libc/string/strerror.3

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.Dd December 17, 2020
.Dt STRERROR 3
.Os
.Sh NAME
.Nm perror ,
.Nm strerror ,
.Nm strerror_l ,
.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"
.Vt extern const char * const sys_errlist[] ;
.Vt extern const int sys_nerr ;
.In string.h
.Ft "char *"
.Fn strerror "int errnum"
.Ft "char *"
.Fn strerror_l "int errnum" "locale_t"
.Ft int
.Fn strerror_r "int errnum" "char *strerrbuf" "size_t buflen"
.Sh DESCRIPTION
The
.Fn strerror ,
.Fn strerror_l ,
.Fn strerror_r ,
and
.Fn perror
functions look up the 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
in the current locale.
.Fn strerror
is not thread-safe.
It returns a pointer to an internal static buffer that could be
overwritten by a
.Fn strerror
call from another thread.
.Pp
The
.Fn strerror_l
function accepts
.Fa errnum
error number and
.Fa locale
locale handle arguments and returns a pointer to a string
corresponding to the specified error in the given locale.
.Fn strerror_l
is thread-safe, its result can be only overwritten by
another call to
.Fn strerror_l
from the current thread.
.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 null 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.
.Pp
If the error number is not recognized, these functions return an error message
string containing
.Dq Li "Unknown error:\ "
followed by the error number in decimal.
The
.Fn strerror
and
.Fn strerror_r
functions return
.Er EINVAL
as a warning.
Error numbers recognized by this implementation fall in
the range 0 <
.Fa errnum
<
.Fa sys_nerr .
The number 0 is also recognized, although applications that take advantage of
this are likely to use unspecified values of
.Va errno .
.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 ,
.Fn strerror_l ,
or
.Fn strerror_r
should be used instead.
.Sh EXAMPLES
The following example shows how to use
.Fn perror
to report an error.
.Bd -literal -offset 2n
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>

int
main(void)
{
	int fd;

	if ((fd = open("/nonexistent", O_RDONLY)) == -1) {
		perror("open()");
		exit(1);
	}
	printf("File descriptor: %d\en", fd);
	return (0);
}
.Ed
.Pp
When executed, the program will print an error message along the lines of
.Ql "open(): No such file or directory" .
.Sh SEE ALSO
.Xr intro 2 ,
.Xr err 3 ,
.Xr psignal 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 .
The
.Fn strerror_l
function conforms to
.St -p1003.1-2008 .
.Sh HISTORY
The
.Fn strerror
and
.Fn perror
functions first appeared in
.Bx 4.4 .
The
.Fn strerror_r
function was implemented in
.Fx 4.4
by
.An Wes Peters Aq Mt wes@FreeBSD.org .
The
.Fn strerror_l
function was added in
.Fx 13.0 .
.Sh BUGS
The
.Fn strerror
function returns its result in a static buffer which
will be overwritten by subsequent calls.
.Pp
Programs that use the deprecated
.Va sys_errlist
variable often fail to compile because they declare it
inconsistently.
Size of the
.Va sys_errlist
object might increase during FreeBSD lifetime,
breaking some ABI stability guarantees.