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getentropy(2) [mojave man page]

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

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

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

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 Linux 2017-09-15 GETENTROPY(3)

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