AVOptions
[Data Structures]

AVOptions provide a generic system to declare options on arbitrary structs ("objects"). More...


Data Structures

struct  AVOption
 AVOption. More...

Modules

 Evaluating option strings
 This group of functions can be used to evaluate option strings and get numbers out of them.
 Option setting functions
 Those functions set the field of obj with the given name to value.
 Option getting functions
 Those functions get a value of the option with the given name from an object.

Defines

#define AV_OPT_SEARCH_CHILDREN   0x0001
 Search in possible children of the given object first.
#define AV_OPT_SEARCH_FAKE_OBJ   0x0002
 The obj passed to av_opt_find() is fake -- only a double pointer to AVClass instead of a required pointer to a struct containing AVClass.

Enumerations

enum  AVOptionType {
  AV_OPT_TYPE_FLAGS, AV_OPT_TYPE_INT, AV_OPT_TYPE_INT64, AV_OPT_TYPE_DOUBLE,
  AV_OPT_TYPE_FLOAT, AV_OPT_TYPE_STRING, AV_OPT_TYPE_RATIONAL, AV_OPT_TYPE_BINARY,
  AV_OPT_TYPE_CONST = 128, AV_OPT_TYPE_IMAGE_SIZE = MKBETAG('S','I','Z','E'), AV_OPT_TYPE_PIXEL_FMT = MKBETAG('P','F','M','T'), FF_OPT_TYPE_FLAGS = 0,
  FF_OPT_TYPE_INT, FF_OPT_TYPE_INT64, FF_OPT_TYPE_DOUBLE, FF_OPT_TYPE_FLOAT,
  FF_OPT_TYPE_STRING, FF_OPT_TYPE_RATIONAL, FF_OPT_TYPE_BINARY, FF_OPT_TYPE_CONST = 128
}

Functions

attribute_deprecated const
AVOption * 		av_find_opt (void *obj, const char *name, const char *unit, int mask, int flags)
 Look for an option in obj.
attribute_deprecated int av_set_string3 (void *obj, const char *name, const char *val, int alloc, const AVOption **o_out)
 Set the field of obj with the given name to value.
attribute_deprecated const
AVOption * 		av_set_double (void *obj, const char *name, double n)
attribute_deprecated const
AVOption * 		av_set_q (void *obj, const char *name, AVRational n)
attribute_deprecated const
AVOption * 		av_set_int (void *obj, const char *name, int64_t n)
double av_get_double (void *obj, const char *name, const AVOption **o_out)
AVRational av_get_q (void *obj, const char *name, const AVOption **o_out)
int64_t av_get_int (void *obj, const char *name, const AVOption **o_out)
attribute_deprecated const char * av_get_string (void *obj, const char *name, const AVOption **o_out, char *buf, int buf_len)
attribute_deprecated const
AVOption * 		av_next_option (void *obj, const AVOption *last)
int av_opt_show2 (void *obj, void *av_log_obj, int req_flags, int rej_flags)
 Show the obj options.
void av_opt_set_defaults (void *s)
 Set the values of all AVOption fields to their default values.
attribute_deprecated void av_opt_set_defaults2 (void *s, int mask, int flags)
int av_set_options_string (void *ctx, const char *opts, const char *key_val_sep, const char *pairs_sep)
 Parse the key/value pairs list in opts.
void av_opt_free (void *obj)
 Free all string and binary options in obj.
int av_opt_flag_is_set (void *obj, const char *field_name, const char *flag_name)
 Check whether a particular flag is set in a flags field.
int av_opt_set_dict (void *obj, struct AVDictionary **options)
const AVOption * av_opt_find (void *obj, const char *name, const char *unit, int opt_flags, int search_flags)
 Look for an option in an object.
const AVOption * av_opt_find2 (void *obj, const char *name, const char *unit, int opt_flags, int search_flags, void **target_obj)
 Look for an option in an object.
const AVOption * av_opt_next (void *obj, const AVOption *prev)
 Iterate over all AVOptions belonging to obj.
void * av_opt_child_next (void *obj, void *prev)
 Iterate over AVOptions-enabled children of obj.
const AVClass * av_opt_child_class_next (const AVClass *parent, const AVClass *prev)
 Iterate over potential AVOptions-enabled children of parent.
void * av_opt_ptr (const AVClass *avclass, void *obj, const char *name)
 Gets a pointer to the requested field in a struct.

Detailed Description

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.

