Description
This function sends a small keep-alive packet to an
established persistent connection. This means that
the persistent parameter of the constructor must have
been set to 1, and a connection must have already
been made with connect. If either is not
true, this function will fail.
If persistent connections are desired but the time between
gathering new data may be large, this function can be
used to guarantee that the connection is still active.
If the keep-alive packet cannot be sent successfully,
this function will return with an appropriate error message.
Returns
Returns -1 if an error occurred, or 0 otherwise.
In case of error, the program will croak() with an appriopriate message.