[时间：2017-10] [状态：Open]
[关键词：ffmpeg，avutil，AVOption]
0 引言
AVOptions提供了一种通用的options机制，可以用于任意特定结构的对象。
本文将先介绍下AVOption的实现，然后整理下如何使用AVOption。（我使用的版本是ffmpeg3.4）
1 AVOption实现细节
这部分可能会很枯燥，多数是ffmpeg源码的粘贴和整理，如果读者对此不感兴趣可以调到下一部分
AVOption的定义如下：
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, ///< offset must point to a pointer immediately followed by an int for the length
AV_OPT_TYPE_DICT,
AV_OPT_TYPE_UINT64,
AV_OPT_TYPE_CONST = 128,
AV_OPT_TYPE_IMAGE_SIZE = MKBETAG('S','I','Z','E'), ///< offset must point to two consecutive integers
AV_OPT_TYPE_PIXEL_FMT = MKBETAG('P','F','M','T'),
AV_OPT_TYPE_SAMPLE_FMT = MKBETAG('S','F','M','T'),
AV_OPT_TYPE_VIDEO_RATE = MKBETAG('V','R','A','T'), ///< offset must point to AVRational
AV_OPT_TYPE_DURATION = MKBETAG('D','U','R',' '),
AV_OPT_TYPE_COLOR = MKBETAG('C','O','L','R'),
AV_OPT_TYPE_CHANNEL_LAYOUT = MKBETAG('C','H','L','A'),
AV_OPT_TYPE_BOOL = MKBETAG('B','O','O','L'),
};
typedef struct AVOption {
const char *name;
/* short English help text */
const char *help;
/* option对应的偏移地址和类型 */
int offset;
enum AVOptionType type;
/* 共用体，实际存储数据 */
union {
int64_t i64;
double dbl;
const char *str;
/* TODO those are unused now */
AVRational q;
} default_val;
double min; ///< minimum valid value for the option
double max; ///< maximum valid value for the option
int flags;// option类型属性的标志
const char *unit;
} AVOption;初始化函数av_opt_set_defaults和av_opt_set_defaults2
void av_opt_set_defaults(void *s)
{
av_opt_set_defaults2(s, 0, 0);
}
// 这里的s是包含AVOptions成员的结构体指针
void av_opt_set_defaults2(void *s, int mask, int flags)
{
const AVOption *opt = NULL;
// 遍历AVOption中所有数据
while ((opt = av_opt_next(s, opt))) {
void *dst = ((uint8_t*)s) + opt->offset;
if ((opt->flags & mask) != flags)
continue;
if (opt->flags & AV_OPT_FLAG_READONLY)
continue;
// 根据类型赋值
switch (opt->type) {
case AV_OPT_TYPE_CONST:
/* Nothing to be done here */
break;
case AV_OPT_TYPE_BOOL:
case AV_OPT_TYPE_FLAGS:
case AV_OPT_TYPE_INT:
case AV_OPT_TYPE_INT64:
case AV_OPT_TYPE_UINT64:
case AV_OPT_TYPE_DURATION:
case AV_OPT_TYPE_CHANNEL_LAYOUT:
case AV_OPT_TYPE_PIXEL_FMT:
case AV_OPT_TYPE_SAMPLE_FMT:
write_number(s, opt, dst, 1, 1, opt->default_val.i64);
break;
case AV_OPT_TYPE_DOUBLE:
case AV_OPT_TYPE_FLOAT: {
double val;
val = opt->default_val.dbl;
write_number(s, opt, dst, val, 1, 1);
}
break;
case AV_OPT_TYPE_RATIONAL: {
AVRational val;
val = av_d2q(opt->default_val.dbl, INT_MAX);
write_number(s, opt, dst, 1, val.den, val.num);
}
break;
case AV_OPT_TYPE_COLOR:
set_string_color(s, opt, opt->default_val.str, dst);
break;
case AV_OPT_TYPE_STRING:
set_string(s, opt, opt->default_val.str, dst);
break;
case AV_OPT_TYPE_IMAGE_SIZE:
set_string_image_size(s, opt, opt->default_val.str, dst);
break;
case AV_OPT_TYPE_VIDEO_RATE:
set_string_video_rate(s, opt, opt->default_val.str, dst);
break;
case AV_OPT_TYPE_BINARY:
