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.Dd May 1, 2020
.Dt GETENTROPY 3
.Os
.Sh NAME
.Nm getentropy
.Nd fill a buffer with high quality random data
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn getentropy "void *buf" "size_t buflen"
.In limits.h
.Li #define GETENTROPY_MAX	256
.Sh DESCRIPTION
.Pp
The
.Fn getentropy
function fills a buffer with high quality random data, suitable for seeding
cryptographically secure psuedorandom number generators.
.Pp
.Fn getentropy
is only intended for seeding random number generators and is not intended
for use by regular code which simply needs secure random data.
For this purpose, please use
.Xr arc4random 3 .
.Pp
The maximum value for
.Li buflen
is 256 bytes.
.Sh IMPLEMENTATION NOTES
.Fn getentropy
reads from the
.Xr sysctl 7
variable
.Li kern.arandom .
.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.
.Sh SEE ALSO
.Xr arc4random 3 ,
.Xr rnd 4
.Sh STANDARDS
The
.Fn getentropy
function is non-standard.
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
.Nx 10 .