getrandom(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

GETRANDOM(2)            Linux Programmer's Manual           GETRANDOM(2)

NAME         top

       getrandom - obtain a series of random bytes

SYNOPSIS         top

       #include <sys/random.h>

       ssize_t getrandom(void *buf, size_t buflen, unsigned int flags);

DESCRIPTION         top

       The getrandom() system call fills the buffer pointed to by buf
       with up to buflen random bytes.  These bytes can be used to seed
       user-space random number generators or for cryptographic
       purposes.

       By default, getrandom() draws entropy from the urandom source
       (i.e., the same source as the /dev/urandom device).  This
       behavior can be changed via the flags argument.

       If the urandom source has been initialized, reads of up to 256
       bytes will always return as many bytes as requested and will not
       be interrupted by signals.  No such guarantees apply for larger
       buffer sizes.  For example, if the call is interrupted by a
       signal handler, it may return a partially filled buffer, or fail
       with the error EINTR.

       If the urandom source has not yet been initialized, then
       getrandom() will block, unless GRND_NONBLOCK is specified in
       flags.

       The flags argument is a bit mask that can contain zero or more of
       the following values ORed together:

       GRND_RANDOM
              If this bit is set, then random bytes are drawn from the
              random source (i.e., the same source as the /dev/random
              device) instead of the urandom source.  The random source
              is limited based on the entropy that can be obtained from
              environmental noise.  If the number of available bytes in
              the random source is less than requested in buflen, the
              call returns just the available random bytes.  If no
              random bytes are available, the behavior depends on the
              presence of GRND_NONBLOCK in the flags argument.

       GRND_NONBLOCK
              By default, when reading from the random source,
              getrandom() blocks if no random bytes are available, and
              when reading from the urandom source, it blocks if the
              entropy pool has not yet been initialized.  If the
              GRND_NONBLOCK flag is set, then getrandom() does not block
              in these cases, but instead immediately returns -1 with
              errno set to EAGAIN.

RETURN VALUE         top

       On success, getrandom() returns the number of bytes that were
       copied to the buffer buf.  This may be less than the number of
       bytes requested via buflen if either GRND_RANDOM was specified in
       flags and insufficient entropy was present in the random source
       or the system call was interrupted by a signal.

       On error, -1 is returned, and errno is set to indicate the error.

ERRORS         top

       EAGAIN The requested entropy was not available, and getrandom()
              would have blocked if the GRND_NONBLOCK flag was not set.

       EFAULT The address referred to by buf is outside the accessible
              address space.

       EINTR  The call was interrupted by a signal handler; see the
              description of how interrupted read(2) calls on "slow"
              devices are handled with and without the SA_RESTART flag
              in the signal(7) man page.

       EINVAL An invalid flag was specified in flags.

       ENOSYS The glibc wrapper function for getrandom() determined that
              the underlying kernel does not implement this system call.

VERSIONS         top

       getrandom() was introduced in version 3.17 of the Linux kernel.
       Support was added to glibc in version 2.25.

CONFORMING TO         top

       This system call is Linux-specific.

NOTES         top

       For an overview and comparison of the various interfaces that can
       be used to obtain randomness, see random(7).

       Unlike /dev/random and /dev/urandom, getrandom() does not involve
       the use of pathnames or file descriptors.  Thus, getrandom() can
       be useful in cases where chroot(2) makes /dev pathnames
       invisible, and where an application (e.g., a daemon during start-
       up) closes a file descriptor for one of these files that was
       opened by a library.

   Maximum number of bytes returned
       As of Linux 3.19 the following limits apply:

       *  When reading from the urandom source, a maximum of 33554431
          bytes is returned by a single call to getrandom() on systems
          where int has a size of 32 bits.

       *  When reading from the random source, a maximum of 512 bytes is
          returned.

   Interruption by a signal handler
       When reading from the urandom source (GRND_RANDOM is not set),
       getrandom() will block until the entropy pool has been
       initialized (unless the GRND_NONBLOCK flag was specified).  If a
       request is made to read a large number of bytes (more than 256),
       getrandom() will block until those bytes have been generated and
       transferred from kernel memory to buf.  When reading from the
       random source (GRND_RANDOM is set), getrandom() will block until
       some random bytes become available (unless the GRND_NONBLOCK flag
       was specified).

       The behavior when a call to getrandom() that is blocked while
       reading from the urandom source is interrupted by a signal
       handler depends on the initialization state of the entropy buffer
       and on the request size, buflen.  If the entropy is not yet
       initialized, then the call fails with the EINTR error.  If the
       entropy pool has been initialized and the request size is large
       (buflen > 256), the call either succeeds, returning a partially
       filled buffer, or fails with the error EINTR.  If the entropy
       pool has been initialized and the request size is small
       (buflen <= 256), then getrandom() will not fail with EINTR.
       Instead, it will return all of the bytes that have been
       requested.

       When reading from the random source, blocking requests of any
       size can be interrupted by a signal handler (the call fails with
       the error EINTR).

       Using getrandom() to read small buffers (<= 256 bytes) from the
       urandom source is the preferred mode of usage.

       The special treatment of small values of buflen was designed for
       compatibility with OpenBSD's getentropy(3), which is nowadays
       supported by glibc.

       The user of getrandom() must always check the return value, to
       determine whether either an error occurred or fewer bytes than
       requested were returned.  In the case where GRND_RANDOM is not
       specified and buflen is less than or equal to 256, a return of
       fewer bytes than requested should never happen, but the careful
       programmer will check for this anyway!

BUGS         top

       As of Linux 3.19, the following bug exists:

       *  Depending on CPU load, getrandom() does not react to
          interrupts before reading all bytes requested.

SEE ALSO         top

       getentropy(3), random(4), urandom(4), random(7), signal(7)

COLOPHON         top

       This page is part of release 5.11 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2021-03-22                   GETRANDOM(2)

Pages that refer to this page: syscalls(2)getentropy(3)random(3)uuid_generate(3)random(4)random(7)signal(7)