set_string_binary(s, opt, opt->default_val.str, dst);
break;
case AV_OPT_TYPE_DICT:
/* Cannot set defaults for these types */
break;
default:
av_log(s, AV_LOG_DEBUG, "AVOption type %d of option %s not implemented yet\n",
opt->type, opt->name);
}
}
}初始化的基本逻辑是扫描AVOptions数组的所有成员，然后按照类型依次初始化。其中最主要的是av_opt_next函数。其实现如下：
// 该函数通过直接在AVOptions数组上顺序+1实现遍历
const AVOption *av_opt_next(const void *obj, const AVOption *last)
{
const AVClass *class;
if (!obj)
return NULL;
class = *(const AVClass**)obj;
if (!last && class && class->option && class->option[0].name)
return class->option;
if (last && last[1].name)
return ++last;
return NULL;
}AVOptions数组必须以NULL结束，否则这个函数可能就出现死循环了。
接下来是保存数据的函数，write_number，其实现的代码如下：
static int write_number(void *obj, const AVOption *o, void *dst, double num, int den, int64_t intnum)
{
switch (o->type) {
case AV_OPT_TYPE_PIXEL_FMT:
*(enum AVPixelFormat *)dst = llrint(num / den) * intnum;
break;
case AV_OPT_TYPE_SAMPLE_FMT:
*(enum AVSampleFormat *)dst = llrint(num / den) * intnum;
break;
case AV_OPT_TYPE_BOOL:
case AV_OPT_TYPE_FLAGS:
case AV_OPT_TYPE_INT:
*(int *)dst = llrint(num / den) * intnum;
break;
case AV_OPT_TYPE_DURATION:
case AV_OPT_TYPE_CHANNEL_LAYOUT:
case AV_OPT_TYPE_INT64:{
double d = num / den;
if (intnum == 1 && d == (double)INT64_MAX) {
*(int64_t *)dst = INT64_MAX;
} else
*(int64_t *)dst = llrint(d) * intnum;
break;}
case AV_OPT_TYPE_UINT64:{
double d = num / den;
// We must special case uint64_t here as llrint() does not support values
// outside the int64_t range and there is no portable function which does
// "INT64_MAX + 1ULL" is used as it is representable exactly as IEEE double
// while INT64_MAX is not
if (intnum == 1 && d == (double)UINT64_MAX) {
*(uint64_t *)dst = UINT64_MAX;
} else if (d > INT64_MAX + 1ULL) {
*(uint64_t *)dst = (llrint(d - (INT64_MAX + 1ULL)) + (INT64_MAX + 1ULL))*intnum;
} else {
*(uint64_t *)dst = llrint(d) * intnum;
}
break;}
case AV_OPT_TYPE_FLOAT:
*(float *)dst = num * intnum / den;
break;
case AV_OPT_TYPE_DOUBLE:
*(double *)dst = num * intnum / den;
break;
case AV_OPT_TYPE_RATIONAL:
case AV_OPT_TYPE_VIDEO_RATE:
if ((int) num == num)
*(AVRational *)dst = (AVRational) { num *intnum, den };
else
*(AVRational *)dst = av_d2q(num * intnum / den, 1 << 24);
break;
default:
return AVERROR(EINVAL);
}
return 0;
}write_number仅支持数字形式的参数。其他格式需要单独处理。下面是其他格式的保存函数：
// AV_OPT_TYPE_COLOR:
static int set_string_color(void *obj, const AVOption *o, const char *val, uint8_t *dst)
{
int ret;
if (!val) {
return 0;
} else {
ret = av_parse_color(dst, val, -1, obj);
if (ret < 0)
av_log(obj, AV_LOG_ERROR, "Unable to parse option value \"%s\" as color\n", val);
return ret;
}
return 0;
}
// AV_OPT_TYPE_STRING:
static int set_string(void *obj, const AVOption *o, const char *val, uint8_t **dst)
{
av_freep(dst);
*dst = av_strdup(val);
return *dst ? 0 : AVERROR(ENOMEM);
}
