Standard C library (libc, -lc)
#include <unistd.h> int getentropy(void buffer[.length], size_t length);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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.
On success, this function returns zero. On error, -1 is returned, and errno is set to indicate the error.
Part or all of the buffer specified by buffer and length is not in valid addressable memory.
length is greater than 256.
An unspecified error occurred while trying to overwrite buffer with random data.
This kernel version does not implement the getrandom(2) system call required to implement this function.
glibc 2.25. OpenBSD.
The getentropy() function is implemented using getrandom(2).
Whereas the glibc wrapper makes getrandom(2) a cancelation point, getentropy() 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 initialized.
getrandom(2), urandom(4), random(7)