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Windows System Programming: Working with Windows Registries

Gurkirat Singh
Yet another noob script kiddie and non TPP registered failure :)
・3 min read

Hi, today I will show you how to create and set values in the windows registries. But first of all, let me introduce to you "what are registries windows systems"

A registry is centralised storage of application / system configurations. The data structure used by registry hives is known as graph, which means it's a hierarchical data structure where each key in the registry can have one or more values. You can read more about it from here

There are different types of registry keys on top level

  • HKEY_LOCAL_MACHINE → It is also known as HKLM. It stores physical information about the system and installed softwares.
  • HKEY_USERS → It stores user level configurations
  • HKEY_CURRENT_CONFIG → It contains current settings of users such as fonts and resolutions
  • HKEY_CLASSES_ROOT → It contains mapping information of file extensions with classes.
  • HKEY_CURRENT_USER → It contains users specific configurations.

In this post, I will be using HKEY_CURRENT_USER.

You can use regedit.exe tool to open / search / modify the registries

First of all, you need to create/open the new/existing key in the registry. This can be done by using RegCreateKeyA function declared in winreg.h header file.

LSTATUS RegCreateKeyA(
  HKEY   hKey,
  LPCSTR lpSubKey,
  PHKEY  phkResult
);
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Function parameters definition as follows

  • hKey → A handle to an open registry key or the following predefined keys
    • HKEY_CLASSES_ROOT
    • HKEY_CURRENT_CONFIG
    • HKEY_CURRENT_USER
    • HKEY_LOCAL_MACHINE
    • HKEY_USERS
  • lpSubKey → The name of a key that this function opens or creates. If hKey is one of the predefined keys, lpSubKey may be NULL. In that case, phkResult receives the same hKey handle passed in to the function.
  • phkResult → A pointer to a variable that receives a handle to the opened or created key.

NOTE: They keys are not case sensitive.

Once the function will return ERROR_SUCCESS, you can use the key handle (as passed in third argument) to make set / get values from the registries. So, to set a value you can use RegSetValueA.

LSTATUS RegSetValueA(
  HKEY   hKey,
  LPCSTR lpSubKey,
  DWORD  dwType,
  LPCSTR lpData,
  DWORD  cbData
);
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  • hKey → A handle to an open registry key or the following predefined keys
    • HKEY_CLASSES_ROOT
    • HKEY_CURRENT_CONFIG
    • HKEY_CURRENT_USER
    • HKEY_LOCAL_MACHINE
    • HKEY_USERS
  • lpSubKey → The name of a subkey of the hKey parameter. If lpSubKey does not exist, the function creates it. If this parameter is NULL or points to an empty string, the function sets the default value of the key identified by hKey.
  • dwType → This parameter must be the REG_SZ type. To store other data types, use the RegSetValueExA function.
  • lpData → The data to be stored. This parameter cannot be NULL.
  • cbData → The length of lpData to be written in the registry.

After doing every work or if you get any error make sure you close the key handle. This can be done by RegCloseKey.

LSTATUS RegCloseKey(
  HKEY hKey
);
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It accepts only one parameter, that is the "handle to key".

Complete source code as follows

#include <Windows.h>
#include <string>
#include <iostream>
#include <winreg.h>

int main(int argc, char**argv)
{
    if (argc < 3) {
        printf("usage: %s <key-name> <key-value>\n", argv[0]);
        return 1;
    }

    // create a new or open exisiting key
    HKEY key;
    // https://docs.microsoft.com/en-us/windows/win32/api/winreg/nf-winreg-regcreatekeya
    LSTATUS statKey = RegCreateKeyA(HKEY_CURRENT_USER, argv[1], &key);

    if (statKey != ERROR_SUCCESS) {
        printf("Unable to create key. Error Code: %d", GetLastError());
        // https://docs.microsoft.com/en-us/windows/win32/api/winreg/nf-winreg-regclosekey
        RegCloseKey(key);
        return 1;
    }
    else {
        // https://docs.microsoft.com/en-us/windows/win32/api/winreg/nf-winreg-regsetvaluea
        LSTATUS statVal = RegSetValueA(key, argv[1], REG_SZ, argv[2], strlen(argv[2]));
        if (statVal != ERROR_SUCCESS) {
            printf("Unable to set value to key. Error Code: %d", GetLastError());
            RegCloseKey(key);
            return 1;
        }
        else {
            printf("Check the regedit.exe and verify the value\n");
            printf("You can find the value in Computer\\HKEY_CURRENT_USER\\%s\\%s", argv[1], argv[1]);
            RegCloseKey(key);
            return 0;
        }
    }
}
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