getentropy(3) Library Functions Manual getentropy(3)
NAME
getentropy - fill a buffer with random bytes
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <unistd.h>
int getentropy(void buffer[.length], 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 to indicate the error.
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.
STANDARDS
None.
HISTORY
glibc 2.25. OpenBSD.
NOTES
The getentropy() function is implemented using getrandom(2).
Whereas the glibc wrapper makes getrandom(2) a cancelation point, geten-
tropy() is not a cancelation 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 initial-
ized.
SEE ALSO
getrandom(2), urandom(4), random(7)
Linux man-pages 6.9.1 2024-05-02 getentropy(3)
Generated by dwww version 1.16 on Tue Dec 16 04:51:38 CET 2025.