getrandom(2) System Calls Manual getrandom(2)
NAME
getrandom - obtain a series of random bytes
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/random.h>
ssize_t getrandom(void buf[.buflen], size_t buflen, unsigned int flags);
DESCRIPTION
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 ran-
dom 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 inter-
rupted by signals. No such guarantees apply for larger buffer sizes.
For example, if the call is interrupted by a signal handler, it may re-
turn 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 re-
quested 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 getran-
dom() does not block in these cases, but instead immediately re-
turns -1 with errno set to EAGAIN.
RETURN VALUE
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 en-
tropy was present in the random source or the system call was inter-
rupted by a signal.
On error, -1 is returned, and errno is set to indicate the error.
ERRORS
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.
STANDARDS
Linux.
HISTORY
Linux 3.17, glibc 2.25.
NOTES
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 re-
turned.
Interruption by a signal handler
When reading from the urandom source (GRND_RANDOM is not set), getran-
dom() 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 re-
quest size is large (buflen > 256), the call either succeeds, returning
a partially filled buffer, or fails with the error EINTR. If the en-
tropy pool has been initialized and the request size is small (bu-
flen <= 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 compat-
ibility 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 re-
turned. 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
As of Linux 3.19, the following bug exists:
• Depending on CPU load, getrandom() does not react to interrupts be-
fore reading all bytes requested.
SEE ALSO
getentropy(3), random(4), urandom(4), random(7), signal(7)
Linux man-pages 6.9.1 2024-05-02 getrandom(2)
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