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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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RAND(3P) POSIX Programmer's Manual RAND(3P)
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.
rand, rand_r, srand — pseudo-random number generator
#include <stdlib.h>
int rand(void);
int rand_r(unsigned *seed);
void srand(unsigned seed);
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.1‐2017 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.1‐2017 calls rand() or srand().
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.
No errors are defined.
The following sections are informative.
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;
}
The drand48() and random() functions provide much more elaborate
pseudo-random number generators.
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.
These functions should be avoided whenever non-trivial
requirements (including safety) have to be fulfilled.
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.
The rand_r() function may be removed in a future version.
drand48(3p), initstate(3p)
The Base Definitions volume of POSIX.1‐2017, stdlib.h(0p)
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 .
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 .
IEEE/The Open Group 2017 RAND(3P)
Pages that refer to this page: stdlib.h(0p), drand48(3p), initstate(3p), srand(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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