.\"	$OpenBSD: getentropy.2,v 1.8 2015/01/31 00:20:12 schwarze Exp $
.\"
.\" Copyright (c) 2018 Conrad Meyer <cem@FreeBSD.org>
.\" Copyright (c) 2014 Theo de Raadt
.\"
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.\" purpose with or without fee is hereby granted, provided that the above
.\" copyright notice and this permission notice appear in all copies.
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.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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.\"
.Dd November 20, 2024
.Dt GETENTROPY 3
.Os
.Sh NAME
.Nm getentropy
.Nd get entropy
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn getentropy "void *buf" "size_t buflen"
.Sh DESCRIPTION
.Fn getentropy
fills a buffer with high-quality random data.
.Pp
The maximum
.Fa buflen
permitted is 256 bytes.
.Pp
If it does not produce an error,
.Fn getentropy
always provides the requested number of bytes of random data.
.Pp
Similar to reading from
.Pa /dev/urandom
just after boot,
.Fn getentropy
may block until the system has collected enough entropy to seed the CSPRNG.
.Sh IMPLEMENTATION NOTES
The
.Fn getentropy
function is implemented using
.Xr getrandom 2 .
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
.Fn getentropy
will succeed unless:
.Bl -tag -width Er
.It Bq Er EFAULT
The
.Fa buf
parameter points to an
invalid address.
.It Bq Er EIO
Too many bytes requested, or some other fatal error occurred.
.El
.Sh SEE ALSO
.Xr getrandom 2 ,
.Xr arc4random 3 ,
.Xr random 4
.Sh STANDARDS
.Fn getentropy
nearly conforms to
.St -p1003.1-2024 .
.Sh HISTORY
The
.Fn getentropy
function appeared in
.Ox 5.6 .
The
.Fx
libc compatibility shim first appeared in
.Fx 12.0 .
.Sh BUGS
.In limits.h
does not define
.Dv GETENTROPY_MAX .
Some error values do not match
.St -p1003.1-2024 .