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.\"     @(#)strerror.3	8.1 (Berkeley) 6/9/93
.Dd May 9, 2015
.Dt STRERROR 3
.Os
.Sh NAME
.Nm perror ,
.Nm strerror ,
.Nm strerror_r ,
.Nm sys_errlist ,
.Nm sys_nerr
.Nd system error messages
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdio.h
.Ft void
.Fn perror "const char *string"
.In errno.h
.Vt extern const char * const sys_errlist[] ;
.Vt extern const int sys_nerr ;
.In string.h
.Ft "char *"
.Fn strerror "int errnum"
.Ft int
.Fn strerror_r "int errnum" "char *strerrbuf" "size_t buflen"
.Sh DESCRIPTION
The
.Fn strerror ,
.Fn strerror_r ,
and
.Fn perror
functions look up the language-dependent error message
string corresponding to an error number.
.Pp
The
.Fn strerror
function accepts an error number argument
.Fa errnum
and returns a pointer to the corresponding
message string.
.Pp
The
.Fn strerror_r
function renders the same result into
.Fa strerrbuf
for a maximum of
.Fa buflen
characters and returns 0 upon success.
.Pp
The
.Fn perror
function finds the error message corresponding to the current
value of the global variable
.Va errno
.Pq Xr intro 2
and writes it, followed by a newline, to the
standard error file descriptor.
If the argument
.Fa string
is
.Pf non- Dv NULL
and does not point to the nul character,
this string is prepended to the message
string and separated from it by
a colon and space
.Pq Dq Li ":\ " ;
otherwise, only the error message string is printed.
Note that in most cases the
.Xr err 3
and
.Xr warn 3
family of functions is preferable to
.Fn perror ;
they are more flexible and also print the program name.
.Pp
If the error number is not recognized, these functions pass an error message
string containing
.Dq Li "Unknown error:\ "
followed by the error number in decimal.
To warn about this,
.Fn strerror
sets
.Dv errno
to
.Er EINVAL ,
and
.Fn strerror_r
returns
.Er EINVAL .
Error numbers recognized by this implementation fall in
the range 0 <
.Fa errnum
<
.Fa sys_nerr .
.Pp
If insufficient storage is provided in
.Fa strerrbuf
(as specified in
.Fa buflen )
to contain the error string,
.Fn strerror_r
returns
.Er ERANGE
and
.Fa strerrbuf
will contain an error message that has been truncated and
.Dv NUL
terminated to fit the length specified by
.Fa buflen .
.Pp
The message strings can be accessed directly using the external
array
.Va sys_errlist .
The external value
.Va sys_nerr
contains a count of the messages in
.Va sys_errlist .
The use of these variables is deprecated;
.Fn strerror
or
.Fn strerror_r
should be used instead.
.Sh SEE ALSO
.Xr intro 2 ,
.Xr err 3 ,
.Xr psignal 3 ,
.Xr warn 3
.Sh STANDARDS
The
.Fn perror
and
.Fn strerror
functions conform to
.St -isoC-99 .
The
.Fn strerror_r
function conforms to
.St -p1003.1-2001 .
.Sh HISTORY
The
.Fn perror
function first appeared in
.At v4 .
The
.Fn strerror
function first appeared in
.Bx 4.3 Reno .
The
.Fn strerror_r
function first appeared in
.Nx 4.0 .
.Sh BUGS
For unknown error numbers, the
.Fn strerror
function will return its result in a static buffer which
may be overwritten by subsequent calls.
.Pp
The return type for
.Fn strerror
is missing a type-qualifier; it should actually be
.Vt const char * .
.Pp
Programs that use the deprecated
.Va sys_errlist
variable often fail to compile because they declare it
inconsistently.