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Asterisk developer's documentation


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/lock.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     ao2_container *c;
00091 
00092     c = ao2_container_alloc(MAX_BUCKETS, my_hash_fn, my_cmp_fn, my_dump_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 - my_dump_fn() is a helper function used only for debugging.
00102 
00103 A container knows little or nothing about the object itself,
00104 other than the fact that it has been created by ao2_alloc()
00105 All knowledge of the (user-defined) internals of the object
00106 is left to the (user-supplied) functions passed as arguments
00107 to ao2_container_alloc().
00108 
00109 If we want to insert the object in the container, we should
00110 initialize its fields -- especially, those used by my_hash_fn() --
00111 to compute the bucket to use.
00112 Once done, we can link an object to a container with
00113 
00114     ao2_link(c, o);
00115 
00116 The function returns NULL in case of errors (and the object
00117 is not inserted in the container). Other values mean success
00118 (we are not supposed to use the value as a pointer to anything).
00119 
00120 \note inserting the object in the container grabs the reference
00121 to the object (which is now owned by the container) so we do not
00122 need to drop ours when we are done.
00123 
00124 \note While an object o is in a container, we expect that
00125 my_hash_fn(o) will always return the same value. The function
00126 does not lock the object to be computed, so modifications of
00127 those fields that affect the computation of the hash should
00128 be done by extractiong the object from the container, and
00129 reinserting it after the change (this is not terribly expensive).
00130 
00131 \note A container with a single buckets is effectively a linked
00132 list. However there is no ordering among elements.
00133 
00134 Objects implement a reference counter keeping the count
00135 of the number of references that reference an object.
00136 
00137 When this number becomes zero the destructor will be
00138 called and the object will be free'd.
00139  */
00140 
00141 /*!
00142  * Invoked just before freeing the memory for the object.
00143  * It is passed a pointer to user data.
00144  */
00145 typedef void (*ao2_destructor_fn)(void *);
00146 
00147 void ao2_bt(void);   /* backtrace */
00148 /*!
00149  * Allocate and initialize an object.
00150  * 
00151  * \param data_size The sizeof() of user-defined structure.
00152  * \param destructor_fn The function destructor (can be NULL)
00153  * \return A pointer to user data. 
00154  *
00155  * Allocates a struct astobj2 with sufficient space for the
00156  * user-defined structure.
00157  * \notes:
00158  * - storage is zeroed; XXX maybe we want a flag to enable/disable this.
00159  * - the refcount of the object just created is 1
00160  * - the returned pointer cannot be free()'d or realloc()'ed;
00161  *   rather, we just call ao2_ref(o, -1);
00162  */
00163 void *ao2_alloc(const size_t data_size, ao2_destructor_fn destructor_fn);
00164 
00165 /*!
00166  * Reference/unreference an object and return the old refcount.
00167  *
00168  * \param o A pointer to the object
00169  * \param delta Value to add to the reference counter.
00170  * \return The value of the reference counter before the operation.
00171  *
00172  * Increase/decrease the reference counter according
00173  * the value of delta.
00174  *
00175  * If the refcount goes to zero, the object is destroyed.
00176  *
00177  * \note The object must not be locked by the caller of this function, as
00178  *       it is invalid to try to unlock it after releasing the reference.
00179  *
00180  * \note if we know the pointer to an object, it is because we
00181  * have a reference count to it, so the only case when the object
00182  * can go away is when we release our reference, and it is
00183  * the last one in existence.
00184  */
00185 int ao2_ref(void *o, int delta);
00186 
00187 /*!
00188  * Lock an object.
00189  * 
00190  * \param a A pointer to the object we want lock.
00191  * \return 0 on success, other values on error.
00192  */
00193 int ao2_lock(void *a);
00194 
00195 /*!
00196  * Unlock an object.
00197  * 
00198  * \param a A pointer to the object we want unlock.
00199  * \return 0 on success, other values on error.
00200  */
00201 int ao2_unlock(void *a);
00202 
00203 /*!
00204  *
00205  * Containers
00206 
00207 containers are data structures meant to store several objects,
00208 and perform various operations on them.
