Updated: 2025/Nov/16

Please read Privacy Policy. It's for your privacy.

GETENTROPY(3)              Library Functions Manual              GETENTROPY(3)

NAME
     getentropy - generate uniform random seeds from system entropy for
     cryptography

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <unistd.h>

     int
     getentropy(void *buf, size_t buflen);

     #include <limits.h>

     #define GETENTROPY_MAX 256

DESCRIPTION
     The getentropy function fills buf with exactly buflen independent uniform
     random bytes derived from the system's entropy pool.

     The output of getentropy is meant to be unpredictable to an adversary and
     fit for use in cryptography.  See CAVEATS below.

     getentropy is meant for seeding random number generators, not for direct
     use by applications; most applications should use arc4random(3).

     buflen must be at most 256.

RETURN VALUES
     The getentropy() function returns the value 0 if successful; 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 argument points to an invalid memory address.

     [EINVAL]           More than 256 bytes were requested.

CAVEATS
     Security can only be guaranteed relative to whatever unpredictable
     physical processes or secret seed material are available to the system;
     see entropy(7).

     On systems which have no hardware random number generator and which have
     not had secret seed material loaded, NetBSD makes a reasonable effort to
     incorporate samples from various physical processes available to it that
     might be unpredictable from random jitter in timing.

     However, the getentropy 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.

     NetBSD attempts to reseed the system entropy pool when it has detected
     the system has been cloned as a guest in a virtual machine, so that
     subsequent calls to getentropy in the clones yield independent outputs.
     However, this relies on the virtual machine host to notify the guest,
     e.g. through the acpivmgenid(4) device, and even so there is an
     unavoidable small window of time between when the virtual machine is
     actually cloned and when the system is reseeded during which getentropy
     may yield identical outputs in the clones.

SEE ALSO
     arc4random(3), rnd(4), entropy(7)

STANDARDS
     The getentropy function conforms to IEEE Std 1003.1-2024 ("POSIX.1").

HISTORY
     The getentropy function first appeared in OpenBSD 5.6, then in
     FreeBSD 12.0, and in NetBSD 10.0.

NetBSD 11.99                    August 28, 2024                   NetBSD 11.99