random_r(3) — Linux manual page


random_r(3)             Library Functions Manual             random_r(3)

NAME         top

       random_r, srandom_r, initstate_r, setstate_r - reentrant random
       number generator

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <stdlib.h>

       int random_r(struct random_data *restrict buf,
                    int32_t *restrict result);
       int srandom_r(unsigned int seed, struct random_data *buf);

       int initstate_r(unsigned int seed, char statebuf[restrict .statelen],
                    size_t statelen, struct random_data *restrict buf);
       int setstate_r(char *restrict statebuf,
                    struct random_data *restrict buf);

   Feature Test Macro Requirements for glibc (see

       random_r(), srandom_r(), initstate_r(), setstate_r():
           /* glibc >= 2.19: */ _DEFAULT_SOURCE
               || /* glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE

DESCRIPTION         top

       These functions are the reentrant equivalents of the functions
       described in random(3).  They are suitable for use in
       multithreaded programs where each thread needs to obtain an
       independent, reproducible sequence of random numbers.

       The random_r() function is like random(3), except that instead of
       using state information maintained in a global variable, it uses
       the state information in the argument pointed to by buf, which
       must have been previously initialized by initstate_r().  The
       generated random number is returned in the argument result.

       The srandom_r() function is like srandom(3), except that it
       initializes the seed for the random number generator whose state
       is maintained in the object pointed to by buf, which must have
       been previously initialized by initstate_r(), instead of the seed
       associated with the global state variable.

       The initstate_r() function is like initstate(3) except that it
       initializes the state in the object pointed to by buf, rather
       than initializing the global state variable.  Before calling this
       function, the buf.state field must be initialized to NULL.  The
       initstate_r() function records a pointer to the statebuf argument
       inside the structure pointed to by buf.  Thus, statebuf should
       not be deallocated so long as buf is still in use.  (So, statebuf
       should typically be allocated as a static variable, or allocated
       on the heap using malloc(3) or similar.)

       The setstate_r() function is like setstate(3) except that it
       modifies the state in the object pointed to by buf, rather than
       modifying the global state variable.  state must first have been
       initialized using initstate_r() or be the result of a previous
       call of setstate_r().

RETURN VALUE         top

       All of these functions return 0 on success.  On error, -1 is
       returned, with errno set to indicate the error.

ERRORS         top

       EINVAL A state array of less than 8 bytes was specified to

       EINVAL The statebuf or buf argument to setstate_r() was NULL.

       EINVAL The buf or result argument to random_r() was NULL.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       │ Interface                  Attribute     Value            │
       │ random_r(), srandom_r(),   │ Thread safety │ MT-Safe race:buf │
       │ initstate_r(),             │               │                  │
       │ setstate_r()               │               │                  │

STANDARDS         top


BUGS         top

       The initstate_r() interface is confusing.  It appears that the
       random_data type is intended to be opaque, but the implementation
       requires the user to either initialize the buf.state field to
       NULL or zero out the entire structure before the call.

SEE ALSO         top

       drand48(3), rand(3), random(3)

Linux man-pages (unreleased)     (date)                      random_r(3)

Pages that refer to this page: random(3)