getrandom - obtain a series of random bytes


   #include <linux/random.h>

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


   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:

          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.

          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.


   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.


   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

   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.


   getrandom() was introduced in version 3.17 of the Linux kernel.


   This system call is Linux-specific.


   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

   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

   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() system call.

   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!

   Emulating OpenBSD's getentropy()
   The  getentropy()  system  call  in  OpenBSD  can be emulated using the
   following function:

       getentropy(void *buf, size_t buflen)
           int ret;

           if (buflen > 256)
               goto failure;
           ret = getrandom(buf, buflen, 0);
           if (ret < 0)
               return ret;
           if (ret == buflen)
               return 0;
           errno = EIO;
           return -1;


   As of Linux 3.19, the following bug exists:

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


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


   This  page  is  part of release 4.09 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


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