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Re: Changing parent process environment variable

by cdarke (Prior)
on Dec 20, 2007 at 21:30 UTC ( #658278=note: print w/replies, xml ) Need Help??

in reply to 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 ;-)
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Re^2: Changing parent process environment variable
by cdarke (Prior) on Dec 20, 2007 at 21:42 UTC
    If it helps, this is how I have changed the PATH env. var. using some other weird obscure language (OK, C++). Sorry it is not in Perl, I hope this is enough for you to get the gist:
    #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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