ability to pass custom environments to processes

[graphitemaster]

would be nice if you could pass custom environments to the processes by passing a list of environment variable strings or NULL to inherit the current processes environment.

[sheredom]

Yeah its on my TODO list (in my head) - worry not!

[sheredom]

Quick question - it's not thread safe to inherit the environment of the parent and have the child process append to the environment of the parent process's environment.

Would it be ok for your usecases that the child process would not inherit the parent's environment, and instead if you wanted that as a user you'd have to pass along the environment variables yourself?

[graphitemaster]

That isn't the approach you would do. The process construction methods themselves allow you to pass an environment list, when you pass NULL they inherit by default the parent's environment. On windows this would be done by creating an environment block which is just a single block of string separated by null terminator characters. The end of the block must be an empty string (so the block end has two null termination characters.). On POSIX systems there's two ways, the easy way and the correct way, the easy way would be to just pass everything through /bin/env as a string piping it to the actual process you create with execl. The correct way however is to just specify a list of strings (terminated by an empty string) and use execvpe instead. You do not have to call any of the setenv/putenv/etc functions here.

[sheredom]

Right but the issue is that if you specify any environment variables at all then any environment variables that were set in the parent are not passed down.

I'm thinking of the case where you want to spawn a process but specify MY_SPECIAL_ENV_VAR but also be able to read the PATH variable that was in the parent's context - the ways in which you do this in Wiindows/Linux are both inherently thread unsafe (you basically have to change the env of the parent process, spawn the child, then reset the env of the parent from my experience!).

[graphitemaster]

That isn't a useful operation. You can always read the parents's environment and copy them when you make your list before spawning the child process. This is actually required on Windows because Window's environment block does not permit you to override an environment variable that is already in the block. If you have FOO already and then supply your own FOO, it won't override. You have to meticulously hand craft the block not to contain duplicates. So it's not a 'safe' operation to blindly inherit the parent's environment and also accept user supplied ones that could conflict with what is already there. It's all or none as far as I understand. You eiher have all the parent's environment block upon child process construction or none of it.

[sheredom]

Gotya! Glad we had some back and forth on this, I'm pretty sure we're on the same page now 😄

I think I'll add some option flags var that controls whether you inherit the parents environment or not - at present we are always inheriting which doesn't seem great!

[sheredom]

As the first part of this, I've created an MR that allows you to specify whether the parent environment should be inherited or not https://github.com/sheredom/process.h/pull/6.

I'll then do a follow-up for setting individual environment variables.

[ha11owed]

I also needed this feature on Linux and created a quick an dirty way to pass the env variables. The environment variables are ignored when the inherit option is set.

The approach that I took can be found here: https://github.com/ha11owed/subprocess.h/commit/c95cf4e8a1910f956bded48f4d726d247a441c40

As a note: I was also considering creating a dedicated struct for all the parameters that need to be passed (to allow further extensibility in the future, for example should a current dir option also be added). The MSVC part is missing, as I do not have access to a windows system right now.