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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON |
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GETRANDOM(2) Linux Programmer's Manual GETRANDOM(2)
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_RANDOM
If 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_NONBLOCK
By 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 appropriately.
EAGAIN The requested entropy was not available, and getrandom() would
have blocked if the GRND_NONBLOCK flag was not set.
EFAULT The address referred to by buf is outside the accessible
address space.
EINTR 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.
EINVAL An invalid flag was specified in flags.
ENOSYS 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.
This system call is Linux-specific.
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.
Maximum number of bytes returned
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.
Interruption by a signal handler
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.
getentropy(3), random(4), urandom(4), random(7), signal(7)
This page is part of release 4.15 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/.
Linux 2017-09-15 GETRANDOM(2)
Pages that refer to this page: syscalls(2), getentropy(3), random(3), random(4), random(7), signal(7)
Copyright and license for this manual page
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