DOKK / manpages / debian 12 / libmemcached-dev / memcached_callback_set.3.en
MEMCACHED_CALLBACK_SET(3) libmemcached-awesome MEMCACHED_CALLBACK_SET(3)

memcached_callback_set - libmemcached Documentation

#include <libmemcached/memcached.h>
Compile and link with -lmemcached

  • ptr -- pointer to initialized memcached_st struct
  • flag -- memcached_callback_t flag to set
  • data -- data corresponding to flag

memcached_return_t indicating success


  • ptr -- pointer to initialized memcached_st struct
  • flag -- memcached_callback_t flag to query
  • error -- pointer to memcached_return_t indicating success

the data previously set



When memcached_free() or memcached_reset is called this function will be executed. At the point of its execution all connections are closed.

Its signature is:



When memcached_clone() is called this function will be executed.

Its signature is:



See MEMCACHED_CALLBACK_NAMESPACE.

You can set a value which will be used to create a domain for your keys. The value specified here will be prefixed to each of your keys. The value can not be greater then MEMCACHED_MAX_NAMESPACE - 1 and will reduce MEMCACHED_MAX_KEY by the value of your key.

The prefix key is only applied to the primary key, not the master key. MEMCACHED_FAILURE will be returned if no key is set. In the case of a key which is too long, MEMCACHED_BAD_KEY_PROVIDED will be returned.

If you set a value with the value being NULL then the prefix key is disabled.


This allows you to store a pointer to a specific piece of data. This can be retrieved from inside of memcached_fetch_execute(). Cloning a memcached_st will copy the pointer to the clone.

Deprecated since version <0.32: Use memcached_set_memory_allocators() instead.


Deprecated since version <0.32: Use memcached_set_memory_allocators() instead.


Deprecated since version <0.32: Use memcached_set_memory_allocators() instead.


This function implements the read through cache behavior. On failure of retrieval this callback will be called.

You are responsible for populating the result object provided. This result object will then be stored in the server and returned to the calling process.

You must clone the memcached_st in order to make use of it. The value will be stored only if you return MEMCACHED_SUCCESS or MEMCACHED_BUFFERED. Returning MEMCACHED_BUFFERED will cause the object to be buffered and not sent immediately (if this is the default behavior based on your connection setup this will happen automatically).

The prototype for this is:



This function implements a trigger upon successful deletion of a key. The memcached_st structure will need to be cloned in order to make use of it.

The prototype for this is:




libmemcached can have callbacks set key execution points. These either provide function calls at points in the code, or return pointers to structures for particular usages.

memcached_callback_get() takes a callback flag and returns the structure or function set by memcached_callback_set().

memcached_callback_set() changes the function/structure assigned by a callback flag. No connections are reset.

You can use MEMCACHED_CALLBACK_USER_DATA to provide custom context if required for any of the callbacks.

memcached_callback_get() returns the function or structure that was provided. Upon error, nothing is set, NULL is returned, and the memcached_return_t argument is set to MEMCACHED_FAILURE.

memcached_callback_set() returns MEMCACHED_SUCCESS upon successful setting, otherwise MEMCACHED_FAILURE on error.

memcached(1) libmemcached(3) memcached_strerror(3)

March 6, 2023 1.1