Write your custom filter methods using NSAPI functions. For a summary of the NSAPI functions specific to filter development, seeOverview of NSAPI Functions for Filter Development and Filter Methods for the filter method prototypes.
The filter must be created by a call to filter_create. Typically, each plug-in defines an nsapi_module_init function that is used to call filter_create and perform any other initialization tasks. For more information, see nsapi_module_init() Function and filter_create() Function.
Filter methods are invoked whenever the server or an SAF calls certain NSAPI functions such as net_write or filter_insert. As a result, filter methods can be invoked from any thread and should only block using NSAPI functions. For example, crit_enter and net_read. If a filter method blocks using other functions, for example, the Windows WaitForMultipleObjects and ReadFile functions, the server could hang. Also, shared objects that define filters should be loaded with the NativeThread="no" flag, as described in Loading and Initializing the Filter.
If a filter method must block using a non-NSAPI function, KernelThreads 1 should be set in magnus.conf. For more information about KernelThreads, see the description in the chapter Syntax and Use of magnus.conf in the Sun Java System Web Server 7.0 Update 4 Administrator’s Configuration File Reference.
Keep the following in mind when writing your filter:
Write thread-safe code.
IO should only be performed using the NSAPI functions documented in File I/O.
Thread synchronization should only be performed using NSAPI functions documented in Threads.
Blocking might affect performance.
Carefully check and handle all errors.
For examples of custom filters, see Chapter 4, Examples of Custom SAFs and Filters.