SSL_get_fd - get file descriptor linked to an SSL object
Synopsis #include <openssl/ssl.h> int
SSL_get_fd(SSL *ssl); int SSL_get_rfd(SSL *ssl); int
SSL_get_wfd(SSL *ssl);
DESCRIPTIONSSL_get_fd() returns the file descriptor which is linked to
ssl. SSL_get_rfd() and SSL_get_wfd() return the file descriptors
for the read or the write channel, which can be different. If the
read and the write channel are different, SSL_get_fd() will return
the file descriptor of the read channel. RETURN VALUESThe following return values can occur:
-1 The operation failed, because the underlying BIO is not of
the correct type (suitable for file descriptors).
HP Open Source Security for OpenVMS Volume 2:... 
SSL Application Programming Interface (API) Reference
