GETRANDOM(2)              (2017-09-15)               GETRANDOM(2)

     NAME
          getrandom - obtain a series of random bytes

     SYNOPSIS
          #include <sys/random.h>

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

     DESCRIPTION
          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 crypto-
          graphic 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 avail-
               able 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

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               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
          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 appropriately.

     ERRORS
          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 accessi-
               ble 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 sys-
               tem call.

     VERSIONS
          getrandom() was introduced in version 3.17 of the Linux ker-
          nel.  Support was added to glibc in version 2.25.

     CONFORMING TO
          This system call is Linux-specific.

     NOTES
          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

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          /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 avail-
          able (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

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

     COLOPHON
          This page is part of release 5.10 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/.

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