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astobj2.h

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00001 /*
00002  * astobj2 - replacement containers for asterisk data structures.
00003  *
00004  * Copyright (C) 2006 Marta Carbone, Luigi Rizzo - Univ. di Pisa, Italy
00005  *
00006  * See http://www.asterisk.org for more information about
00007  * the Asterisk project. Please do not directly contact
00008  * any of the maintainers of this project for assistance;
00009  * the project provides a web site, mailing lists and IRC
00010  * channels for your use.
00011  *
00012  * This program is free software, distributed under the terms of
00013  * the GNU General Public License Version 2. See the LICENSE file
00014  * at the top of the source tree.
00015  */
00016 
00017 #ifndef _ASTERISK_ASTOBJ2_H
00018 #define _ASTERISK_ASTOBJ2_H
00019 
00020 #include "asterisk/compat.h"
00021 
00022 /*! \file 
00023  *
00024  * \brief Object Model implementing objects and containers.
00025 
00026 These functions implement an abstraction for objects (with
00027 locks and reference counts) and containers for these user-defined objects,
00028 supporting locking, reference counting and callbacks.
00029 
00030 The internal implementation of the container is opaque to the user,
00031 so we can use different data structures as needs arise.
00032 
00033 At the moment, however, the only internal data structure is a hash
00034 table. When other structures will be implemented, the initialization
00035 function may change.
00036 
00037 USAGE - OBJECTS
00038 
00039 An object is a block of memory that must be allocated with the
00040 function ao2_alloc(), and for which the system keeps track (with
00041 abit of help from the programmer) of the number of references around.
00042 When an object has no more references, it is destroyed, by first
00043 invoking whatever 'destructor' function the programmer specifies
00044 (it can be NULL), and then freeing the memory.
00045 This way objects can be shared without worrying who is in charge
00046 of freeing them.
00047 
00048 Basically, creating an object requires the size of the object and
00049 and a pointer to the destructor function:
00050  
00051     struct foo *o;
00052  
00053     o = ao2_alloc(sizeof(struct foo), my_destructor_fn);
00054 
00055 The object returned has a refcount = 1.
00056 Note that the memory for the object is allocated and zeroed.
00057 - We cannot realloc() the object itself.
00058 - We cannot call free(o) to dispose of the object; rather we
00059   tell the system that we do not need the reference anymore:
00060 
00061     ao2_ref(o, -1)
00062 
00063   causing the destructor to be called (and then memory freed) when
00064   the refcount goes to 0. This is also available as ao2_unref(o),
00065   and returns NULL as a convenience, so you can do things like
00066    o = ao2_unref(o);
00067   and clean the original pointer to prevent errors.
00068 
00069 - ao2_ref(o, +1) can be used to modify the refcount on the
00070   object in case we want to pass it around.
00071    
00072 
00073 - other calls on the object are ao2_lock(obj), ao2_unlock(),
00074   ao2_trylock(), to manipulate the lock.
00075 
00076 
00077 USAGE - CONTAINERS
00078 
00079 A containers is an abstract data structure where we can store
00080 objects, search them (hopefully in an efficient way), and iterate
00081 or apply a callback function to them. A container is just an object
00082 itself.
00083 
00084 A container must first be allocated, specifying the initial
00085 parameters. At the moment, this is done as follows:
00086 
00087     <b>Sample Usage:</b>
00088     \code
00089 
00090     struct ao2_container *c;
00091 
00092     c = ao2_container_alloc(MAX_BUCKETS, my_hash_fn, my_cmp_fn);
00093 
00094 where
00095 - MAX_BUCKETS is the number of buckets in the hash table,
00096 - my_hash_fn() is the (user-supplied) function that returns a
00097   hash key for the object (further reduced moduly MAX_BUCKETS
00098   by the container's code);
00099 - my_cmp_fn() is the default comparison function used when doing
00100   searches on the container,
00101 
00102 A container knows little or nothing about the object itself,
00103 other than the fact that it has been created by ao2_alloc()
00104 All knowledge of the (user-defined) internals of the object
00105 is left to the (user-supplied) functions passed as arguments
00106 to ao2_container_alloc().
