1.\" 2.\" Sun Microsystems, Inc. gratefully acknowledges The Open Group for 3.\" permission to reproduce portions of its copyrighted documentation. 4.\" Original documentation from The Open Group can be obtained online at 5.\" http://www.opengroup.org/bookstore/. 6.\" 7.\" The Institute of Electrical and Electronics Engineers and The Open 8.\" Group, have given us permission to reprint portions of their 9.\" documentation. 10.\" 11.\" In the following statement, the phrase ``this text'' refers to portions 12.\" of the system documentation. 13.\" 14.\" Portions of this text are reprinted and reproduced in electronic form 15.\" in the SunOS Reference Manual, from IEEE Std 1003.1, 2004 Edition, 16.\" Standard for Information Technology -- Portable Operating System 17.\" Interface (POSIX), The Open Group Base Specifications Issue 6, 18.\" Copyright (C) 2001-2004 by the Institute of Electrical and Electronics 19.\" Engineers, Inc and The Open Group. In the event of any discrepancy 20.\" between these versions and the original IEEE and The Open Group 21.\" Standard, the original IEEE and The Open Group Standard is the referee 22.\" document. The original Standard can be obtained online at 23.\" http://www.opengroup.org/unix/online.html. 24.\" 25.\" This notice shall appear on any product containing this material. 26.\" 27.\" The contents of this file are subject to the terms of the 28.\" Common Development and Distribution License (the "License"). 29.\" You may not use this file except in compliance with the License. 30.\" 31.\" You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE 32.\" or http://www.opensolaris.org/os/licensing. 33.\" See the License for the specific language governing permissions 34.\" and limitations under the License. 35.\" 36.\" When distributing Covered Code, include this CDDL HEADER in each 37.\" file and include the License file at usr/src/OPENSOLARIS.LICENSE. 38.\" If applicable, add the following below this CDDL HEADER, with the 39.\" fields enclosed by brackets "[]" replaced with your own identifying 40.\" information: Portions Copyright [yyyy] [name of copyright owner] 41.\" 42.\" 43.\" Portions Copyright (c) 1992, X/Open Company Limited. All Rights Reserved. 44.\" Copyright (c) 2002, Sun Microsystems, Inc. All Rights Reserved. 45.\" Copyright 2022 Oxide Computer Company 46.\" 47.Dd December 1, 2022 48.Dt ISATTY 3C 49.Os 50.Sh NAME 51.Nm isatty 52.Nd test for a terminal device 53.Sh LIBRARY 54.Lb libc 55.Sh SYNOPSIS 56.In unistd.h 57.Ft int 58.Fo isatty 59.Fa "int filedes" 60.Fc 61.Sh DESCRIPTION 62The 63.Fn isatty 64function tests whether 65.Fa fildes , 66an open file descriptor, is associated with a terminal device. 67.Sh RETURN VALUES 68The 69.Fn isatty 70function returns 71.Sy 1 72if 73.Fa fildes 74is associated with a terminal; otherwise it returns 75.Sy 0 76and 77.Va errno 78is set to indicate the error. 79.Sh ERRORS 80The 81.Fn isatty 82function may fail if: 83.Bl -tag -width Er 84.It Er EBADF 85The 86.Fa fildes 87argument is not a valid open file descriptor. 88.It Er ENOTTY 89The 90.Fa fildes 91argument is not associated with a terminal. 92.El 93.Sh USAGE 94The 95.Fn isatty 96function does not necessarily indicate that a human being is available for 97interaction via 98.Fa fildes . 99It is quite possible that non-terminal devices are connected to the 100communications line. 101.Pp 102There are a few portability concerns to be aware of. 103POSIX does not strictly require that the 104.Fn isatty 105function actually set and update 106.Va errno . 107The only portable and reliable thing to do is to check the return value. 108Older versions of this implementation did not set 109.Va errno 110and erroneously preserved 111.Va errno 112when the 113.Fn isatty 114function returned 0. 115In cases where true portability is required, one should set 116.Va errno 117to zero before calling this function to potentially allow for and distinguish 118this case. 119.Sh INTERFACE STABILITY 120.Sy Committed 121.Sh MT-LEVEL 122.Sy MT-Safe 123.Sh SEE ALSO 124.Xr ttyname 3C , 125.Xr attributes 7 , 126.Xr standards 7 127