00209 Internally, objects are stored in lists, hash tables or other
00210 data structures depending on the needs.
00211 
00212 NOTA BENE: at the moment the only container we support is the
00213 hash table and its degenerate form, the list.
00214 
00215 Operations on container include:
00216 
00217     c = ao2_container_alloc(size, cmp_fn, hash_fn)
00218    allocate a container with desired size and default compare
00219    and hash function
00220 
00221     ao2_find(c, arg, flags)
00222    returns zero or more element matching a given criteria
00223    (specified as arg). Flags indicate how many results we
00224    want (only one or all matching entries), and whether we
00225    should unlink the object from the container.
00226 
00227     ao2_callback(c, flags, fn, arg)
00228    apply fn(obj, arg) to all objects in the container.
00229    Similar to find. fn() can tell when to stop, and
00230    do anything with the object including unlinking it.
00231    Note that the entire operation is run with the container
00232    locked, so noone else can change its content while we work on it.
00233    However, we pay this with the fact that doing
00234    anything blocking in the callback keeps the container
00235    blocked.
00236    The mechanism is very flexible because the callback function fn()
00237    can do basically anything e.g. counting, deleting records, etc.
00238    possibly using arg to store the results.
00239    
00240     iterate on a container
00241    this is done with the following sequence
00242 
00243        ao2_container *c = ... // our container
00244        ao2_iterator i;
00245        void *o;
00246 
00247        i = ao2_iterator_init(c, flags);
00248      
00249        while ( (o = ao2_iterator_next(&i)) ) {
00250       ... do something on o ...
00251       ao2_ref(o, -1);
00252        }
00253 
00254    The difference with the callback is that the control
00255    on how to iterate is left to us.
00256 
00257     ao2_ref(c, -1)
00258    dropping a reference to a container destroys it, very simple!
00259  
00260 Containers are astobj2 object themselves, and this is why their
00261 implementation is simple too.
00262 
00263  */
00264 
00265 /*!
00266  * We can perform different operation on an object. We do this
00267  * according the following flags.
00268  */
00269 enum search_flags {
00270    /*! unlink the object found */
00271    OBJ_UNLINK   = (1 << 0),
00272    /*! on match, don't return the object or increase its reference count. */
00273    OBJ_NODATA   = (1 << 1),
00274    /*! don't stop at the first match 
00275     *  \note This is not fully implemented. */
00276    OBJ_MULTIPLE = (1 << 2),
00277    /*! obj is an object of the same type as the one being searched for.
00278     *  This implies that it can be passed to the object's hash function
00279     *  for optimized searching. */
00280    OBJ_POINTER  = (1 << 3),
00281 };
00282 
00283 /*!
00284  * Type of a generic function to generate a hash value from an object.
00285  *
00286  */
00287 typedef int (*ao2_hash_fn)(const void *obj, const int flags);
00288 
00289 /*!
00290  * valid callback results:
00291  * We return a combination of
00292  * CMP_MATCH when the object matches the request,
00293  * and CMP_STOP when we should not continue the search further.
00294  */
00295 enum _cb_results {
00296    CMP_MATCH   = 0x1,
00297    CMP_STOP = 0x2,
00298 };
00299 
00300 /*!
00301  * generic function to compare objects.
00302  * This, as other callbacks, should return a combination of
00303  * _cb_results as described above.
00304  *
00305  * \param o object from container
00306  * \param arg  search parameters (directly from ao2_find)
00307  * \param flags   passed directly from ao2_find
00308  * XXX explain.
00309  */
00310 
00311 /*!
00312  * Type of a generic callback function
00313  * \param obj  pointer to the (user-defined part) of an object.
00314  * \param arg callback argument from ao2_callback()
00315  * \param flags flags from ao2_callback()
00316  * The return values are the same as a compare function.
00317  * In fact, they are the same thing.
00318  */
00319 typedef int (*ao2_callback_fn)(void *obj, void *arg, int flags);
00320 
00321 /*!
00322  * Here start declarations of containers.
00323  */
00324 
00325 /*!
00326  * This structure contains the total number of buckets 
00327  * and variable size array of object pointers.
00328  * It is opaque, defined in astobj2.c, so we only need
00329  * a type declaration.