00107 
00108 If we want to insert the object in the container, we should
00109 initialize its fields -- especially, those used by my_hash_fn() --
00110 to compute the bucket to use.
00111 Once done, we can link an object to a container with
00112 
00113     ao2_link(c, o);
00114 
00115 The function returns NULL in case of errors (and the object
00116 is not inserted in the container). Other values mean success
00117 (we are not supposed to use the value as a pointer to anything).
00118 
00119 \note While an object o is in a container, we expect that
00120 my_hash_fn(o) will always return the same value. The function
00121 does not lock the object to be computed, so modifications of
00122 those fields that affect the computation of the hash should
00123 be done by extractiong the object from the container, and
00124 reinserting it after the change (this is not terribly expensive).
00125 
00126 \note A container with a single buckets is effectively a linked
00127 list. However there is no ordering among elements.
00128 
00129 Objects implement a reference counter keeping the count
00130 of the number of references that reference an object.
00131 
00132 When this number becomes zero the destructor will be
00133 called and the object will be free'd.
00134  */
00135 
00136 /*!
00137  * Invoked just before freeing the memory for the object.
00138  * It is passed a pointer to user data.
00139  */
00140 typedef void (*ao2_destructor_fn)(void *);
00141 
00142 void ao2_bt(void);   /* backtrace */
00143 /*!
00144  * Allocate and initialize an object.
00145  * 
00146  * \param data_size The sizeof() of user-defined structure.
00147  * \param destructor_fn The function destructor (can be NULL)
00148  * \return A pointer to user data. 
00149  *
00150  * Allocates a struct astobj2 with sufficient space for the
00151  * user-defined structure.
00152  * \notes:
00153  * - storage is zeroed; XXX maybe we want a flag to enable/disable this.
00154  * - the refcount of the object just created is 1
00155  * - the returned pointer cannot be free()'d or realloc()'ed;
00156  *   rather, we just call ao2_ref(o, -1);
00157  */
00158 void *ao2_alloc(const size_t data_size, ao2_destructor_fn destructor_fn);
00159 
00160 /*!
00161  * Reference/unreference an object and return the old refcount.
00162  *
00163  * \param o A pointer to the object
00164  * \param delta Value to add to the reference counter.
00165  * \return The value of the reference counter before the operation.
00166  *
00167  * Increase/decrease the reference counter according
00168  * the value of delta.
00169  *
00170  * If the refcount goes to zero, the object is destroyed.
00171  *
00172  * \note The object must not be locked by the caller of this function, as
00173  *       it is invalid to try to unlock it after releasing the reference.
00174  *
00175  * \note if we know the pointer to an object, it is because we
00176  * have a reference count to it, so the only case when the object
00177  * can go away is when we release our reference, and it is
00178  * the last one in existence.
00179  */
00180 int ao2_ref(void *o, int delta);
00181 
00182 /*!
00183  * Lock an object.
00184  * 
00185  * \param a A pointer to the object we want lock.
00186  * \return 0 on success, other values on error.
00187  */
00188 int ao2_lock(void *a);
00189 
00190 /*!
00191  * Unlock an object.
00192  * 
00193  * \param a A pointer to the object we want unlock.
00194  * \return 0 on success, other values on error.
00195  */
00196 int ao2_unlock(void *a);
00197 
00198 /*!
00199  *
00200  * Containers
00201 
00202 containers are data structures meant to store several objects,
00203 and perform various operations on them.
00204 Internally, objects are stored in lists, hash tables or other
00205 data structures depending on the needs.
00206 
00207 NOTA BENE: at the moment the only container we support is the
00208 hash table and its degenerate form, the list.
00209 
00210 Operations on container include:
00211 
00212     c = ao2_container_alloc(size, cmp_fn, hash_fn)
00213    allocate a container with desired size and default compare
00214    and hash function
00215 
00216     ao2_find(c, arg, flags)
00217    returns zero or more element matching a given criteria
00218    (specified as arg). Flags indicate how many results we
00219    want (only one or all matching entries), and whether we
00220    should unlink the object from the container.
00221 
00222     ao2_callback(c, flags, fn, arg)
00223    apply fn(obj, arg) to all objects in the container.
00224    Similar to find. fn() can tell when to stop, and
00225    do anything with the object including unlinking it.
