'\" et
.TH RAND "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
rand,
rand_r,
srand
\(em pseudo-random number generator
.SH SYNOPSIS
.LP
.nf
#include <stdlib.h>
.P
int rand(void);
int rand_r(unsigned *\fIseed\fP);
void srand(unsigned \fIseed\fP);
.fi
.SH DESCRIPTION
For
\fIrand\fR()
and
\fIsrand\fR():
The functionality described on this reference page is aligned with the
ISO\ C standard. Any conflict between the requirements described here and the
ISO\ C standard is unintentional. This volume of POSIX.1\(hy2017 defers to the ISO\ C standard.
.P
The
\fIrand\fR()
function shall compute a sequence of pseudo-random integers in the
range [0,\c
{RAND_MAX}]
with a period of at least 2\u\s-332\s0\d.
.P
The
\fIrand\fR()
function need not be thread-safe.
.P
The
\fIrand_r\fR()
function shall compute a sequence of pseudo-random integers in
the range [0,\c
{RAND_MAX}].
(The value of the
{RAND_MAX}
macro shall be at least 32\|767.)
.P
If
\fIrand_r\fR()
is called with the same initial value for the object pointed to by
.IR seed
and that object is not modified between successive returns and calls to
\fIrand_r\fR(),
the same sequence shall be generated.
.P
The
\fIsrand\fR()
function uses the argument as a seed for a new sequence of
pseudo-random numbers to be returned by subsequent calls to
\fIrand\fR().
If
\fIsrand\fR()
is then called with the same seed value, the sequence of pseudo-random
numbers shall be repeated. If
\fIrand\fR()
is called before any calls to
\fIsrand\fR()
are made, the same sequence shall be generated as when
\fIsrand\fR()
is first called with a seed value of 1.
.P
The implementation shall behave as if no function defined in this volume of POSIX.1\(hy2017
calls
\fIrand\fR()
or
\fIsrand\fR().
.SH "RETURN VALUE"
The
\fIrand\fR()
function shall return the next pseudo-random number in the sequence.
.P
The
\fIrand_r\fR()
function shall return a pseudo-random integer.
.P
The
\fIsrand\fR()
function shall not return a value.
.SH ERRORS
No errors are defined.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.SS "Generating a Pseudo-Random Number Sequence"
.P
The following example demonstrates how to generate a sequence of
pseudo-random numbers.
.sp
.RS 4
.nf

#include <stdio.h>
#include <stdlib.h>
\&...
    long count, i;
    char *keystr;
    int elementlen, len;
    char c;
\&...
/* Initial random number generator. */
    srand(1);
.P
    /* Create keys using only lowercase characters */
    len = 0;
    for (i=0; i<count; i++) {
        while (len < elementlen) {
            c = (char) (rand() % 128);
            if (islower(c))
                keystr[len++] = c;
        }
.P
        keystr[len] = \(aq\e0\(aq;
        printf("%s Element%0*ld\en", keystr, elementlen, i);
        len = 0;
    }
.fi
.P
.RE
.SS "Generating the Same Sequence on Different Machines"
.P
The following code defines a pair of functions that could be
incorporated into applications wishing to ensure that the same sequence
of numbers is generated across different machines.
.sp
.RS 4
.nf

static unsigned long next = 1;
int myrand(void)  /* RAND_MAX assumed to be 32767. */
{
    next = next * 1103515245 + 12345;
    return((unsigned)(next/65536) % 32768);
}
.P
void mysrand(unsigned seed)
{
    next = seed;
}
.fi
.P
.RE
.SH "APPLICATION USAGE"
The
\fIdrand48\fR()
and
\fIrandom\fR()
functions provide much more elaborate pseudo-random number generators.
.P
The limitations on the amount of state that can be carried between one
function call and another mean the
\fIrand_r\fR()
function can never be implemented in a way which satisfies all of the
requirements on a pseudo-random number generator.
.P
These functions should be avoided whenever non-trivial requirements
(including safety) have to be fulfilled.
.SH RATIONALE
The ISO\ C standard
\fIrand\fR()
and
\fIsrand\fR()
functions allow per-process pseudo-random streams shared by all
threads. Those two functions need not change, but there
has to be mutual-exclusion that prevents interference between two
threads concurrently accessing the random number generator.
.P
With regard to
\fIrand\fR(),
there are two different behaviors that may be wanted in a multi-threaded
program:
.IP " 1." 4
A single per-process sequence of pseudo-random numbers that is shared
by all threads that call
\fIrand\fR()
.IP " 2." 4
A different sequence of pseudo-random numbers for each thread that
calls
\fIrand\fR()
.P
This is provided by the modified thread-safe function based on whether
the seed value is global to the entire process or local to each
thread.
.P
This does not address the known deficiencies of the
\fIrand\fR()
function implementations, which have been approached by maintaining
more state. In effect, this specifies new thread-safe forms of a
deficient function.
.SH "FUTURE DIRECTIONS"
The
\fIrand_r\fR()
function may be removed in a future version.
.SH "SEE ALSO"
.IR "\fIdrand48\fR\^(\|)",
.IR "\fIinitstate\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<stdlib.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .