Stdin takes in values from before my code even starts. How do I prevent this?
Stdin takes in values from before my code even starts. How do I prevent this?
For context, I am using the libraries bevy and print_typewriter.
I noticed that before the program even starts, I am able to type in characters. This is bad, since when I ask for the user's input, the previous characters are included inside of it.
How do I make sure that only the user's input after the program starts, and after I print a question gets in?
In general allowing content to be supplied in advance on stdin is desirable behavior, because it allows a developer to (for example) write applications that work as pipes that can have content queued on the input stream ready for processing.
If that behaviour doesn’t suit your use case and you need to only accept input after a certain point, you could read() and simply ignore/discard the current content of stdin before you write your question/accept the answer from the user.
Thanks! So far it kinda works, but since I'm using print_typewriter, the characters that I'm printing are printed one by one, and user input can slip in between them. I'm not sure how to prevent them from showing up in the first place, and not make them appear in stdin.
Or maybe in this case I shouldn't use the terminal, right?
For an interactive terminal program with the characteristics you want, you need to do two things:
- Flush
stdinbefore using it, similar to what things like sudo do. (Basically, once your program is all started up and ready to go, read everything that's already in there and throw it away.) - Disable the terminal's default echoing behaviour which traces back to when the terminal was a completely separate device possibly half-way around the world from the machine you were logged into on the other side of a slow modem and you didn't want the latency from waiting for the machine on the far end to handle echo. (See this if you want more context.)
Windows and POSIX (Linux, the BSDs, macOS, and basically everything else of note) have different APIs for it. On the Linux side, you want something that wraps the
curseslibrary, which can put your terminal in "raw mode" or some other configuration that operates unbuffered and lacking terminal-side echo. On the windows side, it can either be done by wrapping the Windows APIs directly or by using thepdcurseslibrary.Something like termwiz should do for both... though you'll probably need to reimplement
print_typewriterbut that should be trivial from what I see of its README.- Flush
I’m sure there are ways to suppress output from stdin presenting in the terminal but I couldn’t tell you how to do it without looking it up myself.
The easiest entry point to this problem that I can think of off the top of my head is password input masking (e.g., when you run sudo and type your password, it prevents character output even though the characters are read by the application).
There is almost certainly a much better and more appropriate mechanism to prevent stdin characters from printing directly to the terminal (perhaps some kind of special character? A TTY control option?) but I don’t know it off-hand.