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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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getrandom(2) System Calls Manual getrandom(2)
getrandom - obtain a series of random bytes
Standard C library (libc, -lc)
#include <sys/random.h>
ssize_t getrandom(void buf[.buflen], 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 to indicate the error.
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.
Linux.
Linux 3.17, glibc 2.25.
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 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), 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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.9.1 2024-05-02 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)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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