Ubuntu Manpage: getrandom - obtain a series of random bytes

NAME

       getrandom - obtain a series of random bytes

SYNOPSIS

       #include <linux/random.h>

       int 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.

getrandom() relies on entropy gathered from device drivers and other sources of
environmental noise. Unnecessarily reading large quantities of data will have a negative
impact on other users of the /dev/random and /dev/urandom devices. Therefore, getrandom()
should not be used for Monte Carlo simulations or other programs/algorithms which are
doing probabilistic sampling.

By default, getrandom() draws entropy from the /dev/urandom pool. This behavior can be
changed via the flags argument. If the /dev/urandom pool 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 pool has not yet been initialized, then the call blocks, 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 /dev/random pool instead
of the /dev/urandom pool. The /dev/random pool is limited based on the entropy
that can be obtained from environmental noise. If the number of available bytes in
/dev/random 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 /dev/random, getrandom() blocks if no random bytes
are available, and when reading from /dev/urandom, 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

       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  GRND_RANDOM  was
       specified in flags and insufficient entropy was present in the /dev/random pool, or if the
       system call was interrupted by a signal.

       On error, -1 is returned, and errno is set appropriately.

ERRORS

       EINVAL An invalid flag was specified in flags.

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

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

       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.

VERSIONS

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

CONFORMING TO

       This system call is Linux-specific.

NOTES

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

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

       *  When reading from /dev/random, a maximum of 512 bytes is returned.

   Initialization of the entropy pool
       The kernel collects bits of entropy from environment.  When a sufficient number of  random
       bits  has  been  collected, the /dev/urandom entropy pool is considered to be initialized.
       This state is normally reached early in the system bootstrap phase.

   Interruption by a signal handler
       When reading from /dev/urandom (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 (more than 256) of bytes,  getrandom()  will  block
       until  those  bytes  have  been generated and transferred from kernel memory to buf.  When
       reading from /dev/random (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 /dev/urandom 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 /dev/random, blocking requests of any  size  can  be  interrupted  by  a
       signal (the call fails with the error EINTR).

       Calling  getrandom()  to  read  /dev/urandom  for  small  values (<= 256) of buflen 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!

   Choice of random device
       Unless you are doing long-term key generation (and perhaps not even  then),  you  probably
       shouldn't  be  using  GRND_RANDOM.  The cryptographic algorithms used for /dev/urandom are
       quite conservative, and so should be sufficient for all  purposes.   The  disadvantage  of
       GRND_RANDOM  is  that  it  can  block.   Furthermore, dealing with the partially fulfilled
       getrandom() requests that can occur when using GRND_RANDOM increases code complexity.

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

           int
           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;
           failure:
               errno = EIO;
               return -1;
           }

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.

COLOPHON

       This  page  is  part of release 4.04 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 http://www.kernel.org/doc/man-pages/.