GETRANDOM(2) Linux Programmer's Manual GETRANDOM(2)
getrandom - obtain a series of random bytes
int 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.
getrandom() relies on entropy gathered from device drivers and other
sources of environmental noise. Unnecessarily reading large quantities
of data will have a negative impact on other users of the /dev/random
and /dev/urandom devices. Therefore, getrandom() should not be used
for Monte Carlo simulations or other programs/algorithms which are
doing probabilistic sampling.
By default, getrandom() draws entropy from the /dev/urandom pool. This
behavior can be changed via the flags argument. If the /dev/urandom
pool 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 pool has not yet
been initialized, then the call blocks, unless GRND_NONBLOCK is speci-
fied in flags.
The flags 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
/dev/random pool instead of the /dev/urandom pool. The
/dev/random pool is limited based on the entropy that can be
obtained from environmental noise. If the number of available
bytes in /dev/random 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 /dev/random, getrandom() blocks if
no random bytes are available, and when reading from /dev/uran-
dom, 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 GRND_RANDOM was specified in flags and insufficient
entropy was present in the /dev/random pool, or if the system call was
interrupted by a signal.
On error, -1 is returned, and errno is set appropriately.
EINVAL An invalid flag was specified in flags.
EFAULT The address referred to by buf is outside the accessible address
EAGAIN The requested entropy was not available, and getrandom() would
have blocked if the GRND_NONBLOCK flag was not set.
EINTR The call was interrupted by a signal handler; see the descrip-
tion of how interrupted read(2) calls on "slow" devices are han-
dled with and without the SA_RESTART flag in the signal(7) man
getrandom() was introduced in version 3.17 of the Linux kernel.
This system call is Linux-specific.
Maximum number of bytes returned
As of Linux 3.19 the following limits apply:
* When reading from /dev/urandom, a maximum of 33554431 bytes is
returned by a single call to getrandom() on a system where int has a
size of 32 bits.
* When reading from /dev/random, a maximum of 512 bytes is returned.
Initialization of the entropy pool
The kernel collects bits of entropy from environment. When a suffi-
cient number of random bits has been collected, the /dev/urandom
entropy pool is considered to be initialized. This state is normally
reached early in the system bootstrap phase.
Interruption by a signal handler
When reading from /dev/urandom (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 (more than 256) of bytes, getrandom() will block until
those bytes have been generated and transferred from kernel memory to
buf. When reading from /dev/random (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 /dev/urandom 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 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, return-
ing 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 /dev/random, blocking requests of any size can be
interrupted by a signal (the call fails with the error EINTR).
Calling getrandom() to read /dev/urandom for small values (<= 256) of
buflen is the preferred mode of usage.
The special treatment of small values of buflen was designed for com-
patibility with OpenBSD's getentropy() system call.
The user of getrandom() must always check the return value, to deter-
mine 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!
Choice of random device
Unless you are doing long-term key generation (and perhaps not even
then), you probably shouldn't be using GRND_RANDOM. The cryptographic
algorithms used for /dev/urandom are quite conservative, and so should
be sufficient for all purposes. The disadvantage of GRND_RANDOM is
that it can block. Furthermore, dealing with the partially fulfilled
getrandom() requests that can occur when using GRND_RANDOM increases
Emulating OpenBSD's getentropy()
The getentropy() system call in OpenBSD can be emulated using the fol-
getentropy(void *buf, size_t buflen)
if (buflen > 256)
ret = getrandom(buf, buflen, 0);
if (ret < 0)
if (ret == buflen)
errno = EIO;
As of Linux 3.19, the following bug exists:
* Depending on CPU load, getrandom() does not react to interrupts
before reading all bytes requested.
random(4), urandom(4), signal(7)
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