getrandom

GETRANDOM(2)               Linux Programmer's Manual              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 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  inter-
       rupted  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  immedi-
              ately 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 insuffi-
       cient 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 accessible address
              space.

       EINTR  The  call  was interrupted by a signal handler; see the descrip-
              tion of how interrupted read(2) calls on "slow" devices are han-
              dled  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
       getrandom()  was  introduced in version 3.17 of the Linux kernel.  Sup-
       port 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 /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),  getran-
       dom()  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), getran-
       dom() 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, return-
       ing  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 com-
       patibility with OpenBSD's getentropy(3), which is nowadays supported by
       glibc.

       The  user  of getrandom() must always check the return value, to deter-
       mine 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
       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 4.15 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                             2017-09-15                      GETRANDOM(2)
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