AVOptions: add documentation.

libavutil/opt.h

@@ -32,6 +32,189 @@
 #include "dict.h"
 #include "log.h"
 
+/**
+ * @defgroup avoptions AVOptions
+ * @{
+ * AVOptions provide a generic system to declare options on arbitrary structs
+ * ("objects"). An option can have a help text, a type and a range of possible
+ * values. Options may then be enumerated, read and written to.
+ *
+ * @section avoptions_implement Implementing AVOptions
+ * This section describes how to add AVOptions capabilities to a struct.
+ *
+ * All AVOptions-related information is stored in an AVClass. Therefore
+ * the first member of the struct must be a pointer to an AVClass describing it.
+ * The option field of the AVClass must be set to a NULL-terminated static array
+ * of AVOptions. Each AVOption must have a non-empty name, a type, a default
+ * value and for number-type AVOptions also a range of allowed values. It must
+ * also declare an offset in bytes from the start of the struct, where the field
+ * associated with this AVOption is located. Other fields in the AVOption struct
+ * should also be set when applicable, but are not required.
+ *
+ * The following example illustrates an AVOptions-enabled struct:
+ * @code
+ * typedef struct test_struct {
+ *     AVClass *class;
+ *     int      int_opt;
+ *     char    *str_opt;
+ *     uint8_t *bin_opt;
+ *     int      bin_len;
+ * } test_struct;
+ *
+ * static const AVOption options[] = {
+ *   { "test_int", "This is a test option of int type.", offsetof(test_struct, int_opt),
+ *     AV_OPT_TYPE_INT, { -1 }, INT_MIN, INT_MAX },
+ *   { "test_str", "This is a test option of string type.", offsetof(test_struct, str_opt),
+ *     AV_OPT_TYPE_STRING },
+ *   { "test_bin", "This is a test option of binary type.", offsetof(test_struct, bin_opt),
+ *     AV_OPT_TYPE_BINARY },
+ *   { NULL },
+ * };
+ *
+ * static const AVClass test_class = {
+ *     .class_name = "test class",
+ *     .item_name  = av_default_item_name,
+ *     .option     = options,
+ *     .version    = LIBAVUTIL_VERSION_INT,
+ * };
+ * @endcode
+ *
+ * Next, when allocating your struct, you must ensure that the AVClass pointer
+ * is set to the correct value. Then, av_opt_set_defaults() must be called to
+ * initialize defaults. After that the struct is ready to be used with the
+ * AVOptions API.
+ *
+ * When cleaning up, you may use the av_opt_free() function to automatically
+ * free all the allocated string and binary options.
+ *
+ * Continuing with the above example:
+ *
+ * @code
+ * test_struct *alloc_test_struct(void)
+ * {
+ *     test_struct *ret = av_malloc(sizeof(*ret));
+ *     ret->class = &test_class;
+ *     av_opt_set_defaults(ret);
+ *     return ret;
+ * }
+ * void free_test_struct(test_struct **foo)
+ * {
+ *     av_opt_free(*foo);
+ *     av_freep(foo);
+ * }
+ * @endcode
+ *
+ * @subsection avoptions_implement_nesting Nesting
+ *      It may happen that an AVOptions-enabled struct contains another
+ *      AVOptions-enabled struct as a member (e.g. AVCodecContext in
+ *      libavcodec exports generic options, while its priv_data field exports
+ *      codec-specific options). In such a case, it is possible to set up the
+ *      parent struct to export a child's options. To do that, simply
+ *      implement AVClass.child_next() and AVClass.child_class_next() in the
+ *      parent struct's AVClass.
+ *      Assuming that the test_struct from above now also contains a
+ *      child_struct field:
+ *
+ *      @code
+ *      typedef struct child_struct {
+ *          AVClass *class;
+ *          int flags_opt;
+ *      } child_struct;
+ *      static const AVOption child_opts[] = {
+ *          { "test_flags", "This is a test option of flags type.",
+ *            offsetof(child_struct, flags_opt), AV_OPT_TYPE_FLAGS, { 0 }, INT_MIN, INT_MAX },
+ *          { NULL },
+ *      };
+ *      static const AVClass child_class = {
+ *          .class_name = "child class",
+ *          .item_name  = av_default_item_name,
+ *          .option     = child_opts,
+ *          .version    = LIBAVUTIL_VERSION_INT,
+ *      };
+ *
+ *      void *child_next(void *obj, void *prev)
+ *      {
+ *          test_struct *t = obj;
+ *          if (!prev && t->child_struct)
+ *              return t->child_struct;