00226    Note that the entire operation is run with the container
00227    locked, so noone else can change its content while we work on it.
00228    However, we pay this with the fact that doing
00229    anything blocking in the callback keeps the container
00230    blocked.
00231    The mechanism is very flexible because the callback function fn()
00232    can do basically anything e.g. counting, deleting records, etc.
00233    possibly using arg to store the results.
00234    
00235     iterate on a container
00236    this is done with the following sequence
00237 
00238        struct ao2_container *c = ... // our container
00239        struct ao2_iterator i;
00240        void *o;
00241 
00242        i = ao2_iterator_init(c, flags);
00243      
00244        while ( (o = ao2_iterator_next(&i)) ) {
00245       ... do something on o ...
00246       ao2_ref(o, -1);
00247        }
00248 
00249    The difference with the callback is that the control
00250    on how to iterate is left to us.
00251 
00252     ao2_ref(c, -1)
00253    dropping a reference to a container destroys it, very simple!
00254  
00255 Containers are astobj2 object themselves, and this is why their
00256 implementation is simple too.
00257 
00258  */
00259 
00260 /*!
00261  * We can perform different operation on an object. We do this
00262  * according the following flags.
00263  */
00264 enum search_flags {
00265    /*! unlink the object found */
00266    OBJ_UNLINK   = (1 << 0),
00267    /*! on match, don't return the object or increase its reference count. */
00268    OBJ_NODATA   = (1 << 1),
00269    /*! don't stop at the first match 
00270     *  \note This is not fully implemented. */
00271    OBJ_MULTIPLE = (1 << 2),
00272    /*! obj is an object of the same type as the one being searched for.
00273     *  This implies that it can be passed to the object's hash function
00274     *  for optimized searching. */
00275    OBJ_POINTER  = (1 << 3),
00276    /*! 
00277     * \brief Continue if a match is not found in the hashed out bucket
00278     *
00279     * This flag is to be used in combination with OBJ_POINTER.  This tells
00280     * the ao2_callback() core to keep searching through the rest of the
00281     * buckets if a match is not found in the starting bucket defined by
00282     * the hash value on the argument.
00283     */
00284    OBJ_CONTINUE = (1 << 4),
00285 
00286 };
00287 
00288 /*!
00289  * Type of a generic function to generate a hash value from an object.
00290  *
00291  */
00292 typedef int (*ao2_hash_fn)(const void *obj, const int flags);
00293 
00294 /*!
00295  * valid callback results:
00296  * We return a combination of
00297  * CMP_MATCH when the object matches the request,
00298  * and CMP_STOP when we should not continue the search further.
00299  */
00300 enum _cb_results {
00301    CMP_MATCH   = 0x1,
00302    CMP_STOP = 0x2,
00303 };
00304 
00305 /*!
00306  * generic function to compare objects.
00307  * This, as other callbacks, should return a combination of
00308  * _cb_results as described above.
00309  *
00310  * \param o object from container
00311  * \param arg  search parameters (directly from ao2_find)
00312  * \param flags   passed directly from ao2_find
00313  * XXX explain.
00314  */
00315 
00316 /*!
00317  * Type of a generic callback function
00318  * \param obj  pointer to the (user-defined part) of an object.
00319  * \param arg callback argument from ao2_callback()
00320  * \param flags flags from ao2_callback()
00321  * The return values are the same as a compare function.
00322  * In fact, they are the same thing.
00323  */
00324 typedef int (*ao2_callback_fn)(void *obj, void *arg, int flags);
00325 
00326 /*!
00327  * Here start declarations of containers.
00328  */
00329 struct ao2_container;
00330 
00331 /*!
00332  * Allocate and initialize a container 
00333  * with the desired number of buckets.
00334  * 
00335  * We allocate space for a struct astobj_container, struct container
00336  * and the buckets[] array.
00337  *
00338  * \param my_hash_fn Pointer to a function computing a hash value.
00339  * \param my_cmp_fn Pointer to a function comparating key-value 
00340  *          with a string. (can be NULL)
00341  * \return A pointer to a struct container.
00342  *
00343  * destructor is set implicitly.
