getrandom — obtain a series of random bytes
#include <sys/random.h>
ssize_t
getrandom( |
void *buf, |
| size_t buflen, | |
unsigned int flags); |
The getrandom() system call
fills the buffer pointed to by buf with up to buflen random bytes. These
bytes can be used to seed user-space random number generators
or for cryptographic purposes.
By default, getrandom()
draws entropy from the urandom source (i.e., the
same source as the /dev/urandom
device). This behavior can be changed via the flags argument.
If the urandom
source has been initialized, reads of up to 256 bytes will
always return as many bytes as requested and will not be
interrupted by signals. No such guarantees apply for larger
buffer sizes. For example, if the call is interrupted by a
signal handler, it may return a partially filled buffer, or
fail with the error EINTR.
If the urandom
source has not yet been initialized, then getrandom() will block, unless GRND_NONBLOCK is specified in flags.
The flags argument
is a bit mask that can contain zero or more of the following
values ORed together:
GRND_RANDOMIf this bit is set, then random bytes are drawn from
the random
source (i.e., the same source as the /dev/random device) instead of the
urandom
source. The random source is
limited based on the entropy that can be obtained from
environmental noise. If the number of available bytes
in the random
source is less than requested in buflen, the call returns
just the available random bytes. If no random bytes are
available, the behavior depends on the presence of
GRND_NONBLOCK in the
flags
argument.
GRND_NONBLOCKBy default, when reading from the random source,
getrandom() blocks if no
random bytes are available, and when reading from the
urandom
source, it blocks if the entropy pool has not yet been
initialized. If the GRND_NONBLOCK flag is set, then
getrandom() does not
block in these cases, but instead immediately returns
−1 with errno set to
EAGAIN.
On success, getrandom()
returns the number of bytes that were copied to the buffer
buf. This may be less
than the number of bytes requested via buflen if either GRND_RANDOM was specified in flags and insufficient entropy
was present in the random source or the system
call was interrupted by a signal.
On error, −1 is returned, and errno is set to indicate the error.
The requested entropy was not available, and
getrandom() would have
blocked if the GRND_NONBLOCK flag was not set.
The address referred to by buf is outside the
accessible address space.
The call was interrupted by a signal handler; see
the description of how interrupted read(2) calls on
"slow" devices are handled with and without the
SA_RESTART flag in the
signal(7) man
page.
An invalid flag was specified in flags.
The glibc wrapper function for getrandom() determined that the
underlying kernel does not implement this system
call.
getrandom() was introduced
in version 3.17 of the Linux kernel. Support was added to
glibc in version 2.25.
For an overview and comparison of the various interfaces that can be used to obtain randomness, see random(7).
Unlike /dev/random and
/dev/urandom, getrandom() does not involve the use of
pathnames or file descriptors. Thus, getrandom() can be useful in cases where
chroot(2) makes
/dev pathnames invisible, and
where an application (e.g., a daemon during start-up) closes
a file descriptor for one of these files that was opened by a
library.
As of Linux 3.19 the following limits apply:
When reading from the urandom source, a
maximum of 33554431 bytes is returned by a single
call to getrandom() on
systems where int has a
size of 32 bits.
When reading from the random source, a
maximum of 512 bytes is returned.
When reading from the urandom source
(GRND_RANDOM is not set),
getrandom() will block until
the entropy pool has been initialized (unless the
GRND_NONBLOCK flag was
specified). If a request is made to read a large number of
bytes (more than 256), getrandom() will block until those bytes
have been generated and transferred from kernel memory to
buf. When reading
from the random
source (GRND_RANDOM is set),
getrandom() will block until
some random bytes become available (unless the GRND_NONBLOCK flag was specified).
The behavior when a call to getrandom() that is blocked while reading
from the urandom
source is interrupted by a signal handler depends on the
initialization state of the entropy buffer and on the
request size, buflen. If the entropy is not
yet initialized, then the call fails with the EINTR error. If the entropy pool has
been initialized and the request size is large (buflen > 256), the call
either succeeds, returning a partially filled buffer, or
fails with the error EINTR.
If the entropy pool has been initialized and the request
size is small (buflen <= 256), then
getrandom() will not fail
with EINTR. Instead, it will
return all of the bytes that have been requested.
When reading from the random source, blocking
requests of any size can be interrupted by a signal handler
(the call fails with the error EINTR).
Using getrandom() to read
small buffers (<= 256 bytes) from the urandom source is the
preferred mode of usage.
The special treatment of small values of buflen was designed for
compatibility with OpenBSD's getentropy(3), which is
nowadays supported by glibc.
The user of getrandom()
must always check
the return value, to determine whether either an error
occurred or fewer bytes than requested were returned. In
the case where GRND_RANDOM is
not specified and buflen is less than or equal
to 256, a return of fewer bytes than requested should never
happen, but the careful programmer will check for this
anyway!
As of Linux 3.19, the following bug exists:
Depending on CPU load, getrandom() does not react to
interrupts before reading all bytes requested.
This page is part of release 5.11 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/.
|
Copyright (C) 2014, Theodore Ts'o <tytsomit.edu> Copyright (C) 2014,2015 Heinrich Schuchardt <xypron.glpkgmx.de> Copyright (C) 2015, Michael Kerrisk <mtk.manpagesgmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume. no responsibility for errors or omissions, or for damages resulting. from the use of the information contained herein. The author(s) may. not have taken the same level of care in the production of this. manual, which is licensed free of charge, as they might when working. professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END |