// AV_OPT_TYPE_IMAGE_SIZE:
static int set_string_image_size(void *obj, const AVOption *o, const char *val, int *dst)
{
int ret;
if (!val || !strcmp(val, "none")) {
dst[0] =
dst[1] = 0;
return 0;
}
ret = av_parse_video_size(dst, dst + 1, val);
if (ret < 0)
av_log(obj, AV_LOG_ERROR, "Unable to parse option value \"%s\" as image size\n", val);
return ret;
}
// AV_OPT_TYPE_VIDEO_RATE:
static int set_string_video_rate(void *obj, const AVOption *o, const char *val, AVRational *dst)
{
int ret;
if (!val) {
ret = AVERROR(EINVAL);
} else {
ret = av_parse_video_rate(dst, val);
}
if (ret < 0)
av_log(obj, AV_LOG_ERROR, "Unable to parse option value \"%s\" as video rate\n", val);
return ret;
}
// AV_OPT_TYPE_BINARY:
static int set_string_binary(void *obj, const AVOption *o, const char *val, uint8_t **dst)
{
int *lendst = (int *)(dst + 1);
uint8_t *bin, *ptr;
int len;
av_freep(dst);
*lendst = 0;
if (!val || !(len = strlen(val)))
return 0;
if (len & 1)
return AVERROR(EINVAL);
len /= 2;
ptr = bin = av_malloc(len);
if (!ptr)
return AVERROR(ENOMEM);
while (*val) {
int a = hexchar2int(*val++);
int b = hexchar2int(*val++);
if (a < 0 || b < 0) {
av_free(bin);
return AVERROR(EINVAL);
}
*ptr++ = (a << 4) | b;
}
*dst = bin;
*lendst = len;
return 0;
}当然，AVOptions还有两个构造函数，有兴趣的可以看看ffmpeg中的源代码，其声明如下：
int av_set_options_string(void *ctx, const char *opts,
const char *key_val_sep, const char *pairs_sep);
int av_opt_set_from_string(void *ctx, const char *opts,
const char *const *shorthand,
const char *key_val_sep, const char *pairs_sep);销毁函数av_opt_free
该函数的实现主要依赖于av_opt_next，遍历所有AVOptions，释放初始化时申请的内存。
void av_opt_free(void *obj)
{
const AVOption *o = NULL;
while ((o = av_opt_next(obj, o))) {
switch (o->type) {
case AV_OPT_TYPE_STRING:
case AV_OPT_TYPE_BINARY:
av_freep((uint8_t *)obj + o->offset);
break;
case AV_OPT_TYPE_DICT:
av_dict_free((AVDictionary **)(((uint8_t *)obj) + o->offset));
break;
default:
break;
}
}
}属性查找av_opt_find和av_opt_find2
属性查找依赖于遍历的接口函数，其具体实现如下：
const AVOption *av_opt_find(void *obj, const char *name, const char *unit,
int opt_flags, int search_flags)
{
return av_opt_find2(obj, name, unit, opt_flags, search_flags, NULL);
}
const AVOption *av_opt_find2(void *obj, const char *name, const char *unit,
int opt_flags, int search_flags, void **target_obj)
{
const AVClass *c;
const AVOption *o = NULL;
if(!obj)
return NULL;
c= *(AVClass**)obj;
if (!c)
return NULL;
// 判断是否需要检查子对象属性
if (search_flags & AV_OPT_SEARCH_CHILDREN) {
if (search_flags & AV_OPT_SEARCH_FAKE_OBJ) { // 假对象
const AVClass *child = NULL;
while (child = av_opt_child_class_next(c, child))
if (o = av_opt_find2(&child, name, unit, opt_flags, search_flags, NULL))
return o;
} else {
void *child = NULL;
while (child = av_opt_child_next(obj, child))
if (o = av_opt_find2(child, name, unit, opt_flags, search_flags, target_obj))
return o;
}
}
// 遍历当前AVClass的所有属性（这里的主要区别是遍历函数不同）
