Question

I am beginning to design a shell application to run within a Linux terminal for a class I am taking.

This, of course, will involve reading variable-length input strings (commands) from the user. I know that I can simply read each command into a buffer of a size that I would consider appropriate, but this has the potential to either a) truncate the command or b) cause a buffer overflow.

If possible, how can way limit the length of user input to the console?

Say, if I set the command length to 3, 123 would be allowed, but if 123 were already present in the input string (before the user has pressed enter) and the user attempted to add 4, no character would print to the console, perhaps even with an ‘error ping’.

I realize that I could design such functionality, but if that is needed, I am not sure where to start to do such a thing.

Either a pre-existing solution or advice on implementing my own solution would be greatly appreciated.

Edit:

I suppose a cheap and easy solution would be to read a command on character at a time until an enter signal is reached or the maximum length is reached. Would problems arise with a solution of this sort?

Answer 1

I have little experience with readline, but here’s what you could try:

• Write a function that checks rl_end (the number of characters in rl_line_buffer)
  • If you want to allow more, just return rl_getc
  • If not, you can use rl_ding
• Set the rl_getc_function to call your function as described above

As a side note, if you do use readline, you don’t need to limit the input at all (the library manages its memory as it goes). Another (simpler) function you might be interested in is getline.