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.