Detect in C if outputting to a terminal

Question

I'm writing a C program for OS X and Linux, and I want to tweak the output based on whether or not it's going to a terminal. I know we've covered how to do this in a shell script, e.g. here:

http://stackoverflow.com/questions/911816/detecting-the-output-stream-type-of-a-shell-script

But how do I do it in a C program?

Answer 1

Use isatty():

$ man isatty
ISATTY(3)                  Linux Programmer's Manual                 ISATTY(3)

NAME
       isatty - does this descriptor refer to a terminal

SYNOPSIS
       #include <unistd.h>

       int isatty(int desc);

DESCRIPTION
       returns  1  if  desc is an open file descriptor connected to a terminal
       and 0 otherwise.

Since stdout is always file descriptor 1, you can do:

if(isatty(1))
    // stdout is a terminal

Answer 2

if (isatty (1))
    fprintf (stdout, "Outputting to a terminal.");
else
    fprintf (stdout, "Not outputting to a terminal.");