00344  */
00345 struct ao2_container *ao2_container_alloc(const unsigned int n_buckets,
00346       ao2_hash_fn hash_fn, ao2_callback_fn cmp_fn);
00347 
00348 /*!
00349  * Returns the number of elements in a container.
00350  */
00351 int ao2_container_count(struct ao2_container *c);
00352 
00353 /*
00354  * Here we have functions to manage objects.
00355  *
00356  * We can use the functions below on any kind of 
00357  * object defined by the user.
00358  */
00359 
00360 /*!
00361  * \brief Add an object to a container.
00362  *
00363  * \param c the container to operate on.
00364  * \param newobj the object to be added.
00365  *
00366  * \return NULL on errors, other values on success.
00367  *
00368  * This function inserts an object in a container according its key.
00369  *
00370  * \note Remember to set the key before calling this function.
00371  *
00372  * \note This function automatically increases the reference count to
00373  *       account for the reference to the object that the container now holds.
00374  *
00375  * For Asterisk 1.4 only, there is a dirty hack here to ensure that chan_iax2
00376  * can have objects linked in to the container at the head instead of tail
00377  * when it is just a linked list.  This is to maintain some existing behavior
00378  * where the order must be maintained as it was before this conversion so that
00379  * matching behavior doesn't change.
00380  */
00381 #define ao2_link(c, o) __ao2_link(c, o, 0)
00382 void *__ao2_link(struct ao2_container *c, void *newobj, int iax2_hack);
00383 
00384 /*!
00385  * \brief Remove an object from the container
00386  *
00387  * \arg c the container
00388  * \arg obj the object to unlink
00389  *
00390  * \retval NULL, always
00391  *
00392  * \note The object requested to be unlinked must be valid.  However, if it turns
00393  *       out that it is not in the container, this function is still safe to
00394  *       be called.
00395  *
00396  * \note If the object gets unlinked from the container, the container's
00397  *       reference to the object will be automatically released.
00398  */
00399 void *ao2_unlink(struct ao2_container *c, void *obj);
00400 
00401 /*! \struct Used as return value if the flag OBJ_MULTIPLE is set */
00402 struct ao2_list {
00403    struct ao2_list *next;
00404    void *obj;  /* pointer to the user portion of the object */
00405 };
00406 
00407 /*!
00408  * ao2_callback() and astob2_find() are the same thing with only one difference:
00409  * the latter uses as a callback the function passed as my_cmp_f() at
00410  * the time of the creation of the container.
00411  * 
00412  * \param c A pointer to the container to operate on.
00413  * \param arg passed to the callback.
00414  * \param flags A set of flags specifying the operation to perform,
00415    partially used by the container code, but also passed to
00416    the callback.
00417  * \return  A pointer to the object found/marked, 
00418  *       a pointer to a list of objects matching comparison function,
00419  *       NULL if not found.
00420  * If the function returns any objects, their refcount is incremented,
00421  * and the caller is in charge of decrementing them once done.
00422  * Also, in case of multiple values returned, the list used
00423  * to store the objects must be freed by the caller.
00424  *
00425  * This function searches through a container and performs operations
00426  * on objects according on flags passed.
00427  * XXX describe better
00428  * The comparison is done calling the compare function set implicitly. 
00429  * The p pointer can be a pointer to an object or to a key, 
00430  * we can say this looking at flags value.
00431  * If p points to an object we will search for the object pointed
00432  * by this value, otherwise we serch for a key value.
00433  * If the key is not uniq we only find the first matching valued.
00434  * If we use the OBJ_MARK flags, we mark all the objects matching 
00435  * the condition.
00436  *
00437  * The use of flags argument is the follow:
00438  *
00439  * OBJ_UNLINK     unlinks the object found
00440  * OBJ_NODATA     on match, do return an object
00441  *          Callbacks use OBJ_NODATA as a default
00442  *          functions such as find() do
00443  * OBJ_MULTIPLE      return multiple matches
00444  *          Default for _find() is no.
00445  *          to a key (not yet supported)
00446  * OBJ_POINTER       the pointer is an object pointer
00447  *
00448  * In case we return a list, the callee must take care to destroy 
00449  * that list when no longer used.
