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/.

Linux                                              2015-01-22                                       GETRANDOM(2)