.TH ERRSTR 3
.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
.MR print (3)
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.
.PP
The error string is maintained in parallel with the Unix
error number
.IR errno .
Changing
.I errno
will reset the error string,
and changing the error string via
.I errstr
or
.I werrstr
will reset
.IR errno .
.SH SOURCE
.B \*9/src/lib9/errstr.c
.SH DIAGNOSTICS
.I Errstr
always returns 0.
.SH SEE ALSO
.MR intro (3) ,
.MR perror (3)
.SH BUGS
The implementation sets
.I errno
to the (somewhat arbitrary) 
constant 0x19283745 when
the error string is valid.
When
.I errno
is set to other values, the error string
is synthesized using
.MR strerror (3) .