man-pages/man3p/rand.3p

.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "RAND" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" rand 
.SH NAME
rand, rand_r, srand \- pseudo-random number generator
.SH SYNOPSIS
.LP
\fB#include <stdlib.h>
.br
.sp
int rand(void);
.br
\fP
.LP
\fBint rand_r(unsigned *\fP\fIseed\fP\fB); \fP
\fB
.br
void srand(unsigned\fP \fIseed\fP\fB);
.br
\fP
.SH DESCRIPTION
.LP
The \fIrand\fP() function shall compute a sequence of pseudo-random
integers in the range [0, {RAND_MAX}]   \ with a
period of at least 2**32. 
.LP
The
\fIrand\fP() function need not be reentrant. A function that is not
required to be reentrant is not required to be thread-safe.
.LP
The \fIrand_r\fP() function shall compute a sequence of pseudo-random
integers in the range [0, {RAND_MAX}]. (The value of the
{RAND_MAX} macro shall be at least 32767.)
.LP
If \fIrand_r\fP() is called with the same initial value for the object
pointed to by \fIseed\fP and that object is not
modified between successive returns and calls to \fIrand_r\fP(), the
same sequence shall be generated. 
.LP
The \fIsrand\fP() function uses the argument as a seed for a new sequence
of pseudo-random numbers to be returned by subsequent
calls to \fIrand\fP(). If \fIsrand\fP() is then called with the same
seed value, the sequence of pseudo-random numbers shall be
repeated. If \fIrand\fP() is called before any calls to \fIsrand\fP()
are made, the same sequence shall be generated as when
\fIsrand\fP() is first called with a seed value of 1.
.LP
The implementation shall behave as if no function defined in this
volume of IEEE\ Std\ 1003.1-2001 calls \fIrand\fP()
or \fIsrand\fP().
.SH RETURN VALUE
.LP
The \fIrand\fP() function shall return the next pseudo-random number
in the sequence.
.LP
The \fIrand_r\fP() function shall return a pseudo-random integer.
.LP
The \fIsrand\fP() function shall not return a value.
.SH ERRORS
.LP
No errors are defined.
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.SS Generating a Pseudo-Random Number Sequence
.LP
The following example demonstrates how to generate a sequence of pseudo-random
numbers.
.sp
.RS
.nf

\fB#include <stdio.h>
#include <stdlib.h>
\&...
    long count, i;
    char *keystr;
    int elementlen, len;
    char c;
\&...
/* Initial random number generator. */
    srand(1);
.sp

    /* 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;
        }
.sp

        keystr[len] = '\\0';
        printf("%s Element%0*ld\\n", keystr, elementlen, i);
        len = 0;
    }
\fP
.fi
.RE
.SS Generating the Same Sequence on Different Machines
.LP
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
.nf

\fBstatic unsigned long next = 1;
int myrand(void)  /* RAND_MAX assumed to be 32767. */
{
    next = next * 1103515245 + 12345;
    return((unsigned)(next/65536) % 32768);
}
.sp

void mysrand(unsigned seed)
{
    next = seed;
}
\fP
.fi
.RE
.SH APPLICATION USAGE
.LP
The \fIdrand48\fP() function provides a much more elaborate random
number
generator.
.LP
The limitations on the amount of state that can be carried between
one function call and another mean the \fIrand_r\fP()
function can never be implemented in a way which satisfies all of
the requirements on a pseudo-random number generator. Therefore
this function should be avoided whenever non-trivial requirements
(including safety) have to be fulfilled.
.SH RATIONALE
.LP
The ISO\ C standard \fIrand\fP() and \fIsrand\fP() 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.
.LP
With regard to \fIrand\fP(), 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\fP()
.LP
.IP " 2." 4
A different sequence of pseudo-random numbers for each thread that
calls \fIrand\fP()
.LP
.LP
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.
.LP
This does not address the known deficiencies of the \fIrand\fP() 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
.LP
None.
.SH SEE ALSO
.LP
\fIdrand48\fP() , the Base Definitions volume of IEEE\ Std\ 1003.1-2001,
\fI<stdlib.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .