activate_non_blocking

Activating non blocking mode

Before using any non-blocking operation, it is necessary to enable it first by setting the MYSQL_OPT_NONBLOCK option via mysql_optionsv() api function:

mysql_optionsv(mysql, MYSQL_OPT_NONBLOCK, 0);

This call can be made at any time — typically it will be done at the start, before mysql_real_connect(), but it can be done at any time to start using non-blocking operations.

If a non-blocking operation is attempted without setting the MYSQL_OPT_NONBLOCK option, the program will typically crash with a NULL pointer exception.

The argument for MYSQL_OPT_NONBLOCK is the size of the stack used to save the state of a non-blocking operation while it is waiting for I/O and the application is doing other processing. Normally, applications will not have to change this, and it can be passed as zero to use the default value.

Mixing blocking and non-blocking operation

It is possible to freely mix blocking and non-blocking calls on the same MYSQL connection.

Thus, an application can do a normal blocking mysql_real_connect() and subsequently do a non-blocking mysql_real_query_start(). Or vice versa, do a non-blocking mysql_real_connect_start(), and later do a blocking mysql_real_query() on the resulting connection.

Mixing can be useful to allow code to use the simpler blocking API in parts of the program where waiting is not a problem. For example establishing the connection(s) at program startup, or doing small quick queries between large, long-running ones.

The only restriction is that any previous non-blocking operation must have finished before starting a new blocking (or non-blocking) operation, see the next section: "Terminating a non-blocking operation early" below.

Terminating a non-blocking operation early

When a non-blocking operation is started with mysql_real_query_start() or another _start() function, it must be allowed to finish before starting a new operation. Thus, the application must continue calling 'mysql_real_query_cont()` until zero is returned, indicating that the operation is completed. It is not allowed to leave one operation "hanging" in the middle of processing and then start a new one on top of it.

It is, however, permissible to terminate the connection completely with mysql_close() in the middle of processing a non-blocking call. A new connection must then be initiated with mysql_real_connect() before new queries can be run, either with a new MYSQL object or re-using the old one.

In the future, we may implement an abort facility to force an on-going operation to terminate as quickly as possible (but it will still be necessary to call mysql_real_query_cont() one last time after abort, allowing it to clean up the operation and return immediately with an appropriate error code).