getentropy(2) [mojave man page]

GETENTROPY(2)						      BSD System Calls Manual						     GETENTROPY(2)

NAME

     getentropy -- get entropy

SYNOPSIS

     #include <sys/random.h>

     int
     getentropy(void *buf, size_t buflen);

DESCRIPTION

     getentropy() fills a buffer with random data, which can be used as input for process-context pseudorandom generators like arc4random(3).

     The maximum buffer size permitted is 256 bytes.  If buflen exceeds this, an error of EIO will be indicated.

     getentropy() should be used as a replacement for random(4) when random data derived directly from the kernel random byte generator is
     required.	Unlike the random(4) pseudo-devices, it is not vulnerable to file descriptor exhaustion attacks and is available when sandboxed or
     in a chroot, making it more reliable for security-critical applications.

     However, it should be noted that getentropy() is primarily intended for use in the construction and seeding of userspace PRNGs like
     arc4random(3) or CC_crypto(3).  Clients who simply require random data should use arc4random(3), CCRandomGenerateBytes() from CC_crypto(3),
     or SecRandomCopyBytes() from the Security framework instead of getentropy() or random(4)

RETURN VALUES

     Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the
     error.

ERRORS

     getentropy() will succeed unless:

     [EINVAL]		The buf parameter points to an invalid address.

     [EIO]		Too many bytes requested, or some other fatal error occurred.

SEE ALSO

     arc4random(3) CC_crypto(3) random(4)

HISTORY

     The getentropy() function appeared in OSX 10.12

BSD
								  October 2 2015							       BSD

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

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

       This page is part of release 4.15 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							     GETENTROPY(3)