1.\" Copyright (c) 1980, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" This code is derived from software contributed to Berkeley by 5.\" the American National Standards Committee X3, on Information 6.\" Processing Systems. 7.\" 8.\" Redistribution and use in source and binary forms, with or without 9.\" modification, are permitted provided that the following conditions 10.\" are met: 11.\" 1. Redistributions of source code must retain the above copyright 12.\" notice, this list of conditions and the following disclaimer. 13.\" 2. Redistributions in binary form must reproduce the above copyright 14.\" notice, this list of conditions and the following disclaimer in the 15.\" documentation and/or other materials provided with the distribution. 16.\" 3. Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)strerror.3 8.1 (Berkeley) 6/9/93 33.\" $FreeBSD: head/lib/libc/string/strerror.3 251069 2013-05-28 20:57:40Z emaste $ 34.\" 35.Dd January 30, 2022 36.Dt STRERROR 3 37.Os 38.Sh NAME 39.Nm perror , 40.Nm strerror , 41.Nm strerror_l , 42.Nm strerror_r , 43.Nm sys_errlist , 44.Nm sys_nerr 45.Nd system error messages 46.Sh LIBRARY 47.Lb libc 48.Sh SYNOPSIS 49.In stdio.h 50.Ft void 51.Fn perror "const char *string" 52.Vt extern const char * const sys_errlist[] ; 53.Vt extern const int sys_nerr ; 54.In string.h 55.Ft "char *" 56.Fn strerror "int errnum" 57.Ft "char *" 58.Fn strerror_l "int errnum" "locale_t" 59.Ft int 60.Fn strerror_r "int errnum" "char *strerrbuf" "size_t buflen" 61.Sh DESCRIPTION 62The 63.Fn strerror , 64.Fn strerror_l , 65.Fn strerror_r , 66and 67.Fn perror 68functions look up the error message string corresponding to an 69error number. 70.Pp 71The 72.Fn strerror 73function accepts an error number argument 74.Fa errnum 75and returns a pointer to the corresponding message string 76in the current locale. 77.Fn strerror 78is not thread-safe. 79It returns a pointer to an internal static buffer that could be 80overwritten by a 81.Fn strerror 82call from another thread. 83.Pp 84The 85.Fn strerror_l 86function accepts 87.Fa errnum 88error number and 89.Fa locale 90locale handle arguments and returns a pointer to a string 91corresponding to the specified error in the given locale. 92.Fn strerror_l 93is thread-safe, its result can be only overwritten by 94another call to 95.Fn strerror_l 96from the current thread. 97.Pp 98The 99.Fn strerror_r 100function renders the same result into 101.Fa strerrbuf 102for a maximum of 103.Fa buflen 104characters and returns 0 upon success. 105.Pp 106The 107.Fn perror 108function finds the error message corresponding to the current 109value of the global variable 110.Va errno 111.Pq Xr intro 2 112and writes it, followed by a newline, to the 113standard error file descriptor. 114If the argument 115.Fa string 116is 117.Pf non- Dv NULL 118and does not point to the null character, 119this string is prepended to the message 120string and separated from it by 121a colon and space 122.Pq Dq Li ":\ " ; 123otherwise, only the error message string is printed. 124.Pp 125If the error number is not recognized, these functions return an error message 126string containing 127.Dq Li "Unknown error:\ " 128followed by the error number in decimal. 129The 130.Fn strerror 131and 132.Fn strerror_r 133functions return 134.Er EINVAL 135as a warning. 136Error numbers recognized by this implementation fall in 137the range 0 < 138.Fa errnum 139< 140.Fa sys_nerr . 141The number 0 is also recognized, although applications that take advantage of 142this are likely to use unspecified values of 143.Va errno . 144.Pp 145If insufficient storage is provided in 146.Fa strerrbuf 147(as specified in 148.Fa buflen ) 149to contain the error string, 150.Fn strerror_r 151returns 152.Er ERANGE 153and 154.Fa strerrbuf 155will contain an error message that has been truncated and 156.Dv NUL 157terminated to fit the length specified by 158.Fa buflen . 159.Pp 160The message strings can be accessed directly using the external 161array 162.Va sys_errlist . 163The external value 164.Va sys_nerr 165contains a count of the messages in 166.Va sys_errlist . 167The use of these variables is deprecated; 168.Fn strerror , 169.Fn strerror_l , 170or 171.Fn strerror_r 172should be used instead. 173.Sh SEE ALSO 174.Xr intro 2 , 175.Xr err 3 , 176.Xr psignal 3 177.Sh STANDARDS 178The 179.Fn perror 180and 181.Fn strerror 182functions conform to 183.St -isoC-99 . 184The 185.Fn strerror_r 186function conforms to 187.St -p1003.1-2001 . 188The 189.Fn strerror_l 190function conforms to 191.St -p1003.1-2008 . 192.Sh HISTORY 193The 194.Fn strerror 195and 196.Fn perror 197functions first appeared in 198.Bx 4.4 . 199The 200.Fn strerror_r 201function was implemented in 202.Fx 4.4 203by 204.An Wes Peters Aq Mt wes@FreeBSD.org . 205The 206.Fn strerror_l 207function was added in 208.Fx 13.0 209and ported to 210.Dx 6.3 . 211.Sh BUGS 212The 213.Fn strerror 214function returns its result in a static buffer which 215will be overwritten by subsequent calls. 216.Pp 217Programs that use the deprecated 218.Va sys_errlist 219variable often fail to compile because they declare it 220inconsistently. 221Size of the 222.Va sys_errlist 223object might increase during 224.Dx 225lifetime, breaking some ABI stability guarantees. 226