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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.
NAME
rand, rand_r, srand — pseudo-random number generator
SYNOPSIS
#include <stdlib.h>
int rand(void);
int rand_r(unsigned *seed);
void srand(unsigned seed);
DESCRIPTION
For rand() and srand(): 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.1‐2008 defers to the ISO C standard.
The rand() function shall compute a sequence of pseudo-random integers in the range
[0,{RAND_MAX}] with a period of at least 232.
The rand() function need not be thread-safe.
The rand_r() 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.)
If rand_r() is called with the same initial value for the object pointed to by seed and
that object is not modified between successive returns and calls to rand_r(), the same
sequence shall be generated.
The srand() function uses the argument as a seed for a new sequence of pseudo-random
numbers to be returned by subsequent calls to rand(). If srand() is then called with the
same seed value, the sequence of pseudo-random numbers shall be repeated. If rand() is
called before any calls to srand() are made, the same sequence shall be generated as when
srand() is first called with a seed value of 1.
The implementation shall behave as if no function defined in this volume of POSIX.1‐2008
calls rand() or srand().
RETURN VALUE
The rand() function shall return the next pseudo-random number in the sequence.
The rand_r() function shall return a pseudo-random integer.
The srand() function shall not return a value.
ERRORS
No errors are defined.
The following sections are informative.
EXAMPLES
Generating a Pseudo-Random Number Sequence
The following example demonstrates how to generate a sequence of pseudo-random numbers.
#include <stdio.h>
#include <stdlib.h>
...
long count, i;
char *keystr;
int elementlen, len;
char c;
...
/* Initial random number generator. */
srand(1);
/* 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;
}
keystr[len] = '\0';
printf("%s Element%0*ld\n", keystr, elementlen, i);
len = 0;
}
Generating the Same Sequence on Different Machines
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.
static unsigned long next = 1;
int myrand(void) /* RAND_MAX assumed to be 32767. */
{
next = next * 1103515245 + 12345;
return((unsigned)(next/65536) % 32768);
}
void mysrand(unsigned seed)
{
next = seed;
}
APPLICATION USAGE
The drand48() function provides a much more elaborate random number generator.
The limitations on the amount of state that can be carried between one function call and
another mean the rand_r() 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.
RATIONALE
The ISO C standard rand() and srand() 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.
With regard to rand(), there are two different behaviors that may be wanted in a multi-
threaded program:
1. A single per-process sequence of pseudo-random numbers that is shared by all threads
that call rand()
2. A different sequence of pseudo-random numbers for each thread that calls rand()
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.
This does not address the known deficiencies of the rand() function implementations, which
have been approached by maintaining more state. In effect, this specifies new thread-safe
forms of a deficient function.
FUTURE DIRECTIONS
The rand_r() function may be removed in a future version.
SEE ALSO
drand48()
The Base Definitions volume of POSIX.1‐2008, <stdlib.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
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 .