---
name: "isatty"
section: "3"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/isatty"
---

# isatty(3)

test whether a file descriptor refers to a terminal

## NAME

isatty - test whether a file descriptor refers to a terminal

## LIBRARY

Standard C library (*libc*, *-lc*)

## SYNOPSIS

```
#include <unistd.h>
```

**int isatty(int** *fd***);**

## DESCRIPTION

The **isatty**() function tests whether *fd* is an open file descriptor referring to a terminal.

## RETURN VALUE

**isatty**() returns 1 if *fd* is an open file descriptor referring to a terminal; otherwise 0 is returned, and *errno* is set to indicate the error.

## ERRORS

- **EBADF**

  *fd* is not a valid file descriptor.

- **ENOTTY**

  *fd* refers to a file other than a terminal. On some older kernels, some types of files resulted in the error **EINVAL** in this case (which is a violation of POSIX, which specifies the error **ENOTTY**).

## ATTRIBUTES

For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **isatty**() | Thread safety | MT-Safe |

## STANDARDS

POSIX.1-2008.

## HISTORY

POSIX.1-2001, SVr4, 4.3BSD.

## SEE ALSO

[**fstat**(2)](/fstat/2.md), [**ttyname**(3)](/ttyname/3.md)