How to determine if stderr controls file output?

Question

Is it possible to indicate whether stderr outputs to a file or terminal in a C / C ++ program? I need to display another error message depending on whether the program is called as:

./program

or how:

./program 2 → file

Answer 1

Try using isatty() in the file descriptor:

The isatty() function isatty() file descriptor fd refers to a valid terminal type device.

The fileno() function checks the stream of arguments and returns its integer descriptor.

Note that stderr always in file descriptor 2, so you really don't need fileno() in this particular case.

Answer 2

Yes, you can use isatty(3) to determine if the file descriptor refers to a terminal or to something else (file, etc.). The file descriptor 0 is stdin , 1 is stdout , and 2 is stderr .

 if(isatty(2)) // stderr is a terminal