00330  */
00331 typedef struct __ao2_container ao2_container;
00332 
00333 /*!
00334  * Allocate and initialize a container 
00335  * with the desired number of buckets.
00336  * 
00337  * We allocate space for a struct astobj_container, struct container
00338  * and the buckets[] array.
00339  *
00340  * \param my_hash_fn Pointer to a function computing a hash value.
00341  * \param my_cmp_fn Pointer to a function comparating key-value 
00342  *          with a string. (can be NULL)
00343  * \return A pointer to a struct container.
00344  *
00345  * destructor is set implicitly.
00346  */
00347 ao2_container *ao2_container_alloc(const uint n_buckets,
00348       ao2_hash_fn hash_fn, ao2_callback_fn cmp_fn);
00349 
00350 /*!
00351  * Returns the number of elements in a container.
00352  */
00353 int ao2_container_count(ao2_container *c);
00354 
00355 /*
00356  * Here we have functions to manage objects.
00357  *
00358  * We can use the functions below on any kind of 
00359  * object defined by the user.
00360  */
00361 /*!
00362  * Add an object to a container.
00363  *
00364  * \param c the container to operate on.
00365  * \param obj the object to be added.
00366  * \return NULL on errors, other values on success.
00367  *
00368  * This function insert an object in a container according its key.
00369  *
00370  * \note Remember to set the key before calling this function.
00371  *
00372  * For Asterisk 1.4 only, there is a dirty hack here to ensure that chan_iax2
00373  * can have objects linked in to the container at the head instead of tail
00374  * when it is just a linked list.  This is to maintain some existing behavior
00375  * where the order must be maintained as it was before this conversion so that
00376  * matching behavior doesn't change.
00377  */
00378 #define ao2_link(c, o) __ao2_link(c, o, 0)
00379 void *__ao2_link(ao2_container *c, void *newobj, int iax2_hack);
00380 void *ao2_unlink(ao2_container *c, void *newobj);
00381 
00382 /*! \struct Used as return value if the flag OBJ_MULTIPLE is set */
00383 struct ao2_list {
00384    struct ao2_list *next;
00385    void *obj;  /* pointer to the user portion of the object */
00386 };
00387 
00388 /*!
00389  * ao2_callback() and astob2_find() are the same thing with only one difference:
00390  * the latter uses as a callback the function passed as my_cmp_f() at
00391  * the time of the creation of the container.
00392  * 
00393  * \param c A pointer to the container to operate on.
00394  * \param arg passed to the callback.
00395  * \param flags A set of flags specifying the operation to perform,
00396    partially used by the container code, but also passed to
00397    the callback.
00398  * \return  A pointer to the object found/marked, 
00399  *       a pointer to a list of objects matching comparison function,
00400  *       NULL if not found.
00401  * If the function returns any objects, their refcount is incremented,
00402  * and the caller is in charge of decrementing them once done.
00403  * Also, in case of multiple values returned, the list used
00404  * to store the objects must be freed by the caller.
00405  *
00406  * This function searches through a container and performs operations
00407  * on objects according on flags passed.
00408  * XXX describe better
00409  * The comparison is done calling the compare function set implicitly. 
00410  * The p pointer can be a pointer to an object or to a key, 
00411  * we can say this looking at flags value.
00412  * If p points to an object we will search for the object pointed
00413  * by this value, otherwise we serch for a key value.
00414  * If the key is not uniq we only find the first matching valued.
00415  * If we use the OBJ_MARK flags, we mark all the objects matching 
00416  * the condition.
00417  *
00418  * The use of flags argument is the follow:
00419  *
00420  * OBJ_UNLINK     unlinks the object found
00421  * OBJ_NODATA     on match, do return an object
00422  *          Callbacks use OBJ_NODATA as a default
00423  *          functions such as find() do
00424  * OBJ_MULTIPLE      return multiple matches
00425  *          Default for _find() is no.
00426  *          to a key (not yet supported)
00427  * OBJ_POINTER       the pointer is an object pointer
00428  *
00429  * In case we return a list, the callee must take care to destroy 
00430  * that list when no longer used.
