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36 .\" @(#)strerror.3 8.1 (Berkeley) 6/9/93
37 .\" $FreeBSD: src/lib/libc/string/strerror.3,v 1.7.2.7 2003/01/17 13:39:50 mike Exp $
38 .\" $DragonFly: src/lib/libc/string/strerror.3,v 1.2 2003/06/17 04:26:46 dillon Exp $
39 .\"
40 .Dd December 19, 2002
41 .Dt STRERROR 3
42 .Os
43 .Sh NAME
44 .Nm perror ,
45 .Nm strerror ,
46 .Nm strerror_r ,
47 .Nm sys_errlist ,
48 .Nm sys_nerr
49 .Nd system error messages
50 .Sh LIBRARY
51 .Lb libc
52 .Sh SYNOPSIS
53 .In stdio.h
54 .Ft void
55 .Fn perror "const char *string"
56 .Vt extern const char * const sys_errlist[] ;
57 .Vt extern const int sys_nerr ;
58 .In string.h
59 .Ft "char *"
60 .Fn strerror "int errnum"
61 .Ft int
62 .Fn strerror_r "int errnum" "char *strerrbuf" "size_t buflen"
63 .Sh DESCRIPTION
64 The
65 .Fn strerror ,
66 .Fn strerror_r
67 and
68 .Fn perror
69 functions look up the error message string corresponding to an
70 error number.
71 .Pp
72 The
73 .Fn strerror
74 function accepts an error number argument
75 .Fa errnum
76 and returns a pointer to the corresponding
77 message string.
78 .Pp
79 The
80 .Fn strerror_r
81 function renders the same result into
82 .Fa strerrbuf
83 for a maximum of
84 .Fa buflen
85 characters and returns 0 upon success.
86 .Pp
87 The
88 .Fn perror
89 function finds the error message corresponding to the current
90 value of the global variable
91 .Va errno
92 .Pq Xr intro 2
93 and writes it, followed by a newline, to the
94 standard error file descriptor.
95 If the argument
96 .Fa string
97 is
98 .Pf non- Dv NULL
99 and does not point to the null character,
100 this string is prepended to the message
101 string and separated from it by
102 a colon and space
103 .Pq Dq Li ":\ " ;
104 otherwise, only the error message string is printed.
105 .Pp
106 If
107 .Fa errnum
108 is not a recognized error number,
109 .Fn strerror
110 returns an error message string containing
111 .Dq Li "Unknown error:\ "
112 followed by the error number in decimal, while
113 .Fn strerror_r
114 leaves
115 .Fa strerrbuf
116 unchanged and returns
117 .Er EINVAL .
118 Error numbers recognized by this implementation fall in
119 the range 0 <
120 .Fa errnum
121 <
122 .Fa sys_nerr .
123 .Pp
124 If insufficient storage is provided in
125 .Fa strerrbuf
126 (as specified in
127 .Fa buflen )
128 to contain the error string,
129 .Fn strerror_r
130 returns
131 .Er ERANGE
132 and
133 .Fa strerrbuf
134 will contain an error message that has been truncated and
135 .Dv NUL
136 terminated to fit the length specified by
137 .Fa buflen .
138 .Pp
139 The message strings can be accessed directly using the external
140 array
141 .Va sys_errlist .
142 The external value
143 .Va sys_nerr
144 contains a count of the messages in
145 .Va sys_errlist .
146 The use of these variables is deprecated;
147 .Fn strerror
148 or
149 .Fn strerror_r
150 should be used instead.
151 .Sh SEE ALSO
152 .Xr intro 2 ,
153 .Xr psignal 3
154 .Sh STANDARDS
155 The
156 .Fn perror
157 and
158 .Fn strerror
159 functions conform to
160 .St -isoC-99 .
161 The
162 .Fn strerror_r
163 function conforms to
164 .St -p1003.1-2001 .
165 .Sh HISTORY
166 The
167 .Fn strerror
168 and
169 .Fn perror
170 functions first appeared in
171 .Bx 4.4 .
172 The
173 .Fn strerror_r
174 function was implemented in
175 .Fx 4.4
176 by
177 .An Wes Peters Aq wes@FreeBSD.org .
178 .Sh BUGS
179 For unknown error numbers, the
180 .Fn strerror
181 function will return its result in a static buffer which
182 may be overwritten by subsequent calls.
183 .Pp
184 The return type for
185 .Fn strerror
186 is missing a type-qualifier; it should actually be
187 .Vt const char * .
188 .Pp
189 Programs that use the deprecated
190 .Va sys_errlist
191 variable often fail to compile because they declare it
192 inconsistently.