Using the windows registry

Hello all. I have a question about the windows registry. I've been reading the MSDN on the registry functions and I was trying to learn how to use RegQueryValueEx. Here is the code I have to read the default value of HKEY_LOCAL_MACHINE:

Code:

#include <windows.h>
#include <iostream>

using namespace std;

int main ()
{
 LPDWORD retType;
 LPBYTE retDat;

 RegQueryValueEx (HKEY_LOCAL_MACHINE,
                               "",
                               NULL,
                               retType,
                               retDat,
                               (LPDWORD) 20);
 
 cout << retType;
 cout << retDat;

 return 0;
}

The code compiled fine, but when I run it I get a windows system error and the process terminates. Can anyone explain why I may be getting this type of problem from this code?

RegQueryValueEx expects a pointer to a buffer to save the data in, not an uninitialized pointer.
Remember that everything that starts with LP typically means that it wants a pointer to something, it doesn't mean that you SHOULD pass an actual pointer, but the address of a variable.
The same goes for the last argument. You need to pass the address of a DWORD which will get the size of the data. IF you don't need it, you can pass NULL.

Code:

int main ()
{
	DWORD retType;
	BYTE retDat;
	DWORD dwSize = 1;

	RegQueryValueEx(HKEY_LOCAL_MACHINE, "", NULL, &retType, &retDat, &dwSize);
	cout << retType;
	cout << retDat;

	return 0;
}

Note also that the last argument must contain the size of your buffer, which in your case is one, since you aren't using an array.
Since you're reading a string, you need to make sure the size is the length of the string + 1 for the \0 char.

>>BYTE retDat;


This is only one byte long and will (usually) not be enough to hold the defualt value requested, causing a buffer overrun.

I use a char array and cast to the required type (or send in the type explicitly if I know what to expect ie int value requested, I send a pointer to an int variable).

Code:

char retDat[256]={0};//init to empty

RegQueryValueEx(HKEY_LOCAL_MACHINE, "", NULL, &retType,(unsigned char*) &retDat, &dwSize);

More accurately, it will cause the function to return ERROR_MORE_DATA since I told it my buffer was 1 byte long.
However, it's also possible to query the size of the data using

Code:

RegQueryValueEx(HKEY_LOCAL_MACHINE, "", NULL, &retType, NULL, &dwSize);

Then you can allocate a buffer and fetch the data:

Code:

char* pData;
DWORD dwDataSize = 0;
DWORD retType;
RegQueryValueEx(HKEY_LOCAL_MACHINE, "", NULL, &retType, NULL, &dwSize);
pData = malloc(dwSize);
RegQueryValueEx(HKEY_LOCAL_MACHINE, "", NULL, &retType, pData, &dwSize);
/* Do something */
free(pData)

Also, keep in mind that you must first open the registry before you can query the registry as indicated in the basic sample below.

Code:

//Reg.c
#include <stdio.h>
#include <stdlib.h>
#include <windows.h>

#define  myReg "Software\\Microsoft\\Windows NT\\CurrentVersion"

int main(void)
{
	char* pData = NULL;
	DWORD dwSize = 0;
	DWORD retType;
	HKEY myKey;

	if (RegOpenKeyEx (HKEY_LOCAL_MACHINE, myReg, 0, KEY_ALL_ACCESS, &myKey)	!= ERROR_SUCCESS)
	{
		printf ("Could not open key\n");
		return -1;
	}   
	if(RegQueryValueEx(myKey, "BuildLab", NULL, &retType, NULL, &dwSize) != ERROR_SUCCESS)
	{
		printf("Could not query key for data size\n");
		RegCloseKey(myKey);
		return -1;
	}
	pData =  malloc(dwSize * sizeof(char));
	if(!pData)
	{
		printf("Malloc failed\n");
		RegCloseKey(myKey);
		return -1;
	}
	if(RegQueryValueEx(myKey, "BuildLab", NULL, &retType, pData, &dwSize) != ERROR_SUCCESS)
	{
		printf("Could not query key for data\n");
		RegCloseKey(myKey);
		return -1;
	}
	/* Do something */
	printf("%s\n", pData);
	free(pData);
	RegCloseKey(myKey);
	return 0;
}