Implementation of a function that can change Environment Variable in Windows (actually) by modifying Registry Keys.

change_environment_variables.c

#inlclude <windows.h>
#include <stdio.h>
// function to set a environment variable globally by editing the registry keys.
NTSTATUS SetEnvironmentVariableR(LPCSTR lpName, LPCSTR lpValue){
    HKEY hKey; // this will hold the handle to the registry key.
    
    // We are going to open the registry key for HKEY_CURRENT_USER\Environment.
    RegCreateKey(HKEY_CURRENT_USER, "Environment", &hKey);
    
    // Error check.
    if (hKey == NULL) {
        printf("RegCreateKey failed\n");
        printf("Error code: %d", GetLastError());
        return STATUS_UNSUCCESSFUL;
    }

    // Now we are going to set the value of the environment variable.
    RegSetValueEx(hKey, lpName, 0, REG_SZ, lpValue, lstrlen(lpValue)+1);

    // Checking if the operation was successful by checking if the registry key actually exists.
    if (!RegGetValueA(hKey, "Environment", lpValue, RRF_RT_REG_SZ, NULL, NULL, NULL)){
        printf("RegSetValueEx unsuccessful!");
        printf("Error code: %d", GetLastError());
        return STATUS_UNSUCCESSFUL;
    }
    
    return STATUS_SUCCESS;
}