Question

In my continuing attempt to understand how pseudo-terminals work, I have written a small program to try to run bash.

The problem is, my line-breaking seems to be off. (The shell prompt only appears AFTER I press enter.)

Furthermore, I still cannot properly use ncurses programs, like vi. Can anyone tell me how to setup the pseudo-terminal for this?

My badly written program can be found here, I encourage you to compile it. The operating system is GNU/Linux, thanks.

EDIT: Compile like this: gcc program.c -lutil -o program

EDIT AGAIN: It looks like the issue with weird spacing was due to using printf(), still doesn’t fix the issue with ncurses programs though.

Answer 1

There are several issues in your program. Some are relatively easy to fix – others not so much:

1. forkpty() and its friends come from BSD and are not POSIX compatible. They should be avoided for new programs. From the pty(7) manual page:

   Historically, two pseudoterminal APIs have evolved: BSD and System V. SUSv1 standardized a pseudoterminal API based on the System V API, and this API should be employed in all new programs that use pseudoterminals.

   You should be using posix_openpt() instead. This issue is probably not critical, but you should be aware of it.

2. You are mixing calls to raw system calls (read(), write()) and file-stream (printf(), fgets()) functions. This is a very good way to confuse yourself. In general you should choose one approach and stick with it. In this case, it’s probably best to use the low-level system calls (read(), write()) to avoid any issues that would arise from the presence of the I/O buffers that the C library functions use.

3. You are assuming a line-based paradigm for your terminals, by using printf() and fgets(). This is not always true, especially when dealing with interactive programs like vim.

4. You are assuming a C-style single-byte null-terminated string paradigm. Terminals normally deal with characters and bytes – not strings. And while most character set encodings avoid using a zero byte, not all do so.

5. As a result of (2), (3) and (4) above, you are not using read() and write() correctly. You should be using their return values to determine how many bytes they processed, not string-based functions like strlen().

6. This is the issue that, in my opinion, will be most difficult to solve: You are implicitly assuming that:

   • The terminal (or its driver) is stateless: It is not. Period. There are at least two stateful controls that I suspect are the cause of ncurses-based programs not working correctly: The line mode and the local echo control of the terminal. At least these have to match between the parent/master and the slave terminal in order to avoid various strange artifacts.

   • The control-interface of a terminal can be passed-through just by passing the bytes back and forth: It is not always so. Modern virtual terminals allow for a degree of out-of-band control via ioctl() calls, as described for Linux here.

   The simplest way to deal with this issue is probably to set the parent terminal to raw mode and let the slave pseudo-terminal driver deal with the awkward details.

You may want to have a look at this program which seems to work fine. It comes from the book The Linux Programming Interface and the full source code is here. Disclaimer: I have not read the book, nor am I promoting it – I just found the program using Google.