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RAND(3) Linux Programmer's Manual RAND(3)


     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)):
         Since glibc 2.24:
             _POSIX_C_SOURCE >= 199506L
         Glibc 2.23 and earlier


     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-ran-
     dom 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
     allbox;  lbw25  lb  lb  l  l  l.   Interface Attribute Value T{ rand(),
     rand_r(), srand() T}   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>
         int 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);
             exit(EXIT_SUCCESS); }


     drand48(3), random(3)


     This  page  is  part of release 4.16 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
                                2017-07-13                           RAND(3)
/data/webs/external/dokuwiki/data/pages/man/rand_r.txt · Last modified: 2019/05/17 09:47 by

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