'\" t
.\" Copyright 2008 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\"
.TH random_r 3 2023-03-30 "Linux man-pages 6.04"
.SH NAME
random_r, srandom_r, initstate_r, setstate_r \- reentrant
random number generator
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.PP
.BI "int random_r(struct random_data *restrict " buf ,
.BI "             int32_t *restrict " result );
.BI "int srandom_r(unsigned int " seed ", struct random_data *" buf );
.PP
.BI "int initstate_r(unsigned int " seed ", \
char " statebuf "[restrict ." statelen ],
.BI "             size_t " statelen ", struct random_data *restrict " buf );
.BI "int setstate_r(char *restrict " statebuf ,
.BI "             struct random_data *restrict " buf );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR random_r (),
.BR srandom_r (),
.BR initstate_r (),
.BR setstate_r ():
.nf
    /* glibc >= 2.19: */ _DEFAULT_SOURCE
        || /* glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
.fi
.SH DESCRIPTION
These functions are the reentrant equivalents
of the functions described in
.BR random (3).
They are suitable for use in multithreaded programs where each thread
needs to obtain an independent, reproducible sequence of random numbers.
.PP
The
.BR random_r ()
function is like
.BR random (3),
except that instead of using state information maintained
in a global variable,
it uses the state information in the argument pointed to by
.IR buf ,
which must have been previously initialized by
.BR initstate_r ().
The generated random number is returned in the argument
.IR result .
.PP
The
.BR srandom_r ()
function is like
.BR srandom (3),
except that it initializes the seed for the random number generator
whose state is maintained in the object pointed to by
.IR buf ,
which must have been previously initialized by
.BR initstate_r (),
instead of the seed associated with the global state variable.
.PP
The
.BR initstate_r ()
function is like
.BR initstate (3)
except that it initializes the state in the object pointed to by
.IR buf ,
rather than initializing the global state variable.
Before calling this function, the
.I buf.state
field must be initialized to NULL.
The
.BR initstate_r ()
function records a pointer to the
.I statebuf
argument inside the structure pointed to by
.IR buf .
Thus,
.I statebuf
should not be deallocated so long as
.I buf
is still in use.
(So,
.I statebuf
should typically be allocated as a static variable,
or allocated on the heap using
.BR malloc (3)
or similar.)
.PP
The
.BR setstate_r ()
function is like
.BR setstate (3)
except that it modifies the state in the object pointed to by
.IR buf ,
rather than modifying the global state variable.
\fIstate\fP must first have been initialized
using
.BR initstate_r ()
or be the result of a previous call of
.BR setstate_r ().
.SH RETURN VALUE
All of these functions return 0 on success.
On error, \-1 is returned, with
.I errno
set to indicate the error.
.SH ERRORS
.TP
.B EINVAL
A state array of less than 8 bytes was specified to
.BR initstate_r ().
.TP
.B EINVAL
The
.I statebuf
or
.I buf
argument to
.BR setstate_r ()
was NULL.
.TP
.B EINVAL
The
.I buf
or
.I result
argument to
.BR random_r ()
was NULL.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR random_r (),
.BR srandom_r (),
.BR initstate_r (),
.BR setstate_r ()
T}	Thread safety	MT-Safe race:buf
.TE
.hy
.ad
.sp 1
.SH STANDARDS
GNU.
.\" These functions appear to be on Tru64, but don't seem to be on
.\" Solaris, HP-UX, or FreeBSD.
.SH BUGS
The
.BR initstate_r ()
interface is confusing.
.\" FIXME . https://sourceware.org/bugzilla/show_bug.cgi?id=3662
It appears that the
.I random_data
type is intended to be opaque,
but the implementation requires the user to either initialize the
.I buf.state
field to NULL or zero out the entire structure before the call.
.SH SEE ALSO
.BR drand48 (3),
.BR rand (3),
.BR random (3)