rand(3p) — Linux manual page


RAND(3P)                  POSIX Programmer's Manual                 RAND(3P)

PROLOG         top

       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         top

       rand, rand_r, srand — pseudo-random number generator

SYNOPSIS         top

       #include <stdlib.h>

       int rand(void);
       int rand_r(unsigned *seed);
       void srand(unsigned seed);

DESCRIPTION         top

       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         top

       The rand() function shall return the next pseudo-random number in the

       The rand_r() function shall return a pseudo-random integer.

       The srand() function shall not return a value.

ERRORS         top

       No errors are defined.

       The following sections are informative.

EXAMPLES         top

   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. */

               /* 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() function provides a much more elaborate random number

       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         top

       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


       The rand_r() function may be removed in a future version.

SEE ALSO         top


       The Base Definitions volume of POSIX.1‐2008, stdlib.h(0p)

COPYRIGHT         top

       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 .

IEEE/The Open Group                 2013                            RAND(3P)

Pages that refer to this page: stdlib.h(0p)drand48(3p)initstate(3p)srand(3p)