From collectd Wiki

The value_list_t data structure is used to pass custom information to callback functions, such as read callbacks. The struct contains a void pointer to the actual data and a function pointer. If the function pointer is not NULL, it will be used to free the user data when the callback is not longer used.

[edit] Definition of the struct

The structure is defined in src/plugin.h as follows:

 struct user_data_s
 {
   void *data;
   void (*free_func) (void *);
 };
 typedef struct user_data_s user_data_t;

[edit] Example

 static int my_config (oconfig_item_t *ci)
 {
   my_config_t *my_cfg;
   …
   /* configure and fill in "my_cfg" */
   …
   if (all_went_well)
   {
     user_data_t ud;
 
     memset (&ud, 0, sizeof (ud));
     ud.data = (void *) my_cfg;
     ud.free_func = my_config_free;
 
     plugin_register_complex_read ("my_read",
         /* callback = */ my_read,
         /* interval = */ NULL,
         /* userdata = */ &ud);
   } /* if (all_went_well) */
 
   return (0);
 } /* int my_config */

This example shows how to pass a pointer my_cfg to the callback function my_read. If the callback is deleted, the struct is freed using the my_config_free function.

For real-world examples take a look at src/mysql.c and src/netapp.c.