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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | BUGS | SEE ALSO | COLOPHON |
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random_r(3) Library Functions Manual random_r(3)
random_r, srandom_r, initstate_r, setstate_r - reentrant random
number generator
Standard C library (libc, -lc)
#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
feature_test_macros(7)):
random_r(), srandom_r(), initstate_r(), setstate_r():
/* glibc >= 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).
┌─────────────────────────────┬───────────────┬──────────────────┐
│ Interface │ Attribute │ Value │
├─────────────────────────────┼───────────────┼──────────────────┤
│ random_r(), srandom_r(), │ Thread safety │ MT-Safe race:buf │
│ initstate_r(), setstate_r() │ │ │
└─────────────────────────────┴───────────────┴──────────────────┘
GNU.
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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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man-pages@man7.org
Linux man-pages 6.15 2025-05-17 random_r(3)
Pages that refer to this page: random(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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