GETENTROPY(3)             Linux Programmer's Manual            GETENTROPY(3)

NAME         top

       getentropy - fill a buffer with random bytes

SYNOPSIS         top

       #include <unistd.h>

       int getentropy(void *buffer, size_t length);

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


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 is set appropriately.

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) system
              call required to implement this function.

VERSIONS         top

       The getentropy() function first appeared in glibc 2.25.

CONFORMING TO         top

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

NOTES         top

       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

       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), urandom(4), random(7)

COLOPHON         top

       This page is part of release 4.11 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

Linux                            2017-05-03                    GETENTROPY(3)

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