man3/perror.3

   1 .\" Copyright (c) 1994 Michael Haardt (michael@moria.de), 1994-06-04
   2 .\" Copyright (c) 1995 Michael Haardt
   3 .\"      (michael@cantor.informatik.rwth-aachen.de), 1995-03-16
   4 .\" Copyright (c) 1996 Andries Brouwer (aeb@cwi.nl), 1996-01-13
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  27 .\" 1996-01-13 aeb: merged in some text contributed by Melvin Smith
  28 .\"   (msmith@falcon.mercer.peachnet.edu) and various other changes.
  29 .\" Modified 1996-05-16 by Martin Schulze (joey@infodrom.north.de)
  30 .\"
  31 .TH PERROR 3 2019-03-06 "" "Linux Programmer's Manual"
  32 .SH NAME
  33 perror \- print a system error message
  34 .SH SYNOPSIS
  35 .B #include <stdio.h>
  36 .PP
  37 .BI "void perror(const char *" s );
  38
  39 .B #include <errno.h>
  40 .PP
  41 .BI "const char * const " sys_errlist [];
  42 .br
  43 .BI "int " sys_nerr ;
  44 .br
  45 .BI "int " errno ";       \fR/* Not really declared this way; see errno(3) */"
  46 .PP
  47 .in -4n
  48 Feature Test Macro Requirements for glibc (see
  49 .BR feature_test_macros (7)):
  50 .in
  51 .PP
  52 .IR sys_errlist ,
  53 .IR sys_nerr :
  54     Since glibc 2.19:
  55         _DEFAULT_SOURCE
  56     Glibc 2.19 and earlier:
  57         _BSD_SOURCE
  58 .SH DESCRIPTION
  59 The
  60 .BR perror ()
  61 function produces a message on standard error describing the last
  62 error encountered during a call to a system or library function.
  63 .PP
  64 First (if
  65 .I s
  66 is not NULL and
  67 .I *s
  68 is not a null byte (\(aq\e0\(aq)), the argument string
  69 .I s
  70 is printed, followed by a colon and a blank.
  71 Then an error message corresponding to the current value of
  72 .I errno
  73 and a new-line.
  74 .PP
  75 To be of most use, the argument string should include the name
  76 of the function that incurred the error.
  77 .PP
  78 The global error list
  79 .IR sys_errlist "[],"
  80 which can be indexed by
  81 .IR errno ,
  82 can be used to obtain the error message without the newline.
  83 The largest message number provided in the table is
  84 .IR sys_nerr "\-1."
  85 Be careful when directly accessing this list, because new error values
  86 may not have been added to
  87 .IR sys_errlist "[]."
  88 The use of
  89 .IR sys_errlist "[]"
  90 is nowadays deprecated; use
  91 .BR strerror (3)
  92 instead.
  93 .PP
  94 When a system call fails, it usually returns \-1 and sets the
  95 variable
  96 .I errno
  97 to a value describing what went wrong.
  98 (These values can be found in
  99 .IR <errno.h> .)
 100 Many library functions do likewise.
 101 The function
 102 .BR perror ()
 103 serves to translate this error code into human-readable form.
 104 Note that
 105 .I errno
 106 is undefined after a successful system call or library function call:
 107 this call may well change this variable, even though it succeeds,
 108 for example because it internally used some other library function that failed.
 109 Thus, if a failing call is not immediately followed by a call to
 110 .BR perror (),
 111 the value of
 112 .I errno
 113 should be saved.
 114 .SH ATTRIBUTES
 115 For an explanation of the terms used in this section, see
 116 .BR attributes (7).
 117 .TS
 118 allbox;
 119 lb lb lb
 120 l l l.
 121 Interface       Attribute       Value
 122 T{
 123 .BR perror ()
 124 T}      Thread safety   MT-Safe race:stderr
 125 .TE
 126 .sp 1
 127 .SH CONFORMING TO
 128 .BR perror (),
 129 .IR errno :
 130 POSIX.1-2001, POSIX.1-2008, C89, C99, 4.3BSD.
 131 .PP
 132 The externals
 133 .I sys_nerr
 134 and
 135 .I sys_errlist
 136 derive from BSD, but are not specified in POSIX.1.
 137 .SH NOTES
 138 The externals
 139 .I sys_nerr
 140 and
 141 .I sys_errlist
 142 are defined by glibc, but in
 143 .IR <stdio.h> .
 144 .\" and only when _BSD_SOURCE is defined.
 145 .\" When
 146 .\" .B _GNU_SOURCE
 147 .\" is defined, the symbols
 148 .\" .I _sys_nerr
 149 .\" and
 150 .\" .I _sys_errlist
 151 .\" are provided.
 152 .SH SEE ALSO
 153 .BR err (3),
 154 .BR errno (3),
 155 .BR error (3),
 156 .BR strerror (3)