rand, rand_r, srand - pseudo-random number generator


   #include <stdlib.h>

   int rand(void);

   int rand_r(unsigned int *seedp);

   void srand(unsigned int seed);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

   rand_r(): _POSIX_C_SOURCE


   The  rand()  function returns a pseudo-random integer in the range 0 to
   RAND_MAX inclusive (i.e., the mathematical range [0, RAND_MAX]).

   The srand() function sets its argument as the seed for a  new  sequence
   of  pseudo-random  integers  to be returned by rand().  These sequences
   are repeatable by calling srand() with the same seed value.

   If no seed value is provided,  the  rand()  function  is  automatically
   seeded with a value of 1.

   The  function  rand() is not reentrant, since it uses hidden state that
   is modified on each call.  This might just be the seed value to be used
   by the next call, or it might be something more elaborate.  In order to
   get reproducible behavior in a threaded application, this state must be
   made explicit; this can be done using the reentrant function rand_r().

   Like  rand(),  rand_r()  returns  a  pseudo-random integer in the range
   [0, RAND_MAX].  The seedp argument is a pointer to an unsigned int that
   is  used  to store state between calls.  If rand_r() is called with the
   same initial value for the integer pointed to by seedp, and that  value
   is  not  modified  between  calls, then the same pseudo-random sequence
   will result.

   The value pointed to by the seedp argument of rand_r() provides only  a
   very  small  amount  of  state, so this function will be a weak pseudo-
   random generator.  Try drand48_r(3) instead.


   The rand() and rand_r() functions return a value between 0 and RAND_MAX
   (inclusive).  The srand() function returns no value.


   For   an   explanation   of   the  terms  used  in  this  section,  see

   Interface                  Attribute      Value   
   rand(), rand_r(), srand()  Thread safety  MT-Safe 


   The functions rand() and srand() conform to  SVr4,  4.3BSD,  C89,  C99,
   POSIX.1-2001.     The   function   rand_r()   is   from   POSIX.1-2001.
   POSIX.1-2008 marks rand_r() as obsolete.


   The versions of rand() and srand() in the Linux C Library use the  same
   random number generator as random(3) and srandom(3), so the lower-order
   bits should be as random as the higher-order bits.  However,  on  older
   rand()  implementations,  and  on  current implementations on different
   systems, the lower-order bits are much less  random  than  the  higher-
   order  bits.   Do  not use this function in applications intended to be
   portable when good randomness is needed.  (Use random(3) instead.)


   POSIX.1-2001 gives the following example of an implementation of rand()
   and  srand(),  possibly  useful when one needs the same sequence on two
   different machines.

       static unsigned long next = 1;

       /* RAND_MAX assumed to be 32767 */
       int myrand(void) {
           next = next * 1103515245 + 12345;
           return((unsigned)(next/65536) % 32768);

       void mysrand(unsigned int seed) {
           next = seed;

   The following program can be used to display the pseudo-random sequence
   produced by rand() when given a particular seed.

       #include <stdlib.h>
       #include <stdio.h>

       main(int argc, char *argv[])
           int j, r, nloops;
           unsigned int seed;

           if (argc != 3) {
               fprintf(stderr, "Usage: %s <seed> <nloops>\n", argv[0]);

           seed = atoi(argv[1]);
           nloops = atoi(argv[2]);

           for (j = 0; j < nloops; j++) {
               r =  rand();
               printf("%d\n", r);



   drand48(3), random(3)


   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at

                              2016-03-15                           RAND(3)


Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.

Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.

Free Software

Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.

Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.

Free Books

The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.

Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.


Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.

Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.