man-pages/man2/getrandom.2

.\" Copyright (C) 2014, Theodore Ts'o <tytso@mit.edu>
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.TH GETRANDOM 2 2015-01-22 "Linux" "Linux Programmer's Manual"
.SH NAME
getrandom \- obtain a series of random bytes
.SH SYNOPSIS
.B #include <linux/random.h>
.sp
.BI "int getrandom(void *"buf ", size_t " buflen ", unsigned int " flags );
.SH DESCRIPTION
The
.BR getrandom ()
system call fills the buffer pointed to by
.I buf
with up to
.I buflen
random bytes.
These can be used to seed user-space random number generators
or for cryptographic purposes.
.PP
.BR 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
.I /dev/random
and
.I /dev/urandom
devices.
Therefore
.BR getrandom ()
should not be used for Monte Carlo simulations or other
programs/algorithms which are doing probabilistic sampling.

By default,
.BR getrandom ()
draws entropy from the
.IR /dev/urandom
pool.
This behavior can be changed via the
.I flags
argument.
If the
.IR /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
.BR EINTR .
If the pool has not yet been initialized, then the call blocks, unless
.B GRND_RANDOM
is specified in
.IR flags .

The
.I flags
argument is a bit mask that can contain zero or more of the following values
ORed together:
.TP
.B GRND_RANDOM
If this bit is set, then random bytes are drawn from the
.I /dev/random
pool instead of the
.I /dev/urandom
pool.
The
.I /dev/random
pool is limited based on the entropy that can be obtained from environmental
noise. 
If the number of available bytes in
.I /dev/random
is less than requested in
.IR buflen ,
the call returns just the available random bytes.
If no random bytes are available, the behavior depends on the presence of
.B GRND_NONBLOCK
in the
.I flags
argument.
.TP
.B GRND_NONBLOCK
By default, if there are no random bytes available at all (when reading from
.IR /dev/random ),
or the entropy pool has not yet been initialized (when reading from
.IR /dev/urandom ),
.BR getrandom ()
blocks until data is available.
If the
.B GRND_NONBLOCK
flag is set, then
.BR getrandom ()
instead immediately returns \-1 with
.I errno
set to
.BR EAGAIN .
.SH RETURN VALUE
On success,
.BR getrandom ()
returns the number of bytes that were copied to the buffer
.IR buf .
This may be less than the number of bytes requested via
.I buflen
if
.BR GRND_RANDOM
was specified in
.IR flags
and insufficient entropy was present in the
.IR /dev/random
pool, or if the system call was interrupted by a signal.
.PP
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EINVAL
An invalid flag was specified in
.IR flags .
.TP
.B EFAULT
The address referred to by
.I buf
is outside the accessible address space.
.TP
.B EAGAIN
The requested entropy was not available, and
.BR getrandom ()
would have blocked if the
.B GRND_NONBLOCK
flag was not set.
.TP
.B EINTR
While blocked waiting for entropy, the call was interrupted by a signal
handler; see the description of how interrupted
.BR read (2)
calls on "slow" devices are handled with and without the
.B SA_RESTART
flag in the
.BR signal (7)
man page.
.SH VERSIONS
.BR getrandom ()
was introduced in version 3.17 of the Linux kernel.
.SH CONFORMING TO
This system call is Linux-specific.
.SH NOTES
.SS Maximum number of bytes returned
As of Linux 3.19 the following limits apply:
.IP * 3
When reading from
.IR /dev/urandom ,
a maximum of 33554431 bytes is returned by a single call to
.BR getrandom ()
on a system where
.I int
has a size of 32 bits.
.IP *
When reading from
.IR /dev/random ,
a maximum of 512 bytes is returned.
.SS Initialization of the entropy pool
The kernel collects bits of entropy from environment.
When a sufficient number of random bits has been collected, the
.I /dev/urandom
entropy pool is considered to be initialized.
This state is normally reached early in the system bootstrap phase.
.SS Interruption by a signal handler
A call to
.BR getrandom ()
can block only when called without the
.B GRND_NONBLOCK
flag.
When reading from
.I /dev/urandom
.RB ( GRND_RANDOM
is not set),
blocking occurs only if the entropy pool has not been initialized yet.
When reading from
.I /dev/random
.RB ( GRND_RANDOM
is set),
blocking occurs if not enough random bytes are available.

The behavior when a call to
.BR getrandom ()
that is blocked while reading from
.I /dev/urandom
is interrupted by a signal handler
depends on the initialization state of the entropy buffer
and on the request size,
.IR buflen .
If the entropy is not yet initialized, then the call will fail with the
.B EINTR
error.
If the entropy pool has been initialized
and the request size is large
.RI ( buflen "\ >\ 256),"
the call either succeeds, returning a partially filled buffer,
or fails with the error
.BR EINTR.
If the entropy pool has been initialized and the request size is small
.RI ( buflen "\ <=\ 256),"
then
.BR getrandom ()
will not fail with
.BR EINTR .
Instead, it will return all of the bytes that have been requested.

When reading from
.IR /dev/random ,
blocking requests of any size can be interrupted by a signal
(the call fails with the error
.BR EINTR ).

Calling
.BR getrandom ()
to read
.I /dev/urandom
for small values (<=\ 256) of
.I buflen
is the preferred mode of usage.
.PP
The special treatment of small values of
.I buflen
was designed for compatibility with
OpenBSD's
.BR getentropy ()
system call.
.PP
The user of
.BR getrandom ()
.I must
always check the return value,
to determine whether either an error occurred
or fewer bytes than requested were returned.
In the case where
.B GRND_RANDOM
is not specified and
.I 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!
.SS Choice of random device
Unless you are doing long-term key generation (and perhaps not even
then), you probably shouldn't be using
.B GRND_RANDOM.
The cryptographic algorithms used for
.I /dev/urandom
are quite conservative, and so should be sufficient for all purposes.
The disadvantage of
.B GRND_RANDOM
is that it can block.
Furthermore, dealing with the partially fulfilled
.BR getrandom ()
requests that can occur when using
.B GRND_RANDOM
increases code complexity.
.SS Emulating OpenBSD's getentropy()
The
.BR getentropy ()
system call in OpenBSD can be emulated using the following
function:

.in +4n
.nf
int
getentropy(void *buf, size_t buflen)
{
    int ret;

    if (buflen > 256)
        goto failure;
    ret = getrandom(buf, buflen, 0);
    if (ret < 0)
        return ret;
    if (ret == buflen)
        return 0;
failure:
    errno = EIO;
    return \-1;
}
.fi
.in
.SH BUGS
As of Linux 3.19, the following bug exists:
.\" FIXME patch proposed https://lkml.org/lkml/2014/11/29/16
.IP * 3
Depending on CPU load,
.BR getrandom ()
does not react to interrupts before reading all bytes requested.
.SH SEE ALSO
.BR random (4),
.BR urandom (4)