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.Dd January 12, 2020
.Dt GETRANDOM 2
.Os
.Sh NAME
.Nm getrandom
.Nd get random data
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/random.h
.Ft ssize_t
.Fn getrandom "void *buf" "size_t buflen" "unsigned int flags"
.Sh DESCRIPTION
.Fn getrandom
fills
.Fa buf
with up to
.Fa buflen
bytes of random data.
.Pp
The
.Fa flags
argument may include zero or more of the following:
.Bl -tag -width _GRND_NONBLOCK_
.It Ql GRND_NONBLOCK
Return
.Er EAGAIN
instead of blocking, if the
.Xr random 4
device has not yet been seeded.
By default,
.Fn getrandom
will block until the device is seeded.
.It Ql GRND_RANDOM
This flag does nothing on
.Fx .
.Pa /dev/random
and
.Pa /dev/urandom
are identical.
.It Ql GRND_INSECURE
This flag is treated as an alternative name for
.Dv GRND_NONBLOCK .
It is provided solely for API compatibility with Linux.
.El
.Pp
If the
.Xr random 4
device has been seeded, reads of up to 256 bytes will always return as many
bytes as requested and will not be interrupted by signals.
.Sh RETURN VALUES
Upon successful completion, the number of bytes which were actually read is
returned.
For requests larger than 256 bytes, this can be fewer bytes than were
requested.
Otherwise, -1 is returned and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn getrandom
operation returns the following errors:
.Bl -tag -width Er
.It Bq Er EAGAIN
The
.Ql GRND_NONBLOCK
(or
.Ql GRND_INSECURE )
flag was set and the
.Xr random 4
device was not yet seeded.
.It Bq Er EFAULT
The
.Fa buf
parameter points to an invalid address.
.It Bq Er EINTR
The sleep was interrupted by a signal.
.It Bq Er EINVAL
An invalid
.Fa flags
was specified.
.It Bq Er EINVAL
The requested
.Fa buflen
was larger than
.Dv IOSIZE_MAX .
.El
.Sh SEE ALSO
.Xr arc4random 3 ,
.Xr getentropy 3 ,
.Xr random 4
.Sh STANDARDS
.Fn getrandom
is non-standard.
It is present in Linux.
.Sh HISTORY
The
.Fn getrandom
system call first appeared in
.Fx 12.0 .
.Sh CAVEATS
Unlike Linux, the
.Dv GRND_INSECURE
flag on
.Fx
does not produce any output before the
.Xr random 4
device is seeded.