+ *          return NULL
+ *      }
+ *      const AVClass child_class_next(const AVClass *prev)
+ *      {
+ *          return prev ? NULL : &child_class;
+ *      }
+ *      @endcode
+ *      Putting child_next() and child_class_next() as defined above into
+ *      test_class will now make child_struct's options accessible through
+ *      test_struct (again, proper setup as described above needs to be done on
+ *      child_struct right after it is created).
+ *
+ *      From the above example it might not be clear why both child_next()
+ *      and child_class_next() are needed. The distinction is that child_next()
+ *      iterates over actually existing objects, while child_class_next()
+ *      iterates over all possible child classes. E.g. if an AVCodecContext
+ *      was initialized to use a codec which has private options, then its
+ *      child_next() will return AVCodecContext.priv_data and finish
+ *      iterating. OTOH child_class_next() on AVCodecContext.av_class will
+ *      iterate over all available codecs with private options.
+ *
+ * @subsection avoptions_implement_named_constants Named constants
+ *      It is possible to create named constants for options. Simply set the unit
+ *      field of the option the constants should apply to to a string and
+ *      create the constants themselves as options of type AV_OPT_TYPE_CONST
+ *      with their unit field set to the same string.
+ *      Their default_val field should contain the value of the named
+ *      constant.
+ *      For example, to add some named constants for the test_flags option
+ *      above, put the following into the child_opts array:
+ *      @code
+ *      { "test_flags", "This is a test option of flags type.",
+ *        offsetof(child_struct, flags_opt), AV_OPT_TYPE_FLAGS, { 0 }, INT_MIN, INT_MAX, "test_unit" },
+ *      { "flag1", "This is a flag with value 16", 0, AV_OPT_TYPE_CONST, { 16 }, 0, 0, "test_unit" },
+ *      @endcode
+ *
+ * @section avoptions_use Using AVOptions
+ * This section deals with accessing options in an AVOptions-enabled struct.
+ * Such structs in Libav are e.g. AVCodecContext in libavcodec or
+ * AVFormatContext in libavformat.
+ *
+ * @subsection avoptions_use_examine Examining AVOptions
+ * The basic functions for examining options are av_opt_next(), which iterates
+ * over all options defined for one object, and av_opt_find(), which searches
+ * for an option with the given name.
+ *
+ * The situation is more complicated with nesting. An AVOptions-enabled struct
+ * may have AVOptions-enabled children. Passing the AV_OPT_SEARCH_CHILDREN flag
+ * to av_opt_find() will make the function search children recursively.
+ *
+ * For enumerating there are basically two cases. The first is when you want to
+ * get all options that may potentially exist on the struct and its children
+ * (e.g.  when constructing documentation). In that case you should call
+ * av_opt_child_class_next() recursively on the parent struct's AVClass.  The
+ * second case is when you have an already initialized struct with all its
+ * children and you want to get all options that can be actually written or read
+ * from it. In that case you should call av_opt_child_next() recursively (and
+ * av_opt_next() on each result).
+ *
+ * @subsection avoptions_use_get_set Reading and writing AVOptions
+ * When setting options, you often have a string read directly from the
+ * user. In such a case, simply passing it to av_opt_set() is enough. For
+ * non-string type options, av_opt_set() will parse the string according to the
+ * option type.
+ *
+ * Similarly av_opt_get() will read any option type and convert it to a string
+ * which will be returned. Do not forget that the string is allocated, so you
+ * have to free it with av_free().
+ *
+ * In some cases it may be more convenient to put all options into an
+ * AVDictionary and call av_opt_set_dict() on it. A specific case of this
+ * are the format/codec open functions in lavf/lavc which take a dictionary
+ * filled with option as a parameter. This allows to set some options
+ * that cannot be set otherwise, since e.g. the input file format is not known
+ * before the file is actually opened.
+ * @}
+ */
+
 enum AVOptionType{
     AV_OPT_TYPE_FLAGS,
     AV_OPT_TYPE_INT,