getrandom

GETRANDOM(2)                Linux Programmer's Manual               GETRANDOM(2)



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.

SEE ALSO
       getentropy(3), random(4), urandom(4), random(7), signal(7)

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/.



Linux                              2017-09-15                       GETRANDOM(2)