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 <linux/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.

       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         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 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         top

       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         top

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

CONFORMING TO         top

       This system call is Linux-specific.

NOTES         top

   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         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

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

COLOPHON         top

       This page is part of release 4.07 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                            2015-01-22                     GETRANDOM(2)