Simulate Typing with This C Program

by Jim Hall

I recently created a video demonstration of how to do some work at the command line, but as I tried to record my video, I kept running into problems. I'm just not the kind of person who can type commands at a keyboard and talk about it at the same time. I quickly realized I needed a way to simulate typing, so I could create a "canned" demonstration that I could narrate in my video.

After doing some searching, I couldn't find a command on my distribution that would simulate typing. I wasn't surprised; that's not a common thing people need to do. So instead, I rolled my own program to do it.

Writing a program to simulate typing isn't as difficult as it first might seem. I needed my program to act like the echo command, where it displayed output given as command-line parameters. I added command-line options so I could set a delay between the program "typing" each letter, with an additional delay for spaces and newlines. The program basically did this the following for each character in a given string:

1. Insert a delay.
2. Print the character.
3. Flush the output buffer so it shows up on screen.

First, I needed a way to simulate a delay in typing, such as someone typing slowly, or pausing before typing the next word or pressing Enter. The C function to create a delay is usleep(useconds_t usec). You use usleep() with the number of microseconds you want your program to pause. So if you want to wait one second, you would use usleep(1000000).

Working in microseconds means too many zeroes for me to type, so I wrote a simple wrapper called msleep(int millisec) that does the same thing in milliseconds:

int
msleep (int millisec)
{
  useconds_t usec;
  int ret;


  /* wrapper to usleep() but values in milliseconds instead */


  usec = (useconds_t) millisec *1000;
  ret = usleep (usec);
  return (ret);
}

Next, I needed to push characters to the screen after each delay. Normally, you can use putchar(int char) to send a single character to standard output (such as the screen), but you won't actually see the output until you send a newline. To get around this, you need to flush the output buffer manually. The C function fflush(FILE *stream) will flush an output stream for you. If you put a delay() before each fflush(), it will appear that someone is pausing slightly between typing each character.

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