C - How to check if user has sent text file to stdin

Question

I'm currently working on my school project and wondered if there is any way to tell if user has written <textfile.txt to the terminal.

Like this: ./project <textfile.txt

My project reads all the lines from the text file to stdin and via scanf works with them. But when I don't use the <textfile.txt "thing", it starts without the data. Instead, I'd like it to write an error and stop the program.

Thanks a lot.

Answer 1

You might use isatty(3) to detect if your stdin is a terminal.

You could use fstat(2) on STDIN_FILENO, e.g. to detect redirection or pipelines.

Or (on Linux) readlink(2) /proc/self/fd/0 , see proc(5)

I recommend accepting some program argument to override such an auto-detection. Read this.

Be aware that redirection and globbing is done by the shell, see this.