00431  *
00432  * \note When the returned object is no longer in use, ao2_ref() should
00433  * be used to free the additional reference possibly created by this function.
00434  */
00435 /* XXX order of arguments to find */
00436 void *ao2_find(ao2_container *c, void *arg, enum search_flags flags);
00437 void *ao2_callback(ao2_container *c,
00438    enum search_flags flags,
00439    ao2_callback_fn cb_fn, void *arg);
00440 
00441 /*!
00442  *
00443  *
00444  * When we need to walk through a container, we use
00445  * ao2_iterator to keep track of the current position.
00446  * 
00447  * Because the navigation is typically done without holding the
00448  * lock on the container across the loop,
00449  * objects can be inserted or deleted or moved
00450  * while we work. As a consequence, there is no guarantee that
00451  * the we manage to touch all the elements on the list, or it
00452  * is possible that we touch the same object multiple times.
00453  * However, within the current hash table container, the following is true:
00454  *  - It is not possible to miss an object in the container while iterating
00455  *    unless it gets added after the iteration begins and is added to a bucket
00456  *    that is before the one the current object is in.  In this case, even if
00457  *    you locked the container around the entire iteration loop, you still would
00458  *    not see this object, because it would still be waiting on the container
00459  *    lock so that it can be added.
00460  *  - It would be extremely rare to see an object twice.  The only way this can
00461  *    happen is if an object got unlinked from the container and added again 
00462  *    during the same iteration.  Furthermore, when the object gets added back,
00463  *    it has to be in the current or later bucket for it to be seen again.
00464  *
00465  * An iterator must be first initialized with ao2_iterator_init(),
00466  * then we can use o = ao2_iterator_next() to move from one
00467  * element to the next. Remember that the object returned by
00468  * ao2_iterator_next() has its refcount incremented,
00469  * and the reference must be explicitly released when done with it.
00470  *
00471  * Example:
00472  *
00473  *  \code
00474  *
00475  *  ao2_container *c = ... // the container we want to iterate on
00476  *  ao2_iterator i;
00477  *  struct my_obj *o;
00478  *
00479  *  i = ao2_iterator_init(c, flags);
00480  *
00481  *  while ( (o = ao2_iterator_next(&i)) ) {
00482  *     ... do something on o ...
00483  *     ao2_ref(o, -1);
00484  *  }
00485  *
00486  *  \endcode
00487  *
00488  */
00489 
00490 /*!
00491  * You are not supposed to know the internals of an iterator!
00492  * We would like the iterator to be opaque, unfortunately
00493  * its size needs to be known if we want to store it around
00494  * without too much trouble.
00495  * Anyways...
00496  * The iterator has a pointer to the container, and a flags
00497  * field specifying various things e.g. whether the container
00498  * should be locked or not while navigating on it.
00499  * The iterator "points" to the current object, which is identified
00500  * by three values:
00501  * - a bucket number;
00502  * - the object_id, which is also the container version number
00503  *   when the object was inserted. This identifies the object
00504  *   univoquely, however reaching the desired object requires
00505  *   scanning a list.
00506  * - a pointer, and a container version when we saved the pointer.
00507  *   If the container has not changed its version number, then we
00508  *   can safely follow the pointer to reach the object in constant time.
00509  * Details are in the implementation of ao2_iterator_next()
00510  * A freshly-initialized iterator has bucket=0, version = 0.
00511  */
00512 
00513 struct __ao2_iterator {
00514    /*! the container */
00515    ao2_container *c;
00516    /*! operation flags */
00517    int flags;
00518 #define  F_AO2I_DONTLOCK   1  /*!< don't lock when iterating */
00519    /*! current bucket */
00520    int bucket;
00521    /*! container version */
00522    uint c_version;
00523    /*! pointer to the current object */
00524    void *obj;
00525    /*! container version when the object was created */
00526    uint version;
00527 };              
00528 typedef struct __ao2_iterator ao2_iterator;
00529 
00530 ao2_iterator ao2_iterator_init(ao2_container *c, int flags);
00531 
00532 void *ao2_iterator_next(ao2_iterator *a);
00533 
00534 #endif /* _ASTERISK_ASTOBJ2_H */

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