Code change:
<!-- Thank you for filing a bug report! Please provide a short summary of the bug, along with any information you feel relevant to replicating the bug. --I tried this code: App P: App C: Build this in Windows. Since has "windows" subsystem, it will create no console window (i.e. no process). Since is a console app, when the process is created, I expect to see one console window that prints "Hello World" infinitely. App creates its console window but no text is printed. In Windows, spawn() eventually calls to with a , whose  is hard coded to . Additionally, it also with , and set them to the . Since app uses "windows" subsystem, it does not have console. stdout is always . Combining with in causes the unexpected behavior. If I make a call to with same parameters except a "zeroed" , it exhibits the correct behavior. spawn() should only conditionally set the flag if it can obtain those device handles, and never set any of those handles . Like this (pseudo code): <!-- If you're using the stable version of the compiler, you should also check if the bug also exists in the beta or nightly versions. --:
I think this will need a T-libs-api decision as the current documentation for  (and stdin/stderr) is: I would guess that this "inherit by default" is why it's implemented the way it is. But perhaps there's some wiggle room with what it means to inherit handles that don't exist so we can change it. In either case it would be good to have some way to use platform default behaviour here. label -T-libs +T-libs-api
That's exactly my point. If there is nothing to inherit in the first place, just inherit nothing. The current behavior () is simply wrong. I believe fixing it doesn't violate the current specification.
Basically, currently there are only two options when handling the stdout/stderr: inherit or pipe. I think there should be a third option: isolated - make the child handle its console by itself. For example, if user calls and , he basically says "the parent process doesn't care about the child's output". It is the same as the "windows" subsystem case above. In this case, the flag should never be set, regardless if or is called. More importantly, this is de facto the default way of how a process is created in Windows. Defaulting to really makes me confused before reading the internal source code.
Not always. Parent process can still set those handles to something other than . For example, if you launch App P from IntelliJ it actually prints "Hello World!" in a loop but not in the spawned console which is empty, but in IntelliJ internal terminal. returns non invalid handle. This solution would make behaviour different depending on how you launch your app. Also, you can explicitly ask to create new console with even if your app already uses console subsystem. In this case you also likely want to avoid inheriting handles because you want output of the spawned process to be displayed in the new console, not in the existing one. This solution doesn't help with this.
I can't verify your statements since I don't have IntelliJ atm, but even it's true, isn't it IntelliJ setting up the console with maybe some special debugging API? How to you modify the code of P to achieve the same? And realistically speaking, do people actually do that with GUI apps? I think in this case, it's impossible to be 100% sure where the user wants the output at. New option might be the way to go.
Your prediction was dead-on, as it came true in While we can do this, as is proposing, it seems best here that we create a function, or set of functions, that explicitly handle this kind of situation rather than guessing.
Even with new APIs, we still need reasonable default behaviour. And the Command API is documented as inheriting handles by default. However, currently the API neglects the "no handle set" case entirely as that's not really a thing in POSIX or ISO C (well, you can close stdio but that's usually considered a Bad Thing unless you're being very very careful).

Related discussion:
text: let stdin = stdin.to_handle(c::STD_INPUT_HANDLE, &mut pipes.stdin)?; let stdout = stdout.to_handle(c::STD_OUTPUT_HANDLE, &mut pipes.stdout)?; let stderr = stderr.to_handle(c::STD_ERROR_HANDLE, &mut pipes.stderr)?; <del> si.hStdInput = stdin.as_raw_handle(); si.hStdOutput = stdout.as_raw_handle(); si.hStdError = stderr.as_raw_handle(); </del> <ins> let mut si = zeroed_startupinfo(); si.cb = mem::size_of::<c::STARTUPINFO>() as c::DWORD; // If at least one of stdin, stdout or stderr are set (i.e. are non null) // then set the `hStd` fields in `STARTUPINFO`. // Otherwise skip this and allow the OS to apply its default behaviour. // This provides more consistent behaviour between Win7 and Win8+. let is_set = |stdio: &Handle| !stdio.as_raw_handle().is_null(); if is_set(&stderr) || is_set(&stdout) || is_set(&stdin) { si.dwFlags |= c::STARTF_USESTDHANDLES; si.hStdInput = stdin.as_raw_handle(); si.hStdOutput = stdout.as_raw_handle(); si.hStdError = stderr.as_raw_handle(); } </ins> unsafe { cvt(c::CreateProcessW(