NAME

       getentropy - fill a buffer with random bytes

SYNOPSIS

       #include <unistd.h>

       int getentropy(void *buffer, size_t length);

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

       getentropy():
           _DEFAULT_SOURCE

DESCRIPTION

       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

       On success, this function returns zero.  On error,  -1  is  returned,  and  errno  is  set
       appropriately.

ERRORS

       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)  system  call  required  to
              implement this function.

VERSIONS

       The getentropy() function first appeared in glibc 2.25.

CONFORMING TO

       This function is nonstandard.  It is also present on OpenBSD.

NOTES

       The getentropy() function is implemented using getrandom(2).

       Whereas  the  glibc wrapper makes getrandom(2) a cancellation point, getentropy() is not a
       cancellation 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

       getrandom(2), urandom(4), random(7)

COLOPHON

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