while (o = av_opt_next(obj, o)) {
if (!strcmp(o->name, name) && (o->flags & opt_flags) == opt_flags &&
((!unit && o->type != AV_OPT_TYPE_CONST) ||
(unit && o->type == AV_OPT_TYPE_CONST && o->unit && !strcmp(o->unit, unit)))) {
if (target_obj) {
if (!(search_flags & AV_OPT_SEARCH_FAKE_OBJ))
*target_obj = obj;
else
*target_obj = NULL;
}
return o;
}
}
return NULL;
}AVOptions遍历
av_opt_next在前面的初始化函数中介绍过。这里重点关注后边两个：av_opt_child_next和av_opt_child_class_next。
对应的实现代码如下：
// 该函数直接通过AVClass->child_next函数指针访问子对象
void *av_opt_child_next(void *obj, void *prev)
{
const AVClass *c = *(AVClass **)obj;
if (c->child_next)
return c->child_next(obj, prev);
return NULL;
}
// 该函数直接通过AVClass->child_class_next函数指针访问子类
const AVClass *av_opt_child_class_next(const AVClass *parent, const AVClass *prev)
{
if (parent->child_class_next)
return parent->child_class_next(prev);
return NULL;
}属性设置
属性设置函数比较多，这里依次查看该实现：
int av_opt_set(void *obj, const char *name, const char *val, int search_flags)
{
int ret = 0;
void *dst, *target_obj;
// 根据name查找对应的AVOption
const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);
if (!o || !target_obj)
return AVERROR_OPTION_NOT_FOUND;
if (!val && (o->type != AV_OPT_TYPE_STRING &&
o->type != AV_OPT_TYPE_PIXEL_FMT && o->type != AV_OPT_TYPE_SAMPLE_FMT &&
o->type != AV_OPT_TYPE_IMAGE_SIZE && o->type != AV_OPT_TYPE_VIDEO_RATE &&
o->type != AV_OPT_TYPE_DURATION && o->type != AV_OPT_TYPE_COLOR &&
o->type != AV_OPT_TYPE_CHANNEL_LAYOUT && o->type != AV_OPT_TYPE_BOOL))
return AVERROR(EINVAL);
if (o->flags & AV_OPT_FLAG_READONLY)
return AVERROR(EINVAL);
dst = ((uint8_t *)target_obj) + o->offset;
switch (o->type) { // 根据AVOPtion的类型设置该参数
case AV_OPT_TYPE_BOOL:
return set_string_bool(obj, o, val, dst);//这个在初始化函数中有介绍
case AV_OPT_TYPE_STRING:
return set_string(obj, o, val, dst); //这个在初始化函数中有介绍
case AV_OPT_TYPE_BINARY:
return set_string_binary(obj, o, val, dst);//这个在初始化函数中有介绍
case AV_OPT_TYPE_FLAGS:
case AV_OPT_TYPE_INT:
case AV_OPT_TYPE_INT64:
case AV_OPT_TYPE_UINT64:
case AV_OPT_TYPE_FLOAT:
case AV_OPT_TYPE_DOUBLE:
case AV_OPT_TYPE_RATIONAL:
return set_string_number(obj, target_obj, o, val, dst);
case AV_OPT_TYPE_IMAGE_SIZE:
return set_string_image_size(obj, o, val, dst);
case AV_OPT_TYPE_VIDEO_RATE: {
AVRational tmp;
ret = set_string_video_rate(obj, o, val, &tmp);
if (ret < 0)
return ret;
return write_number(obj, o, dst, 1, tmp.den, tmp.num);
}
case AV_OPT_TYPE_PIXEL_FMT:
return set_string_pixel_fmt(obj, o, val, dst);
case AV_OPT_TYPE_SAMPLE_FMT:
return set_string_sample_fmt(obj, o, val, dst);
case AV_OPT_TYPE_DURATION:
if (!val) {
*(int64_t *)dst = 0;
return 0;
} else {
if ((ret = av_parse_time(dst, val, 1)) < 0)
av_log(obj, AV_LOG_ERROR, "Unable to parse option value \"%s\" as duration\n", val);
return ret;
}
break;
case AV_OPT_TYPE_COLOR:
return set_string_color(obj, o, val, dst);
