Detect in C if outputting to a terminal

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?

From stackoverflow

• 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
  

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