.\" Copyright (C) 2014, Theodore Ts'o <tytso@mit.edu>
.\" Copyright (C) 2014, Heinrich Schuchardt <xypron.glpk@gmx.de>
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.TH GETRANDOM 2 2014-10-03 "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 system call
.BR getrandom ()
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 other 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.
.PP
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 fewer random bytes are available than are requested in
.IR buflen ,
the available random bytes will be copied and the call returns.
If no random byte is available, the response will depend on the
presence of
.B GRND_NONBLOCK
in the
.I flags
argument.
.TP
.B GRND_NONBLOCK
If this bit is set and there is no random byte available at all,
.BR getrandom ()
will return -1 with
.I errno
set to
.BR EAGAIN .
If the
.B GRND_NONBLOCK
bit is not set and there is no random byte available at all,
.BR getrandom ()
will block.
.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 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 Interruption by a signal handler
.\" FIXME Here, I think there needs to be an opening paragraph that describes
.\" the cases where getrandom() can block. This should cover the cases with
.\" GRND_RANDOM and without GRND_RANDOM. Reading the existing page, I am
.\" still not completely confident that I know what the cases are.
The reaction of
.BR getrandom ()
in case of an interruption of a blocking call by a signal
when reading from
.I /dev/urandom
.RB ( GRND_RANDOM
is not set)
depends on the initialization state of the entropy buffer
and on the request size
.IR buflen .
If the entropy is not yet initialized or the request size is large
.RI ( buflen "\ >\ 256),"
.B EINTR
will be returned.
If the entropy pool has been initialized and the request size is small
.RI ( buflen "\ <=\ 256),"
.BR getrandom ()
will not return
.BR EINTR .
Instead, it will return all of the bytes that have been requested.
.PP
When reading from
.I /dev/random
.RB ( GRND_RANDOM
is set)
these guarantees do
.I not
apply.
.PP
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 user-space code should 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 partially fulfilled
.BR getrandom ()
requests 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 SEE ALSO
.BR random (4),
.BR urandom (4)