Append to a User Environment Variable with RegSetValueEx() in C++ - c++

Question

I want to append a new path to an existing environment variable with c++. This should be for the current user (if you can provide an example for the Local_Machine, that will also be acceptable).
The path(new) and the variable(already existing) are
variable = Cur
path = E:\Softwares\Setup
What i have tried
void SetUserVariablePath(){
HKEY hkey;
long regOpenResult;
const char key_name[] = "Environment";
const char path[]="E:\\Softwares\\Setup;"; //new path
LPCSTR stuff = "Cur"; //Variable Name
// here we open the variable and set the value
regOpenResult = RegOpenKeyEx(HKEY_CURRENT_USER,key_name, 0, KEY_ALL_ACCESS, &hkey);
RegSetValueEx(hkey,stuff,0,REG_EXPAND_SZ,(BYTE*) path, strlen(path)+1);
// tell other process to update their env values
SendMessageTimeout(HWND_BROADCAST, WM_SETTINGCHANGE, 0, (LPARAM)"Environment", SMTO_BLOCK, 100, NULL);
// close the registery key
RegCloseKey(hkey);
}
I have followed how-to-add-environment-variable-in-c for the above code.The code does work and sets the environment variable for the current user but it also over-rides any existing paths already present, What i want to do is to append to the existing paths.
Things that dont answer my question
I am not taking about setting the variable for the current/child process(that can be done with setenv() or _putenv() function).
I have already looked at these questions and they don't answer my question. Update system environment variable from c++ and Set system variable from C++

Answer 1

I followed up to #john recommendations and got the working code as follows
void appendReg()
{
// commons
static const char sysVar[] = "Cur" ; // Variable Name
const char key_name[] = "Environment";
// first read the current value of registery
static BYTE curRegVal[1000000]; // will store the reg value
DWORD curRegValCP = sizeof(curRegVal) ;
HKEY readKey;
RegOpenKeyExA( HKEY_CURRENT_USER, key_name, 0, KEY_QUERY_VALUE, &readKey);
RegQueryValueExA( readKey, sysVar, nullptr, nullptr, curRegVal, &curRegValCP);
RegCloseKey(readKey);
// curRegVal now has the current reg value
std::cout<<"\nCurrent Reg value is = > "<< reinterpret_cast<const char*>(curRegVal);
// second append the new path value to current value
const char appendedVal[1000000]="C:\\New\\Dir\\To\\Be\\Appended;"; // to be appended
strcat(appendedVal,curRegVal); // concatenate the current and new values
std::cout<<"\nAppended Reg value is => "<<appendedVal;
// third set the new(appended) value for registery and broadcast changes to other processes
HKEY writeKey;
RegOpenKeyEx(HKEY_CURRENT_USER,key_name, 0, KEY_ALL_ACCESS, &writeKey);
RegSetValueEx(writeKey,(LPCSTR)sysVar,0,REG_EXPAND_SZ,(BYTE*) appendedVal, strlen(appendedVal)+1);
SendMessageTimeout(HWND_BROADCAST, WM_SETTINGCHANGE, 0, (LPARAM)key_name, SMTO_BLOCK, 100, NULL); // tell other process to update their env values
RegCloseKey(writeKey);
}
This will append the new path to environment variable of current user.