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GETENTROPY(2) System Calls Manual GETENTROPY(2)

getentropy —
get entropy

#include <unistd.h>

int
getentropy(void *buf, size_t buflen);

getentropy() fills a buffer with high-quality entropy, 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() is not intended for regular code; please use the arc4random(3) family of functions instead.

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.

getentropy() will succeed unless:
[]
The buf parameter points to an invalid address.
[]
Too many bytes requested, or some other fatal error occurred.

arc4random(3)

The getentropy() function appeared in OpenBSD 5.6.
January 31, 2015 OpenBSD-current