Ubuntu Manpage: getrandom - obtain a series of random bytes

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NAME

       getrandom - obtain a series of random bytes

SYNOPSIS

       #include <sys/random.h>

       ssize_t 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.

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 appropriately.

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.

VERSIONS

       getrandom() was introduced in version 3.17 of the Linux  kernel.   Support  was  added  to
       glibc in version 2.25.

CONFORMING TO

       This system call is Linux-specific.

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 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 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.

COLOPHON

       This  page  is  part of release 5.10 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/.