.TH ERRSTR 2
.SH NAME
errstr, rerrstr, werrstr \- description of last system call error
.SH SYNOPSIS
.B #include <u.h>
.br
.B #include <libc.h>
.PP
.B
int errstr(char *err, uint nerr)
.PP
.B
void rerrstr(char *err, uint nerr)
.PP
.B
void werrstr(char *fmt, ...)
.SH DESCRIPTION
When a system call fails it returns \-1 and
records a null terminated string describing the error in a per-process buffer.
.I Errstr
swaps the contents of that buffer with the contents of the array
.IR err .
.I Errstr
will write at most 
.I nerr
bytes into 
.IR err ;
if the per-process error string does not fit,
it is silently truncated at a UTF character boundary.
The returned string is NUL-terminated.
Usually
.I errstr
will be called with an empty string,
but the exchange property provides a mechanism for
libraries to set the return value for the next call to
.IR errstr .
.PP
The per-process buffer is
.B ERRMAX
bytes long.  Any error string provided by the user will
be truncated at 
.B ERRMAX-1
bytes.
.B ERRMAX
is defined in
.BR <libc.h> .
.PP
If no system call has generated an error since the last call to
.I errstr
with an empty string,
the result is an empty string.
.PP
The verb
.B r
in
.IR print (2)
calls
.I errstr
and outputs the error string.
.PP
.I Rerrstr
reads the error string but does not modify the per-process buffer, so
a subsequent
.I errstr
will recover the same string.
.PP
.I Werrstr
takes a
.I print
style format as its argument and uses it to format
a string to pass to
.IR errstr .
The string returned from
.I errstr
is discarded.
.SH SOURCE
.B /sys/src/libc/9syscall
.br
.B /sys/src/libc/9sys/werrstr.c
.SH DIAGNOSTICS
.I Errstr
always returns 0.
.SH SEE ALSO
.IR intro (2),
.IR perror (2)