From collectd Wiki

Category for plugins which (may) register a read callback.

read callbacks are registered with one of the following functions from src/plugin.h:

 typedef int (*plugin_read_cb) (user_data_t *);
 
 int plugin_register_read (const char *name,
     int (*callback) (void));
 int plugin_register_complex_read (const char *name,
     plugin_read_cb callback,
     const struct timespec *interval,
     user_data_t *user_data);

When registered with plugin_register_complex_read, it is possible to pass arbitrary data to the callback functions using the user_data_t structure.