getrandom - obtain a series of random bytes
ssize getrandom(void *buf, size_t buflen,
unsigned int flags);
() 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
(i.e., the same source as the /dev/urandom
device). This behavior can
be changed via the flags
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
() will block, unless GRND_NONBLOCK
is specified in
argument is a bit mask that can contain zero or more of the
following values ORed together:
- 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.
- 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
if either GRND_RANDOM
was specified in flags
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.
- The requested entropy was not available, and
getrandom() would have blocked if the GRND_NONBLOCK flag was
- 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.
() 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)
() does not
involve the use of pathnames or file descriptors. Thus, getrandom
be useful in cases where chroot(2)
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
is not set),
() 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
. When reading from the random
() 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,
. If the entropy is not yet initialized, then the call will fail
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
If the entropy pool has been initialized and the request size is
<= 256), then getrandom
not fail with EINTR
. Instead, it will return all of the bytes that have
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
() to read small buffers (<= 256 bytes) from the
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
The user of getrandom
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
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
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 4.13 of the Linux man-pages
description of the project, information about reporting bugs, and the latest
version of this page, can be found at