Asterisk.org: Developer Documentation (28 Oct 2009)

Wed Oct 28 15:47:49 2009

## 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/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 */