case AV_OPT_TYPE_CHANNEL_LAYOUT:
if (!val || !strcmp(val, "none")) {
*(int64_t *)dst = 0;
} else {
int64_t cl = av_get_channel_layout(val);
if (!cl) {
av_log(obj, AV_LOG_ERROR, "Unable to parse option value \"%s\" as channel layout\n", val);
ret = AVERROR(EINVAL);
}
*(int64_t *)dst = cl;
return ret;
}
break;
}
av_log(obj, AV_LOG_ERROR, "Invalid option type.\n");
return AVERROR(EINVAL);
}下面看一下其中具体调用的函数实现细节（其中多数函数在初始化一节中一节介绍了，这里不做重复）
static int read_number(const AVOption *o, const void *dst, double *num, int *den, int64_t *intnum)
{
switch (o->type) {
case AV_OPT_TYPE_FLAGS:
*intnum = *(unsigned int*)dst;
return 0;
case AV_OPT_TYPE_PIXEL_FMT:
*intnum = *(enum AVPixelFormat *)dst;
return 0;
case AV_OPT_TYPE_SAMPLE_FMT:
*intnum = *(enum AVSampleFormat *)dst;
return 0;
case AV_OPT_TYPE_BOOL:
case AV_OPT_TYPE_INT:
*intnum = *(int *)dst;
return 0;
case AV_OPT_TYPE_CHANNEL_LAYOUT:
case AV_OPT_TYPE_DURATION:
case AV_OPT_TYPE_INT64:
case AV_OPT_TYPE_UINT64:
*intnum = *(int64_t *)dst;
return 0;
case AV_OPT_TYPE_FLOAT:
*num = *(float *)dst;
return 0;
case AV_OPT_TYPE_DOUBLE:
*num = *(double *)dst;
return 0;
case AV_OPT_TYPE_RATIONAL:
*intnum = ((AVRational *)dst)->num;
*den = ((AVRational *)dst)->den;
return 0;
case AV_OPT_TYPE_CONST:
*num = o->default_val.dbl;
return 0;
}
return AVERROR(EINVAL);
}
#define DEFAULT_NUMVAL(opt) ((opt->type == AV_OPT_TYPE_INT64 || \
opt->type == AV_OPT_TYPE_UINT64 || \
opt->type == AV_OPT_TYPE_CONST || \
opt->type == AV_OPT_TYPE_FLAGS || \
opt->type == AV_OPT_TYPE_INT) \
? opt->default_val.i64 \
: opt->default_val.dbl)
static int set_string_number(void *obj, void *target_obj, const AVOption *o, const char *val, void *dst)
{
int ret = 0;
int num, den;
char c;
if (sscanf(val, "%d%*1[:/]%d%c", &num, &den, &c) == 2) {
if ((ret = write_number(obj, o, dst, 1, den, num)) >= 0)
return ret;
ret = 0;
}
// 下面支持表达式的处理逻辑
for (;;) {
int i = 0;
char buf[256];
int cmd = 0;
double d;
int64_t intnum = 1;
if (o->type == AV_OPT_TYPE_FLAGS) {
if (*val == '+' || *val == '-')
cmd = *(val++);
for (; i < sizeof(buf) - 1 && val[i] && val[i] != '+' && val[i] != '-'; i++)
buf[i] = val[i];
buf[i] = 0;
}
{
const AVOption *o_named = av_opt_find(target_obj, i ? buf : val, o->unit, 0, 0);
int res;
int ci = 0;
double const_values[64];
const char * const_names[64];
if (o_named && o_named->type == AV_OPT_TYPE_CONST)
d = DEFAULT_NUMVAL(o_named);
else {
if (o->unit) {
for (o_named = NULL; o_named = av_opt_next(target_obj, o_named); ) {
if (o_named->type == AV_OPT_TYPE_CONST &&
o_named->unit &&
!strcmp(o_named->unit, o->unit)) {
if (ci + 6 >= FF_ARRAY_ELEMS(const_values)) {
av_log(obj, AV_LOG_ERROR, "const_values array too small for %s\n", o->unit);
return AVERROR_PATCHWELCOME;
}
const_names [ci ] = o_named->name;
const_values[ci++] = DEFAULT_NUMVAL(o_named);
}
}
}
const_names [ci ] = "default";
