int RAND_bytes(unsigned char *buf, int num);
int RAND_pseudo_bytes(unsigned char *buf, int num);
RAND_bytes() puts num cryptographically strong pseudo-random bytes into buf. An error occurs if the PRNG has not been seeded with enough randomness to ensure an unpredictable byte sequence.
RAND_pseudo_bytes() puts num pseudo-random bytes into buf. Pseudo-random byte sequences generated by RAND_pseudo_bytes() will be unique if they are of sufficient length, but are not necessarily unpredictable. They can be used for non-cryptographic purposes and for certain purposes in cryptographic protocols, but usually not for key generation etc.
The contents of buf is mixed into the entropy pool before retrieving the new pseudo-random bytes unless disabled at compile time (see FAQ).
RAND_bytes() returns 1 on success, 0 otherwise. The error code can be obtained by ERR_get_error(3). RAND_pseudo_bytes() returns 1 if the bytes generated are cryptographically strong, 0 otherwise. Both functions return -1 if they are not supported by the current RAND method.
RAND_bytes() is available in all versions of SSLeay and OpenSSL. It has a return value since OpenSSL 0.9.5. RAND_pseudo_bytes() was added in OpenSSL 0.9.5.