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34.\"     @(#)strerror.3	8.1 (Berkeley) 6/9/93
35.Dd May 9, 2015
36.Dt STRERROR 3
37.Os
38.Sh NAME
39.Nm perror ,
40.Nm strerror ,
41.Nm strerror_r ,
42.Nm sys_errlist ,
43.Nm sys_nerr
44.Nd system error messages
45.Sh LIBRARY
46.Lb libc
47.Sh SYNOPSIS
48.In stdio.h
49.Ft void
50.Fn perror "const char *string"
51.In errno.h
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 int
58.Fn strerror_r "int errnum" "char *strerrbuf" "size_t buflen"
59.Sh DESCRIPTION
60The
61.Fn strerror ,
62.Fn strerror_r ,
63and
64.Fn perror
65functions look up the language-dependent error message
66string corresponding to an error number.
67.Pp
68The
69.Fn strerror
70function accepts an error number argument
71.Fa errnum
72and returns a pointer to the corresponding
73message string.
74.Pp
75The
76.Fn strerror_r
77function renders the same result into
78.Fa strerrbuf
79for a maximum of
80.Fa buflen
81characters and returns 0 upon success.
82.Pp
83The
84.Fn perror
85function finds the error message corresponding to the current
86value of the global variable
87.Va errno
88.Pq Xr intro 2
89and writes it, followed by a newline, to the
90standard error file descriptor.
91If the argument
92.Fa string
93is
94.Pf non- Dv NULL
95and does not point to the nul character,
96this string is prepended to the message
97string and separated from it by
98a colon and space
99.Pq Dq Li ":\ " ;
100otherwise, only the error message string is printed.
101Note that in most cases the
102.Xr err 3
103and
104.Xr warn 3
105family of functions is preferable to
106.Fn perror ;
107they are more flexible and also print the program name.
108.Pp
109If the error number is not recognized, these functions pass an error message
110string containing
111.Dq Li "Unknown error:\ "
112followed by the error number in decimal.
113To warn about this,
114.Fn strerror
115sets
116.Dv errno
117to
118.Er EINVAL ,
119and
120.Fn strerror_r
121returns
122.Er EINVAL .
123Error numbers recognized by this implementation fall in
124the range 0 \*[Lt]
125.Fa errnum
126\*[Lt]
127.Fa sys_nerr .
128.Pp
129If insufficient storage is provided in
130.Fa strerrbuf
131(as specified in
132.Fa buflen )
133to contain the error string,
134.Fn strerror_r
135returns
136.Er ERANGE
137and
138.Fa strerrbuf
139will contain an error message that has been truncated and
140.Dv NUL
141terminated to fit the length specified by
142.Fa buflen .
143.Pp
144The message strings can be accessed directly using the external
145array
146.Va sys_errlist .
147The external value
148.Va sys_nerr
149contains a count of the messages in
150.Va sys_errlist .
151The use of these variables is deprecated;
152.Fn strerror
153or
154.Fn strerror_r
155should be used instead.
156.Sh SEE ALSO
157.Xr intro 2 ,
158.Xr err 3 ,
159.Xr psignal 3 ,
160.Xr warn 3
161.Sh STANDARDS
162The
163.Fn perror
164and
165.Fn strerror
166functions conform to
167.St -isoC-99 .
168The
169.Fn strerror_r
170function conforms to
171.St -p1003.1-2001 .
172.Sh HISTORY
173The
174.Fn perror
175function first appeared in
176.At v4 .
177The
178.Fn strerror
179function first appeared in
180.Bx 4.3 Reno .
181The
182.Fn strerror_r
183function first appeared in
184.Nx 4.0 .
185.Sh BUGS
186For unknown error numbers, the
187.Fn strerror
188function will return its result in a static buffer which
189may be overwritten by subsequent calls.
190.Pp
191The return type for
192.Fn strerror
193is missing a type-qualifier; it should actually be
194.Vt const char * .
195.Pp
196Programs that use the deprecated
197.Va sys_errlist
198variable often fail to compile because they declare it
199inconsistently.
200