const_values[ci++] = DEFAULT_NUMVAL(o);
const_names [ci ] = "max";
const_values[ci++] = o->max;
const_names [ci ] = "min";
const_values[ci++] = o->min;
const_names [ci ] = "none";
const_values[ci++] = 0;
const_names [ci ] = "all";
const_values[ci++] = ~0;
const_names [ci] = NULL;
const_values[ci] = 0;
res = av_expr_parse_and_eval(&d, i ? buf : val, const_names,
const_values, NULL, NULL, NULL, NULL, NULL, 0, obj);
if (res < 0) {
av_log(obj, AV_LOG_ERROR, "Unable to parse option value \"%s\"\n", val);
return res;
}
}
}
if (o->type == AV_OPT_TYPE_FLAGS) {
read_number(o, dst, NULL, NULL, &intnum);
if (cmd == '+')
d = intnum | (int64_t)d;
else if (cmd == '-')
d = intnum &~(int64_t)d;
}
if ((ret = write_number(obj, o, dst, d, 1, 1)) < 0)
return ret;
val += i;
if (!i || !*val)
return 0;
}
}从上面一部分代码可以看出av_opt_set函数仅仅是根据类型进行数据区域赋值。下面的函数是针对特定类型的赋值：
// int
int av_opt_set_int(void *obj, const char *name, int64_t val, int search_flags)
{
return set_number(obj, name, 1, 1, val, search_flags);
}
// double
int av_opt_set_double(void *obj, const char *name, double val, int search_flags)
{
return set_number(obj, name, val, 1, 1, search_flags);
}
// AVRational
int av_opt_set_q(void *obj, const char *name, AVRational val, int search_flags)
{
return set_number(obj, name, val.num, val.den, 1, search_flags);
}
// binary
int av_opt_set_bin(void *obj, const char *name, const uint8_t *val, int len, int search_flags)
{
void *target_obj;
const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);
uint8_t *ptr;
uint8_t **dst;
int *lendst;
if (!o || !target_obj)
return AVERROR_OPTION_NOT_FOUND;
if (o->type != AV_OPT_TYPE_BINARY || o->flags & AV_OPT_FLAG_READONLY)
return AVERROR(EINVAL);
ptr = len ? av_malloc(len) : NULL;
if (len && !ptr)
return AVERROR(ENOMEM);
dst = (uint8_t **)(((uint8_t *)target_obj) + o->offset);
lendst = (int *)(dst + 1);
av_free(*dst);
*dst = ptr;
*lendst = len;
if (len)
memcpy(ptr, val, len);
return 0;
}
// image_size，字符格式"1024x768"
int av_opt_set_image_size(void *obj, const char *name, int w, int h, int search_flags)
{
void *target_obj;
const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);
if (!o || !target_obj)
return AVERROR_OPTION_NOT_FOUND;
if (o->type != AV_OPT_TYPE_IMAGE_SIZE) {
av_log(obj, AV_LOG_ERROR,
"The value set by option '%s' is not an image size.\n", o->name);
return AVERROR(EINVAL);
}
if (w<0 || h<0) {
av_log(obj, AV_LOG_ERROR,
"Invalid negative size value %dx%d for size '%s'\n", w, h, o->name);
return AVERROR(EINVAL);
}
*(int *)(((uint8_t *)target_obj) + o->offset) = w;
*(int *)(((uint8_t *)target_obj+sizeof(int)) + o->offset) = h;
return 0;
}
// 视频码率
int av_opt_set_video_rate(void *obj, const char *name, AVRational val, int search_flags)
{
void *target_obj;
const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);
if (!o || !target_obj)
return AVERROR_OPTION_NOT_FOUND;