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 should 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: 

 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, { .i64 = -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,
 };

Next, when allocating your struct, you must ensure that the AVClass pointer is set to the correct value. Then, av_opt_set_defaults() can 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:

 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);
 }

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:

      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, { .i64 = 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;
      }
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.

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: 
      { "test_flags", "This is a test option of flags type.",
        offsetof(child_struct, flags_opt), AV_OPT_TYPE_FLAGS, { .i64 = 0 }, INT_MIN, INT_MAX, "test_unit" },
      { "flag1", "This is a flag with value 16", 0, AV_OPT_TYPE_CONST, { .i64 = 16 }, 0, 0, "test_unit" },

Using AVOptions

This section deals with accessing options in an AVOptions-enabled struct. Such structs in FFmpeg are e.g. AVCodecContext in libavcodec or AVFormatContext in libavformat.

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).

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.

Define Documentation

#define AV_OPT_SEARCH_CHILDREN   0x0001

Search in possible children of the given object first.

Definition at line 451 of file opt.h.

Referenced by av_get_string(), av_opt_find2(), ff_rtp_chain_mux_open(), and opt_default().

#define AV_OPT_SEARCH_FAKE_OBJ   0x0002

The obj passed to av_opt_find() is fake -- only a double pointer to AVClass instead of a required pointer to a struct containing AVClass.

This is useful for searching for options without needing to allocate the corresponding object.

Definition at line 459 of file opt.h.

Referenced by av_opt_find2(), av_opt_ptr(), filter_codec_opts(), init(), opt_default(), and opt_input_file().

Enumeration Type Documentation

enum AVOptionType

Enumerator:
AV_OPT_TYPE_FLAGS 
AV_OPT_TYPE_INT 
AV_OPT_TYPE_INT64 
AV_OPT_TYPE_DOUBLE 
AV_OPT_TYPE_FLOAT 
AV_OPT_TYPE_STRING 
AV_OPT_TYPE_RATIONAL 
AV_OPT_TYPE_BINARY  offset must point to a pointer immediately followed by an int for the length
AV_OPT_TYPE_CONST 
AV_OPT_TYPE_IMAGE_SIZE  offset must point to two consecutive integers
AV_OPT_TYPE_PIXEL_FMT 
FF_OPT_TYPE_FLAGS 
FF_OPT_TYPE_INT 
FF_OPT_TYPE_INT64 
FF_OPT_TYPE_DOUBLE 
FF_OPT_TYPE_FLOAT 
FF_OPT_TYPE_STRING 
FF_OPT_TYPE_RATIONAL 
FF_OPT_TYPE_BINARY  offset must point to a pointer immediately followed by an int for the length
FF_OPT_TYPE_CONST 

Definition at line 218 of file opt.h.

Function Documentation

attribute_deprecated const AVOption* av_find_opt ( void *  obj,
const char *  name,
const char *  unit,
int  mask,
int  flags 
)

Look for an option in obj.

Look only for the options which have the flags set as specified in mask and flags (that is, for which it is the case that opt->flags & mask == flags).

Parameters:
[in] obj a pointer to a struct whose first element is a pointer to an AVClass
[in] name the name of the option to look for
[in] unit the unit of the option to look for, or any if NULL
Returns:
a pointer to the option found, or NULL if no option has been found
Deprecated:
use av_opt_find.

Definition at line 41 of file opt.c.

double av_get_double ( void *  obj,
const char *  name,
const AVOption **  o_out 
)

Definition at line 500 of file opt.c.

int64_t av_get_int ( void *  obj,
const char *  name,
const AVOption **  o_out 
)

Definition at line 525 of file opt.c.

Referenced by config_output(), query_formats(), and queue_picture().

AVRational av_get_q ( void *  obj,
const char *  name,
const AVOption **  o_out 
)

Definition at line 511 of file opt.c.

attribute_deprecated const char* av_get_string ( void *  obj,
const char *  name,
const AVOption **  o_out,
char *  buf,
int  buf_len 
)

Parameters:
buf a buffer which is used for returning non string values as strings, can be NULL
buf_len allocated length in bytes of buf

Definition at line 392 of file opt.c.

attribute_deprecated const AVOption* av_next_option ( void *  obj,
const AVOption *  last 
)

Definition at line 54 of file opt.c.

Referenced by av_find_opt(), print_option(), and show_opts().

const AVClass* av_opt_child_class_next ( const AVClass *  parent,
const AVClass *  prev 
)

Iterate over potential AVOptions-enabled children of parent.

