Re: Changing parent process environment variable

It is not possible for a child process to directly alter the parent's environment block without using some shared memory magic - and that is true whatever the language used, compiled or not.

The environment block is set at process creation time and by default is a copy of the parent's. There is no concept of a "global" environment. All this is true on Windows and UNIX.

However on Windows, environment variables are also stored in the Registry, either in HKEY_CURRENT_USER\Environment (user variables) or HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session Manager\Environment (system variables). So you could use one of the (several) Win32 Perl modules to change the registry.

Now the tricky bit: for the parent to pick-up the change you should send a WM_SETTINGCHANGE message. I don't know how to do that from Perl, but there probably is a way. Look at the Win32:: namespace modules.

There are easier ways to pass a text string between programs ;-)

Comment on Re: Changing parent process environment variable

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Re^2: Changing parent process environment variable
by cdarke (Prior) on Dec 20, 2007 at 21:42 UTC

#define ENVNAME "Path"
#define MAXLEN  1024

static void UpdatePath (char *szDir)
{
   char szPath [MAXLEN]={0};
   DWORD dwReturnValue;

   // Get previous value 
   if ( !GetEnvironmentVariable (ENVNAME, szPath, MAX_PATH) )
      ReportError ("GetEnvironmentVariable", GetLastError());

   // Update path 
   strcat (szPath, ";");
   strcat (szPath, szDir);

   // On W2K we need OSVERSIONINFOEX
   OSVERSIONINFO OsVer;
   OsVer.dwOSVersionInfoSize = sizeof (OsVer);
   GetVersionEx (&OsVer);

   if ( OsVer.dwPlatformId == VER_PLATFORM_WIN32_NT )
   {
      // Write to the user environment variables - 
      // only exists on WNT/W2K!

      HKEY hEnv;
      dwReturnValue = RegOpenKeyEx (HKEY_CURRENT_USER, 
                                    "Environment", 0, 
                                    KEY_SET_VALUE, 
                                    &hEnv) ;
      if ( dwReturnValue != ERROR_SUCCESS )
         ReportError ("RegOpenKeyEx", dwReturnValue);

      dwReturnValue = RegSetValueEx (hEnv, ENVNAME, 0, 
                                     REG_SZ, (BYTE*)szPath, 
                                     strlen(szPath)+1);

      if ( dwReturnValue != ERROR_SUCCESS )
         ReportError ("RegSetValueEx", dwReturnValue);

      RegFlushKey (hEnv);

      if ( hEnv ) RegCloseKey(hEnv);
   }
   
   // Tell the rest of the world (cmd line)

   if ( !SendMessageTimeout(HWND_BROADCAST, WM_SETTINGCHANGE, 0,
                     (LPARAM) "Environment", SMTO_ABORTIFHUNG,
                     5000, &dwReturnValue) )
   {
      ReportError ("SendMessageTimeout", GetLastError());
   }

   // Tell our children 

   if ( !SetEnvironmentVariable (ENVNAME, szPath) )
      ReportError ("SetEnvironmentVariable", GetLastError());

}  // UpdEnv
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