
RAND(P) POSIX Programmer's Manual RAND(P)
NAME
rand, rand_r, srand  pseudorandom number generator
SYNOPSIS
#include <stdlib.h>
int rand(void);
int rand_r(unsigned *seed);
void srand(unsigned seed);
DESCRIPTION
The rand() function shall compute a sequence of pseudorandom integers in the range [0,
{RAND_MAX}] with a period of at least 2**32.
The rand() function need not be reentrant. A function that is not required to be reentrant
is not required to be threadsafe.
The rand_r() function shall compute a sequence of pseudorandom 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 pseudorandom num
bers to be returned by subsequent calls to rand(). If srand() is then called with the same
seed value, the sequence of pseudorandom 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
IEEE Std 1003.12001 calls rand() or srand().
RETURN VALUE
The rand() function shall return the next pseudorandom number in the sequence.
The rand_r() function shall return a pseudorandom integer.
The srand() function shall not return a value.
ERRORS
No errors are defined.
The following sections are informative.
EXAMPLES
Generating a PseudoRandom Number Sequence
The following example demonstrates how to generate a sequence of pseudorandom 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 applica
tions 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 pseudorandom number generator. Therefore this function should be
avoided whenever nontrivial requirements (including safety) have to be fulfilled.
RATIONALE
The ISO C standard rand() and srand() functions allow perprocess pseudorandom 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 perprocess sequence of pseudorandom numbers that is shared by all threads
that call rand()
2. A different sequence of pseudorandom numbers for each thread that calls rand()
This is provided by the modified threadsafe 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 threadsafe
forms of a deficient function.
FUTURE DIRECTIONS
None.
SEE ALSO
drand48() , the Base Definitions volume of IEEE Std 1003.12001, <stdlib.h>
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) 20012003 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 orig
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 RAND(P) 
