man3/perror.3

   1 '\" t
   2 .\" Copyright (c) 1994 Michael Haardt (michael@moria.de), 1994-06-04
   3 .\" Copyright (c) 1995 Michael Haardt
   4 .\"      (michael@cantor.informatik.rwth-aachen.de), 1995-03-16
   5 .\" Copyright (c) 1996 Andries Brouwer (aeb@cwi.nl), 1996-01-13
   6 .\"
   7 .\" SPDX-License-Identifier: GPL-2.0-or-later
   8 .\"
   9 .\" 1996-01-13 aeb: merged in some text contributed by Melvin Smith
  10 .\"   (msmith@falcon.mercer.peachnet.edu) and various other changes.
  11 .\" Modified 1996-05-16 by Martin Schulze (joey@infodrom.north.de)
  12 .\"
  13 .TH perror 3 (date) "Linux man-pages (unreleased)"
  14 .SH NAME
  15 perror \- print a system error message
  16 .SH LIBRARY
  17 Standard C library
  18 .RI ( libc ", " \-lc )
  19 .SH SYNOPSIS
  20 .nf
  21 .B #include <stdio.h>
  22 .PP
  23 .BI "void perror(const char *" s );
  24 .PP
  25 .B #include <errno.h>
  26 .PP
  27 .BI "int " errno ";       \fR/* Not really declared this way; see errno(3) */"
  28 .PP
  29 .BI "[[deprecated]] const char *const " sys_errlist [];
  30 .BI "[[deprecated]] int " sys_nerr ;
  31 .fi
  32 .PP
  33 .RS -4
  34 Feature Test Macro Requirements for glibc (see
  35 .BR feature_test_macros (7)):
  36 .RE
  37 .PP
  38 .IR sys_errlist ,
  39 .IR sys_nerr :
  40 .nf
  41     From glibc 2.19 to glibc 2.31:
  42         _DEFAULT_SOURCE
  43     glibc 2.19 and earlier:
  44         _BSD_SOURCE
  45 .fi
  46 .SH DESCRIPTION
  47 The
  48 .BR perror ()
  49 function produces a message on standard error describing the last
  50 error encountered during a call to a system or library function.
  51 .PP
  52 First (if
  53 .I s
  54 is not NULL and
  55 .I *s
  56 is not a null byte (\[aq]\e0\[aq])), the argument string
  57 .I s
  58 is printed, followed by a colon and a blank.
  59 Then an error message corresponding to the current value of
  60 .I errno
  61 and a new-line.
  62 .PP
  63 To be of most use, the argument string should include the name
  64 of the function that incurred the error.
  65 .PP
  66 The global error list
  67 .IR sys_errlist "[],"
  68 which can be indexed by
  69 .IR errno ,
  70 can be used to obtain the error message without the newline.
  71 The largest message number provided in the table is
  72 .IR sys_nerr "\-1."
  73 Be careful when directly accessing this list, because new error values
  74 may not have been added to
  75 .IR sys_errlist "[]."
  76 The use of
  77 .IR sys_errlist "[]"
  78 is nowadays deprecated; use
  79 .BR strerror (3)
  80 instead.
  81 .PP
  82 When a system call fails, it usually returns \-1 and sets the
  83 variable
  84 .I errno
  85 to a value describing what went wrong.
  86 (These values can be found in
  87 .IR <errno.h> .)
  88 Many library functions do likewise.
  89 The function
  90 .BR perror ()
  91 serves to translate this error code into human-readable form.
  92 Note that
  93 .I errno
  94 is undefined after a successful system call or library function call:
  95 this call may well change this variable, even though it succeeds,
  96 for example because it internally used some other library function that failed.
  97 Thus, if a failing call is not immediately followed by a call to
  98 .BR perror (),
  99 the value of
 100 .I errno
 101 should be saved.
 102 .SH ATTRIBUTES
 103 For an explanation of the terms used in this section, see
 104 .BR attributes (7).
 105 .ad l
 106 .nh
 107 .TS
 108 allbox;
 109 lbx lb lb
 110 l l l.
 111 Interface       Attribute       Value
 112 T{
 113 .BR perror ()
 114 T}      Thread safety   MT-Safe race:stderr
 115 .TE
 116 .hy
 117 .ad
 118 .sp 1
 119 .SH STANDARDS
 120 .TP
 121 .I errno
 122 .TQ
 123 .BR perror ()
 124 C11, POSIX.1-2008.
 125 .TP
 126 .I sys_nerr
 127 .TQ
 128 .I sys_errlist
 129 BSD.
 130 .SH HISTORY
 131 .TP
 132 .I errno
 133 .TQ
 134 .BR perror ()
 135 POSIX.1-2001, C89, 4.3BSD.
 136 .TP
 137 .I sys_nerr
 138 .TQ
 139 .I sys_errlist
 140 Removed in glibc 2.32.
 141 .SH SEE ALSO
 142 .BR err (3),
 143 .BR errno (3),
 144 .BR error (3),
 145 .BR strerror (3)