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():

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/.