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>

       int 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 appropriately.

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.

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/random, 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
       will fail 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 4.11 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                            2017-03-13                     GETRANDOM(2)

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