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getrandom(2)
getrandom(2) System Calls getrandom(2)
NAME
getrandom, getentropy - retrieve data from the kernel random pool
SYNOPSIS
#include <sys/random.h>
ssize_t getrandom(void *buf, size_t buflen, unsigned int flags);
#include <unistd.h>
int getentropy(void *buf, size_t buflen);
DESCRIPTION
The getrandom() function can be used to request buflen bytes of data
from the kernel random pool, which is to be placed into the buffer,
pointed to by buf. It is recommended to use the getrandom() function
instead of open(2) and read(2) functions on the /dev/random or
/dev/urandom device.
The random data returned by the getrandom() function is processed by a
FIPS 140-2 approved deterministic random bit generator (DRBG).
If the GRND_RANDOM flag is set, then the implementation uses the same
pool as /dev/random, otherwise the /dev/urandom pool is used.
If no entropy is available in the pool, the getrandom() function will
block unless the GRND_NONBLOCK flag is set. In this case, the function
returns -1 and errno is set to EAGAIN. Note that the number of bytes
returned can be less than requested or 0. Callers need to check the
return value to determine if random bytes were returned. This means
this is not an acceptable calling sequence:
(void) getrandom(&buf, sizeof (buf), 0);
The getentropy() function is always a blocking call, it is expected to
be used only to seed a userspace implementation of a random bit genera‐
tor.
RETURN VALUES
Upon successful completion, the getrandom() function returns the number
of bytes written to buf. Otherwise, it returns 0 and sets errno to
indicate the error.
Upon successful completion, the getentropy() function returns 0. Other‐
wise, it returns -1 and sets errno to indicate the error.
ERRORS
The getrandom() and getentropy() functions fail if:
EINVAL Invalid flags or flag combinations
o bufsz is <= 0 or > 1040, when GRND_RANDOM is set
o bufsz is <= 0 or > 133120, when GRND_RANDOM is not
set
EFAULT buf is an invalid address.
EAGAIN No entropy is available and GRND_NONBLOCK is set.
The getentropy() call also fails if:
EIO More than 256 bytes are requested, or the returned amount of
entropy does not match the request.
EXAMPLES
Example 1 Using the getrandom() function
#include <sys/random.h>
#include <stdlib.h>
.
size_t bufsz = 1024;
char *buf;
int ret;
.
...
buf = malloc(bufsz);
...
ret = getrandom(buf, bufsz, GRND_RANDOM);
if (ret != bufsz) {
perror("getrandom failed");
...
}
...
Example 2 Using the getentropy() function
#include <sys/random.h>
#include <stdlib.h>
.
size_t entsz = 128;
char *entropy;
int err;
.
...
entropy = malloc(entsz);
...
err = getentropy(entropy, entsz);
if (err != 0) {
perror("getentropy failed");
...
}
/* Use entropy to seed our RNG */
ATTRIBUTES
See attributes(7) for descriptions of the following attributes:
tab() box; cw(2.75i) |cw(2.75i) lw(2.75i) |lw(2.75i) ATTRIBUTE TYPEAT‐
TRIBUTE VALUE _ Interface StabilityCommitted _ MT-LevelMT-Safe
SEE ALSO
random(4D)
Oracle Solaris 11.4 05 Sep 2019 getrandom(2)