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.
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.
getentropy(3),
random(4),
urandom(4),
random(7),
signal(7)