programmers like myself.

Edit: And the question is even worded: "Is this C code using pointers valid?" So my answer is: no, this C code is not valid, it's not even C code, and its' usage of pointers is one of the few issues.

Damn, 1 little asterisk breaks it. Thanks for pointing that out. Not sure how it worked as expected anyway!

I did try using array notation but couldn't get it to work, so guess I need to read up about that a bit more unless someone is kind enough to show a quick example.

I tagged it as C as I didn't want people suggesting the use of new/delete or vector class etc. I'm using Visual C++ compiler so I confess it may not be *pure C* but for the purposes of this post it served its purpose.

FRex, I'm not sure what you mean by the following points

This code may read freed memory. - I don't access the pointer again after the call to free

This code may double free. - Not sure how\where I'm doing this?

Thanks for the replies and suggestions :)