The filter can submitted as a character string in ECL or as a compiled function in a C library archieve.
The Echo CVS repository contains samples for both cases.

int xxx_filter(void* input, void* output, void* data) {

Filters can be divided into two classes. One class of filters take in the incoming event and decides whether it should be published on the derived channel. The other class of filters act as a tranformer, which means it takes in the incoming event but publish it in a different event format. The two kind of filters are submitted through different APIs.

For ECL filter, there are three predefined parameters: input, output, and data. "input" represents the incoming event, "output" represents the outgoing event, and "data" acts like a static variable which will still persist after each filter invocation. The format of the "input" parameter is specified when the *to-be-derived* channel is created, and the format of "output" and "data" is specified when the *derived* channel is created. For example,

/* when channel is created */
chan = EChannel_typed_create(cc, INPUT_FIELD, INPUT_FIELD_FORMAT_LIST);
TO_BE_DERIVED_CHAN_ID = ECglobal_id(chan);

/* when channel is derived */	
derived_chan = EChannel_typed_derive_data(cc, 
        TO_BE_DERIVED_CHAN_ID, 
        FILTER_NAME, 
        OUTPUT_FIELD, 
        OUTPUT_FIELD_FORMAT_LIST,      			
        DATA_SPEC);

For filter as a function in C library, the parameters are determined by their order of the declaration.
Usually, we use the filter declaration in C like this.

int XXX_FILTER(void* input, void* output, void* data);

So, the "input/output/data" parameter will be the 1st, 2nd and the 3rd parameter, and the function declaration maintains this consistency. For source code access, I remembered the test/ directory in the ECho CVS repository contains some samples for the filter use. Also the filters for smartP can be obtained from the pipegl/pipgl_filters.[ch] from the smartP's CVS repository.