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

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