GETENTROPY(3)          FreeBSD Library Functions Manual          GETENTROPY(3)

NAME
     getentropy - get entropy

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <unistd.h>

     int
     getentropy(void *buf, size_t buflen);

DESCRIPTION
     getentropy() fills a buffer with high-quality random data.

     The maximum buflen permitted is 256 bytes.

     If it does not produce an error, getentropy() always provides the
     requested number of bytes of random data.

     Similar to reading from /dev/urandom just after boot, getentropy() may
     block until the system has collected enough entropy to seed the CSPRNG.

IMPLEMENTATION NOTES
     The getentropy() function is implemented using getrandom(2).

RETURN VALUES
     Upon successful completion, the value 0 is returned; otherwise the
     value -1 is returned and the global variable errno is set to indicate the
     error.

ERRORS
     getentropy() will succeed unless:

     [EFAULT]           The buf parameter points to an invalid address.

     [EIO]              Too many bytes requested, or some other fatal error
                        occurred.

SEE ALSO
     getrandom(2), arc4random(3), random(4)

STANDARDS
     getentropy() is non-standard.  It is present on OpenBSD and Linux.

HISTORY
     The getentropy() function appeared in OpenBSD 5.6.  The FreeBSD libc
     compatibility shim first appeared in FreeBSD 12.0.

FreeBSD 13.1-RELEASE-p6        February 24, 2018       FreeBSD 13.1-RELEASE-p6