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.Dd February 28, 2023
.Dt GETENTROPY 3
.Os
.Sh NAME
.Nm getentropy
.Nd generate uniform random seeds from system entropy for cryptography
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn getentropy "void *buf" "size_t buflen"
.In limits.h
.Pp
.Li #define GETENTROPY_MAX	256
.Sh DESCRIPTION
.Pp
The
.Nm
function fills
.Fa buf
with exactly
.Fa buflen
independent uniform random bytes derived from the system's entropy
pool.
.Pp
The output of
.Nm
is meant to be unpredictable to an adversary and fit for use in
cryptography.
See
.Sx CAVEATS
below.
.Pp
.Nm
is meant for seeding random number generators, not for direct use by
applications; most applications should use
.Xr arc4random 3 .
.Pp
.Fa buflen
must be at most 256.
.Sh RETURN VALUES
.Rv -std getentropy
.Sh ERRORS
.Fn getentropy
will succeed unless:
.Bl -tag -width Er
.It Bq Er EFAULT
The
.Fa buf
argument points to an invalid memory address.
.It Bq Er EINVAL
More than 256 bytes were requested.
.El
.Sh CAVEATS
Security can only be guaranteed relative to whatever unpredictable
physical processes or secret seed material are available to the system;
see
.Xr entropy 7 .
.Pp
On systems which have no hardware random number generator and which
have not had secret seed material loaded,
.Nx
makes a reasonable effort to incorporate samples from various physical
processes available to it that might be unpredictable from random
jitter in timing.
.Pp
However, the
.Nm
interface alone can make no security guarantees without a physical
system configuration that includes random number generation hardware or
secret seed material from such hardware on another machine.
.Sh SEE ALSO
.Xr arc4random 3 ,
.Xr rnd 4 ,
.Xr entropy 7
.Sh STANDARDS
The
.Fn getentropy
function is nonstandard.
However, it is likely to be included in the next revision of POSIX.
.Sh HISTORY
The
.Fn getentropy
function first appeared in
.Ox 5.6 ,
then in
.Fx 12.0 ,
and in
.Nx 10.0 .