Ubuntu Manpage: getrandom - obtain a series of random bytes

NAME

       getrandom - obtain a series of random bytes

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/random.h>

       ssize_t getrandom(void buf[.buflen], 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 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:

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 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  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 to indicate the error.

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 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.

       ENOSYS The  glibc  wrapper  function  for  getrandom()  determined  that  the  underlying kernel does not
              implement this system call.

STANDARDS

       Linux.

HISTORY

       Linux 3.17, glibc 2.25.

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 32Mi-1 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), 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 fails 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 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  compatibility  with  OpenBSD's
       getentropy(3), which is nowadays supported by glibc.

       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!

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.