getentropy(3) — Linux manual page


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


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

       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 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)
              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 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), urandom(4), random(7)

COLOPHON         top

       This page is part of release 5.13 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                          2021-03-22                  GETENTROPY(3)

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