Update system environment variable from c++

Question

Hey folks,

I am currently writing an unmanaged C++ program which works with a system environment variable. I am getting the value with GetEnvironmentVariable(...). Now I have an C# program which may change this variable at any time, e.g. like this:

Environment.SetEnvironmentVariable("CalledPath", System.Windows.Forms.Application.ExecutablePath, EnvironmentVariableTarget.Machine);

The problem is that the C++ program does not update this variable (or its environment block in general) automatically so that I am still working with the old value unless I restart the program which is not really good. Is there a way to update the environment block or preferably another way to read system environment variables?

Thanks in advance, Russo

Answer 1

Look at putenv() - a C-runtime function (or the MS preferred _putenv() and _putenv_s() - for your C++ equivalent for updating.

getenv() and similar reads in an environment string.

K

Answer 2

I believe it's _putenv() on Windows.

Answer 3

You can't. Each process gets a copy of the environment variables of its parent. As it only gets a copy, it can't change the parent's environment, or that of any other running process.

Answer 4

To make a long story short, environment variables won't work dependably as a form of inter-process communication -- you really need to switch to something else.

To work, both programs would need access to some common block of environment variables -- but in reality, each process gets its own, independent copy of a set of environment variables. Worse, most typical (C and C++) standard libraries don't let you even work with that directly -- instead, they make another copy of the environment variables for you to work with. When/if you call getenv() or _putenv(), only the program's internal copy of the environment variable block is involved. This means even if you could change a process' environment variables, the program running in that process still wouldn't get the new data.

So, you really need to re-think what you're doing. There are lots of possibilities for sending data from one program to another, including a Windows message (E.g. WM_COPYDATA), an anonymous or named pipe, a mailslot, a shared memory region, a socket, etc. The list is long, but still doesn't include environment variables.

Answer 5

Thank you guys but I finally figured it out myself. Since the values I receive with GetEnvironmentVariable are not the current ones I read the values directly from the registry. The machine environment variables are stored in

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Environment

I read them via the RegOpenKeyEx(...) and RegQueryValueEx(...) functions which works perfectly well.