Parameters:
prev result of a previous call to this function or NULL
Returns:
AVClass corresponding to next potential child or NULL

Definition at line 877 of file opt.c.

Referenced by av_opt_find2(), and show_help_children().

void* av_opt_child_next ( void *  obj,
void *  prev 
)

Iterate over AVOptions-enabled children of obj.

Parameters:
prev result of a previous call to this function or NULL
Returns:
next AVOptions-enabled child or NULL

Definition at line 869 of file opt.c.

Referenced by av_opt_find2().

const AVOption* av_opt_find ( void *  obj,
const char *  name,
const char *  unit,
int  opt_flags,
int  search_flags 
)

Look for an option in an object.

Consider only options which have all the specified flags set.

Parameters:
[in] obj A pointer to a struct whose first element is a pointer to an AVClass. Alternatively a double pointer to an AVClass, if AV_OPT_SEARCH_FAKE_OBJ search flag is set.
[in] name The name of the option to look for.
[in] unit When searching for named constants, name of the unit it belongs to.
opt_flags Find only options with all the specified flags set (AV_OPT_FLAG).
search_flags A combination of AV_OPT_SEARCH_*.
Returns:
A pointer to the option found, or NULL if no option was found.
Note:
Options found with AV_OPT_SEARCH_CHILDREN flag may not be settable directly with av_set_string3(). Use special calls which take an options AVDictionary (e.g. avformat_open_input()) to set options found with this flag.

Definition at line 822 of file opt.c.

Referenced by av_get_string(), av_opt_flag_is_set(), av_set_double(), av_set_int(), av_set_q(), av_set_string3(), ffserver_opt_default(), filter_codec_opts(), init(), opt_default(), opt_input_file(), and set_string_number().

const AVOption* av_opt_find2 ( void *  obj,
const char *  name,
const char *  unit,
int  opt_flags,
int  search_flags,
void **  target_obj 
)

Look for an option in an object.

Consider only options which have all the specified flags set.

Parameters:
[in] obj A pointer to a struct whose first element is a pointer to an AVClass. Alternatively a double pointer to an AVClass, if AV_OPT_SEARCH_FAKE_OBJ search flag is set.
[in] name The name of the option to look for.
[in] unit When searching for named constants, name of the unit it belongs to.
opt_flags Find only options with all the specified flags set (AV_OPT_FLAG).
search_flags A combination of AV_OPT_SEARCH_*.
[out] target_obj if non-NULL, an object to which the option belongs will be written here. It may be different from obj if AV_OPT_SEARCH_CHILDREN is present in search_flags. This parameter is ignored if search_flags contain AV_OPT_SEARCH_FAKE_OBJ.
Returns:
A pointer to the option found, or NULL if no option was found.

Definition at line 828 of file opt.c.

Referenced by av_opt_find(), av_opt_find2(), av_opt_get(), av_opt_ptr(), av_opt_set(), av_opt_set_bin(), get_number(), and set_number().

int av_opt_flag_is_set ( void *  obj,
const char *  field_name,
const char *  flag_name 
)

Check whether a particular flag is set in a flags field.

Parameters:
field_name the name of the flag field option
flag_name the name of the flag to check
Returns:
non-zero if the flag is set, zero if the flag isn't set, isn't of the right type, or the flags field doesn't exist.

Definition at line 577 of file opt.c.

Referenced by ff_rtp_get_payload_type().

void av_opt_free ( void *  obj  ) 

Free all string and binary options in obj.

Definition at line 793 of file opt.c.

Referenced by av_write_trailer(), avcodec_close(), avformat_free_context(), avresample_free(), channelmap_init(), ffurl_closep(), init(), init_audio(), init_video(), join_init(), movie_uninit(), seg_write_trailer(), segment_end(), uninit(), and writer_close().

const AVOption* av_opt_next ( void *  obj,
const AVOption *  prev 
)

Iterate over all AVOptions belonging to obj.

Parameters:
obj an AVOptions-enabled struct or a double pointer to an AVClass describing it.
prev result of the previous call to av_opt_next() on this object or NULL
Returns:
next AVOption or NULL

Definition at line 60 of file opt.c.

Referenced by av_next_option(), av_opt_find2(), av_opt_free(), av_opt_set_defaults2(), opt_list(), and show_stream().

void* av_opt_ptr ( const AVClass *  avclass,
void *  obj,
const char *  name 
)

Gets a pointer to the requested field in a struct.

