Manpages - random_r.3

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

  #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 *restrict statebuf,
   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 *feature_test_macros*(7)):

*random_r*(), *srandom_r*(), *initstate_r*(), *setstate_r*():

      /* Glibc since 2.19: */ _DEFAULT_SOURCE
          || /* Glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE

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*().

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

EINVAL

A state array of less than 8 bytes was specified to *initstate_r*().

EINVAL

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

EINVAL

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

For an explanation of the terms used in this section, see *attributes*(7).

These functions are nonstandard glibc extensions.

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.

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

This page is part of release 5.13 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 https://www.kernel.org/doc/man-pages/.