.\" Copyright (C) 2017, Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH getentropy 3 2023-03-30 "Linux man-pages 6.04"
.SH NAME
getentropy \- fill a buffer with random bytes
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.BI "int getentropy(void " buffer [. length "], size_t " length );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR getentropy ():
.nf
    _DEFAULT_SOURCE
.fi
.SH DESCRIPTION
The
.BR getentropy ()
function writes
.I length
bytes of high-quality random data to the buffer starting
at the location pointed to by
.IR buffer .
The maximum permitted value for the
.I length
argument is 256.
.PP
A successful call to
.BR getentropy ()
always provides the requested number of bytes of entropy.
.SH RETURN VALUE
On success, this function returns zero.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EFAULT
Part or all of the buffer specified by
.I buffer
and
.I length
is not in valid addressable memory.
.TP
.B EIO
.I length
is greater than 256.
.TP
.B EIO
An unspecified error occurred while trying to overwrite
.I buffer
with random data.
.TP
.B ENOSYS
This kernel version does not implement the
.BR getrandom (2)
system call required to implement this function.
.SH STANDARDS
None.
.SH HISTORY
glibc 2.25.
OpenBSD.
.SH NOTES
The
.BR getentropy ()
function is implemented using
.BR getrandom (2).
.PP
Whereas the glibc wrapper makes
.BR getrandom (2)
a cancelation point,
.BR getentropy ()
is not a cancelation point.
.PP
.BR getentropy ()
is also declared in
.BR <sys/random.h> .
(No feature test macro need be defined to obtain the declaration from
that header file.)
.PP
A call to
.BR getentropy ()
may block if the system has just booted and the kernel has
not yet collected enough randomness to initialize the entropy pool.
In this case,
.BR getentropy ()
will keep blocking even if a signal is handled,
and will return only once the entropy pool has been initialized.
.SH SEE ALSO
.BR getrandom (2),
.BR urandom (4),
.BR random (7)
