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Manual Reference Pages  -  DC_SERVER_NEW (2)


DC_SERVER_set_default_cache, DC_SERVER_set_cache, DC_SERVER_new, DC_SERVER_free, DC_SERVER_items_stored, DC_SERVER_reset_operations, DC_SERVER_num_operations, DC_SERVER_new_client, DC_SERVER_del_client, DC_SERVER_process_client, DC_SERVER_clients_to_sel, DC_SERVER_clients_io - distcache server API



 #include <distcache/dc_server.h>

 DC_SERVER *DC_SERVER_new(unsigned int max_sessions);
 void DC_SERVER_free(DC_SERVER *ctx);
 int DC_SERVER_set_default_cache(void);
 int DC_SERVER_set_cache(const DC_CACHE_cb *impl);
 unsigned int DC_SERVER_items_stored(DC_SERVER *ctx,
                                     const struct timeval *now);
 void DC_SERVER_reset_operations(DC_SERVER *ctx);
 unsigned long DC_SERVER_num_operations(DC_SERVER *ctx);
                                 unsigned int flags);
 int DC_SERVER_del_client(DC_CLIENT *clnt);
 int DC_SERVER_process_client(DC_CLIENT *clnt,
                              const struct timeval *now);
 int DC_SERVER_clients_to_sel(DC_SERVER *ctx, NAL_SELECTOR *sel);
 int DC_SERVER_clients_io(DC_SERVER *ctx, NAL_SELECTOR *sel,
                          const struct timeval *now);


DC_SERVER_new() returns an initialised DC_SERVER object, or NULL for failure.

DC_SERVER_free() and DC_SERVER_reset_operations() have no return value.

DC_SERVER_items_stored() returns the number of cached sessions in a cache (after any session expiry is performed).

DC_SERVER_num_operations() indicates how many operations the cache object has performed.

DC_SERVER_new_client() returns a new DC_CLIENT object, or NULL for failure.

The remaining functions return non-zero for success or zero for failure.


Use of the dc_server.h header requires the "struct timeval" type to be defined. On many systems, this will require that you include the time.h header in advance, though details will vary from system to system. If in doubt, try consulting your system’s gettimeofday(2) man page for information on how to have this system type defined.

These DC_SERVER functions facilitate the implementation a session cache server to be compatible with the distcache protocol. The source code to dc_server(1) provides an example of using this API, and is probably the ideal reference (a single C file of 304 lines). The storage of the cache is provided by a table of handler functions defined by the DC_CACHE_cb structure;

 typedef struct st_DC_CACHE_cb {
         DC_CACHE *   (*cache_new)(unsigned int max_sessions);
         void         (*cache_free)(DC_CACHE *cache);
         int          (*cache_add)(DC_CACHE *cache,
                                   const struct timeval *now,
                                   unsigned long timeout_msecs,
                                   const unsigned char *session_id,
                                   unsigned int session_id_len,
                                   const unsigned char *data,
                                   unsigned int data_len);
         unsigned int (*cache_get)(DC_CACHE *cache,
                                   const struct timeval *now,
                                   const unsigned char *session_id,
                                   unsigned int session_id_len,
                                   unsigned char *store,
                                   unsigned int store_size);
         int          (*cache_remove)(DC_CACHE *cache,
                                      const struct timeval *now,
                                      const unsigned char *session_id,
                                      unsigned int session_id_len);
         int          (*cache_have)(DC_CACHE *cache,
                                    const struct timeval *now,
                                    const unsigned char *session_id,
                                    unsigned int session_id_len);
         unsigned int (*cache_num_items)(DC_CACHE *cache,
                                         const struct timeval *now);
 } DC_CACHE_cb;

libdistcacheserver provides a default implementation that can be enabled by calling DC_SERVER_set_default_cache() prior to DC_SERVER_new(). Alternatively, a customised cache implementation can be specified by DC_SERVER_set_cache(). The reason that one or the other must be specified is so that custom implementations will not need to have the default implementation linked in because they won’t explicitly call DC_SERVER_set_default_cache().

The choice of DC_CACHE_cb implementation will control all manipulations and queries on the session cache. Each handler is passed a struct timeval value to allow it to implicitly handle expiry of old sessions without having to repeatedly query the time on each invokation.

Outside the actual cache implementation, the other subject covered by libdistcacheserver is that of managing client connections and processing their requests. It is assumed that the caller will use libnal to handle the network aspects of the cache server - otherwise the application would be better to use the lower-level DC_PLUG API (see DC_PLUG_new(2)), and the implementation of libdistcacheserver would provide a good reference for this.

New clients of the cache server are created by DC_SERVER_new_client() using the supplied connection object conn. The behaviour of the returned DC_CLIENT object depends on the flags parameter, which is zero or a bitwise combination of the following values;

 #define DC_CLIENT_FLAG_NOFREE_CONN   (unsigned int)0x0001
 #define DC_CLIENT_FLAG_IN_SERVER     (unsigned int)0x0002

If DC_CLIENT_FLAG_NOFREE_CONN is set, then conn will not be destroyed when the DC_CLIENT object is destroyed by DC_SERVER_new_client(). Note, the DC_CLIENT object encapsulates the provided conn object and does not copy it.

If DC_CLIENT_FLAG_IN_SERVER is set, then network traffic and request processing for the client will be implicit in the DC_SERVER_clients_to_sel() and DC_SERVER_clients_io() functions. This includes destroying any clients that have disconnected at the network level or had corruption errors at the data level.

If DC_CLIENT_FLAG_IN_SERVER is not set, then selecting and performing network I/O should be handled by the caller directly using the original conn object, and checking for (and processing of) requests should be handled directly by DC_SERVER_process_client(). A zero return value from this function indicates an error in the client’s processing, and would then require the caller to destroy the client object via DC_SERVER_del_client(). This allows network handling and logical cache handling to be explicitly separated by the implementation if required.

Note that the dc_server(1) implementation is greatly simplified by using DC_CLIENT_FLAG_IN_SERVER and not setting DC_CLIENT_FLAG_NOFREE_CONN. This allows it to forget about NAL_CONNECTION objects after they have been successfully converted into DC_CLIENT objects, and in fact can forget about the resulting DC_CLIENT objects too as they become completely controlled by the DC_SERVER object. If the client is closed, the underlying connection object is destroyed also. If the cache server itself is destroyed, then any remaining clients will likewise be properly cleaned up.

DC_SERVER_clients_to_sel() and DC_SERVER_clients_io() only operate on cache clients that are created with the DC_CLIENT_FLAG_IN_SERVER flag.


DC_PLUG_new(2), DC_PLUG_read(2) - Lower-level asynchronous implementation of the distcache protocol, useful for client and server operation.

dc_server(1) - Runs a cache server listening on a configurable network address.

distcache(8) - Overview of the distcache architecture. - Distcache home page.


This toolkit was designed and implemented by Geoff Thorpe for Cryptographic Appliances Incorporated. Since the project was released into open source, it has a home page and a project environment where development, mailing lists, and releases are organised. For problems with the software or this man page please check for new releases at the project web-site below, mail the users mailing list described there, or contact the author at

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1.4.5 DC_SERVER_NEW (2) 2004.03.23

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