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

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