This function allows accessing a struct even when its fields are moved or renamed since the application making the access has been compiled,

Returns:
a pointer to the field, it can be cast to the correct type and read or written to.

Definition at line 884 of file opt.c.

Referenced by decode_video().

void av_opt_set_defaults ( void *  s  ) 

Set the values of all AVOption fields to their default values.

Parameters:
s an AVOption-enabled struct (its first member must be a pointer to AVClass)

Definition at line 678 of file opt.c.

Referenced by avcodec_get_context_defaults3(), avcodec_open2(), avformat_alloc_output_context2(), avformat_get_context_defaults(), avformat_open_input(), avformat_write_header(), avresample_alloc_context(), channelmap_init(), init(), init_audio(), init_video(), join_init(), movie_common_init(), segment_start(), swr_alloc(), sws_alloc_context(), url_alloc_for_protocol(), and writer_open().

attribute_deprecated void av_opt_set_defaults2 ( void *  s,
int  mask,
int  flags 
)

Definition at line 684 of file opt.c.

Referenced by av_opt_set_defaults(), and avcodec_get_context_defaults3().

int av_opt_set_dict ( void *  obj,
struct AVDictionary **  options 
)

Definition at line 801 of file opt.c.

Referenced by avcodec_open2(), avformat_open_input(), avformat_write_header(), and ffurl_open().

int av_opt_show2 ( void *  obj,
void *  av_log_obj,
int  req_flags,
int  rej_flags 
)

Show the obj options.

Parameters:
req_flags requested flags for the options to show. Show only the options for which it is opt->flags & req_flags.
rej_flags rejected flags for the options to show. Show only the options for which it is !(opt->flags & req_flags).
av_log_obj log context to use for showing the options

Definition at line 666 of file opt.c.

Referenced by show_help_children().

attribute_deprecated const AVOption* av_set_double ( void *  obj,
const char *  name,
double  n 
)

Definition at line 317 of file opt.c.

attribute_deprecated const AVOption* av_set_int ( void *  obj,
const char *  name,
int64_t  n 
)

Definition at line 333 of file opt.c.

int av_set_options_string ( void *  ctx,
const char *  opts,
const char *  key_val_sep,
const char *  pairs_sep 
)

Parse the key/value pairs list in opts.

For each key/value pair found, stores the value in the field in ctx that is named like the key. ctx must be an AVClass context, storing is done using AVOptions.

Parameters:
opts options string to parse, may be NULL
key_val_sep a 0-terminated list of characters used to separate key from value
pairs_sep a 0-terminated list of characters used to separate two pairs from each other
Returns:
the number of successfully set key/value pairs, or a negative value corresponding to an AVERROR code in case of error: AVERROR(EINVAL) if opts cannot be parsed, the error code issued by av_set_string3() if a key/value pair cannot be set

Definition at line 773 of file opt.c.

Referenced by channelmap_init(), init(), init_audio(), init_video(), join_init(), movie_common_init(), set_options(), and writer_open().

attribute_deprecated const AVOption* av_set_q ( void *  obj,
const char *  name,
AVRational  n 
)

Definition at line 325 of file opt.c.

attribute_deprecated int av_set_string3 ( void *  obj,
const char *  name,
const char *  val,
int  alloc,
const AVOption **  o_out 
)

Set the field of obj with the given name to value.

Parameters:
[in] obj A struct whose first element is a pointer to an AVClass.
[in] name the name of the field to set
[in] val The value to set. If the field is not of a string type, then the given string is parsed. SI postfixes and some named scalars are supported. If the field is of a numeric type, it has to be a numeric or named scalar. Behavior with more than one scalar and +- infix operators is undefined. If the field is of a flags type, it has to be a sequence of numeric scalars or named flags separated by '+' or '-'. Prefixing a flag with '+' causes it to be set without affecting the other flags; similarly, '-' unsets a flag.
[out] o_out if non-NULL put here a pointer to the AVOption found
alloc this parameter is currently ignored
Returns:
0 if the value has been set, or an AVERROR code in case of error: AVERROR_OPTION_NOT_FOUND if no matching option exists AVERROR(ERANGE) if the value is out of range AVERROR(EINVAL) if the value is not valid
Deprecated:
use av_opt_set()

Definition at line 228 of file opt.c.

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