getentropy(3) - Linux manual page (original) (raw)

getentropy(3) Library Functions Manual getentropy(3)

NAME top

   getentropy - fill a buffer with random bytes

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#include <unistd.h>**

   **int getentropy(void** _buffer_**[.**_length_**], size_t** _length_**);**

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

   **getentropy**():
       _DEFAULT_SOURCE

DESCRIPTION top

   The **getentropy**() function writes _length_ bytes of high-quality
   random data to the buffer starting at the location pointed to by
   _buffer_.  The maximum permitted value for the _length_ argument is
   256.

   A successful call to **getentropy**() always provides the requested
   number of bytes of entropy.

RETURN VALUE top

   On success, this function returns zero.  On error, -1 is returned,
   and _[errno](../man3/errno.3.html)_ is set to indicate the error.

ERRORS top

   **EFAULT** Part or all of the buffer specified by _buffer_ and _length_ is
          not in valid addressable memory.

   **EIO** _length_ is greater than 256.

   **EIO** An unspecified error occurred while trying to overwrite
          _buffer_ with random data.

   **ENOSYS** This kernel version does not implement the [getrandom(2)](../man2/getrandom.2.html)
          system call required to implement this function.

STANDARDS top

   None.

HISTORY top

   glibc 2.25.  OpenBSD.

NOTES top

   The **getentropy**() function is implemented using [getrandom(2)](../man2/getrandom.2.html).

   Whereas the glibc wrapper makes [getrandom(2)](../man2/getrandom.2.html) a cancelation point,
   **getentropy**() is not a cancelation point.

   **getentropy**() is also declared in **<sys/random.h>**.  (No feature test
   macro need be defined to obtain the declaration from that header
   file.)

   A call to **getentropy**() may block if the system has just booted and
   the kernel has not yet collected enough randomness to initialize
   the entropy pool.  In this case, **getentropy**() will keep blocking
   even if a signal is handled, and will return only once the entropy
   pool has been initialized.

SEE ALSO top

   [getrandom(2)](../man2/getrandom.2.html), [urandom(4)](../man4/urandom.4.html), [random(7)](../man7/random.7.html)

COLOPHON top

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Linux man-pages 6.10 2024-07-23 getentropy(3)

Pages that refer to this page:getrandom(2), random(7)