isatty() — Test if descriptor represents a terminal

Standards

Standards / Extensions	C or C++	Dependencies
POSIX.1 XPG4 XPG4.2 Single UNIX Specification, Version 3	both

Format

#define _POSIX_SOURCE
#include <unistd.h>

int isatty(int fildes);

General description

Determines if a file descriptor, fildes, is associated with a terminal.

isatty() only works in an environment where either a controlling terminal exists, or stdin and stderr refer to tty devices. Specifically, it does not work in a TSO environment.

Returned value

isatty() returns 1 if the given file descriptor is a terminal, or 0 otherwise.

Special behavior for XPG4

isatty() returns 1 if the given file descriptor is a terminal, or 0 otherwise and sets errno to one of the following values:

Error Code: Description
EBADF: The fildes argument is not a valid open file descriptor.
ENOTTY: The fildes argument is not associated with a terminal.

Example

CELEBI03

/* CELEBI03

   This example determines if a file descriptor is
   associated with a terminal.

 */
#define _POSIX_SOURCE
#include <unistd.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

void check_fd(int fd) {
  printf("fd %d is ", fd);
  if (!isatty(fd))
    printf("NOT ");
  puts("a tty");
}

main() {
  int p[2], fd;
  char fn[]="temp.file";

  if (pipe(p) != 0)
    perror("pipe() error");
  else {
    if ((fd = creat(fn, S_IWUSR)) < 0)
      perror("creat() error");
    else {
      check_fd(0);
      check_fd(fileno(stderr));
      check_fd(p[1]);
      check_fd(fd);
      close(fd);
      unlink(fn);
    }
    close(p[0]);
    close(p[1]);
  }
}

Output