00450  *
00451  * \note When the returned object is no longer in use, ao2_ref() should
00452  * be used to free the additional reference possibly created by this function.
00453  */
00454 /* XXX order of arguments to find */
00455 void *ao2_find(struct ao2_container *c, void *arg, enum search_flags flags);
00456 void *ao2_callback(struct ao2_container *c,
00457    enum search_flags flags,
00458    ao2_callback_fn cb_fn, void *arg);
00459 
00460 int ao2_match_by_addr(void *user_data, void *arg, int flags);
00461 /*!
00462  *
00463  *
00464  * When we need to walk through a container, we use
00465  * ao2_iterator to keep track of the current position.
00466  * 
00467  * Because the navigation is typically done without holding the
00468  * lock on the container across the loop,
00469  * objects can be inserted or deleted or moved
00470  * while we work. As a consequence, there is no guarantee that
00471  * the we manage to touch all the elements on the list, or it
00472  * is possible that we touch the same object multiple times.
00473  * However, within the current hash table container, the following is true:
00474  *  - It is not possible to miss an object in the container while iterating
00475  *    unless it gets added after the iteration begins and is added to a bucket
00476  *    that is before the one the current object is in.  In this case, even if
00477  *    you locked the container around the entire iteration loop, you still would
00478  *    not see this object, because it would still be waiting on the container
00479  *    lock so that it can be added.
00480  *  - It would be extremely rare to see an object twice.  The only way this can
00481  *    happen is if an object got unlinked from the container and added again 
00482  *    during the same iteration.  Furthermore, when the object gets added back,
00483  *    it has to be in the current or later bucket for it to be seen again.
00484  *
00485  * An iterator must be first initialized with ao2_iterator_init(),
00486  * then we can use o = ao2_iterator_next() to move from one
00487  * element to the next. Remember that the object returned by
00488  * ao2_iterator_next() has its refcount incremented,
00489  * and the reference must be explicitly released when done with it.
00490  *
00491  * Example:
00492  *
00493  *  \code
00494  *
00495  *  struct ao2_container *c = ... // the container we want to iterate on
00496  *  struct ao2_iterator i;
00497  *  struct my_obj *o;
00498  *
00499  *  i = ao2_iterator_init(c, flags);
00500  *
00501  *  while ( (o = ao2_iterator_next(&i)) ) {
00502  *     ... do something on o ...
00503  *     ao2_ref(o, -1);
00504  *  }
00505  *
00506  *  \endcode
00507  *
00508  */
00509 
00510 /*!
00511  * You are not supposed to know the internals of an iterator!
00512  * We would like the iterator to be opaque, unfortunately
00513  * its size needs to be known if we want to store it around
00514  * without too much trouble.
00515  * Anyways...
00516  * The iterator has a pointer to the container, and a flags
00517  * field specifying various things e.g. whether the container
00518  * should be locked or not while navigating on it.
00519  * The iterator "points" to the current object, which is identified
00520  * by three values:
00521  * - a bucket number;
00522  * - the object_id, which is also the container version number
00523  *   when the object was inserted. This identifies the object
00524  *   univoquely, however reaching the desired object requires
00525  *   scanning a list.
00526  * - a pointer, and a container version when we saved the pointer.
00527  *   If the container has not changed its version number, then we
00528  *   can safely follow the pointer to reach the object in constant time.
00529  * Details are in the implementation of ao2_iterator_next()
00530  * A freshly-initialized iterator has bucket=0, version = 0.
00531  */
00532 
00533 struct ao2_iterator {
00534    /*! the container */
00535    struct ao2_container *c;
00536    /*! operation flags */
00537    int flags;
00538 #define  F_AO2I_DONTLOCK   1  /*!< don't lock when iterating */
00539    /*! current bucket */
00540    int bucket;
00541    /*! container version */
00542    unsigned int c_version;
00543    /*! pointer to the current object */
00544    void *obj;
00545    /*! container version when the object was created */
00546    unsigned int version;
00547 };
00548 
00549 struct ao2_iterator ao2_iterator_init(struct ao2_container *c, int flags);
00550 
00551 void *ao2_iterator_next(struct ao2_iterator *a);
00552 
00553 #endif /* _ASTERISK_ASTOBJ2_H */

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