getrandom(2) - Linux manual page (original) (raw)
getrandom(2) System Calls Manual getrandom(2)
NAME top
getrandom - obtain a series of random bytesLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <sys/random.h>**
**ssize_t getrandom(void** _buf_**[.**_buflen_**], size_t** _buflen_**, unsigned int** _flags_**);**DESCRIPTION top
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](../man3/errno.3.html)_ set to **EAGAIN**.RETURN VALUE top
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](../man3/errno.3.html)_ is set to indicate the error.ERRORS top
**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)](../man2/read.2.html) calls on "slow"
devices are handled with and without the **SA_RESTART** flag in
the [signal(7)](../man7/signal.7.html) 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.STANDARDS top
Linux.HISTORY top
Linux 3.17, glibc 2.25.NOTES top
For an overview and comparison of the various interfaces that can
be used to obtain randomness, see [random(7)](../man7/random.7.html).
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)](../man2/chroot.2.html) 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 32Mi-1 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)](../man3/getentropy.3.html), 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!BUGS top
As of Linux 3.19, the following bug exists:
• Depending on CPU load, **getrandom**() does not react to interrupts
before reading all bytes requested.SEE ALSO top
[getentropy(3)](../man3/getentropy.3.html), [random(4)](../man4/random.4.html), [urandom(4)](../man4/urandom.4.html), [random(7)](../man7/random.7.html), [signal(7)](../man7/signal.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 getrandom(2)
Pages that refer to this page:mcookie(1), syscalls(2), getentropy(3), random(3), sd_id128_randomize(3), uuid_generate(3), random(4), random(7), signal(7)