if (o->type != AV_OPT_TYPE_VIDEO_RATE) {
av_log(obj, AV_LOG_ERROR,
"The value set by option '%s' is not a video rate.\n", o->name);
return AVERROR(EINVAL);
}
if (val.num <= 0 || val.den <= 0)
return AVERROR(EINVAL);
return set_number(obj, name, val.num, val.den, 1, search_flags);
}
// 图像采样格式
int av_opt_set_pixel_fmt(void *obj, const char *name, enum AVPixelFormat fmt, int search_flags)
{
return set_format(obj, name, fmt, search_flags, AV_OPT_TYPE_PIXEL_FMT, "pixel", AV_PIX_FMT_NB);
}
// 音频采样格式
int av_opt_set_sample_fmt(void *obj, const char *name, enum AVSampleFormat fmt, int search_flags)
{
return set_format(obj, name, fmt, search_flags, AV_OPT_TYPE_SAMPLE_FMT, "sample", AV_SAMPLE_FMT_NB);
}
// channel_layout
int av_opt_set_channel_layout(void *obj, const char *name, int64_t cl, int search_flags)
{
void *target_obj;
const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);
if (!o || !target_obj)
return AVERROR_OPTION_NOT_FOUND;
if (o->type != AV_OPT_TYPE_CHANNEL_LAYOUT) {
av_log(obj, AV_LOG_ERROR,
"The value set by option '%s' is not a channel layout.\n", o->name);
return AVERROR(EINVAL);
}
*(int64_t *)(((uint8_t *)target_obj) + o->offset) = cl;
return 0;
}
// 设置为AVDictionary
int av_opt_set_dict_val(void *obj, const char *name, const AVDictionary *val,
int search_flags)
{
void *target_obj;
AVDictionary **dst;
const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);
if (!o || !target_obj)
return AVERROR_OPTION_NOT_FOUND;
if (o->flags & AV_OPT_FLAG_READONLY)
return AVERROR(EINVAL);
dst = (AVDictionary **)(((uint8_t *)target_obj) + o->offset);
av_dict_free(dst);
av_dict_copy(dst, val, 0);
return 0;
}属性获取
属性获取和属性设置是对应的，对应函数一般是成对存在，比如av_opt_get/av_opt_set。下面先查看av_opt_get代码：
int av_opt_get(void *obj, const char *name, int search_flags, uint8_t **out_val)
{
void *dst, *target_obj;
// 通过name查找对应的AVOption
const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);
uint8_t *bin, buf[128];
int len, i, ret;
int64_t i64;
if (!o || !target_obj || (o->offset<=0 && o->type != AV_OPT_TYPE_CONST))
return AVERROR_OPTION_NOT_FOUND;
dst = (uint8_t *)target_obj + o->offset;
buf[0] = 0;
switch (o->type) { // 根据实际类型返回参数
// 对于整型参数，可以直接通过snprintf格式化之
case AV_OPT_TYPE_BOOL:
ret = snprintf(buf, sizeof(buf), "%s", (char *)av_x_if_null(get_bool_name(*(int *)dst), "invalid"));
break;
case AV_OPT_TYPE_FLAGS:
ret = snprintf(buf, sizeof(buf), "0x%08X", *(int *)dst);
break;
case AV_OPT_TYPE_INT:
ret = snprintf(buf, sizeof(buf), "%d", *(int *)dst);
break;
case AV_OPT_TYPE_INT64:
ret = snprintf(buf, sizeof(buf), "%"PRId64, *(int64_t *)dst);
break;
case AV_OPT_TYPE_UINT64:
ret = snprintf(buf, sizeof(buf), "%"PRIu64, *(uint64_t *)dst);
break;
case AV_OPT_TYPE_FLOAT:
ret = snprintf(buf, sizeof(buf), "%f", *(float *)dst);
break;
case AV_OPT_TYPE_DOUBLE:
ret = snprintf(buf, sizeof(buf), "%f", *(double *)dst);
break;
case AV_OPT_TYPE_VIDEO_RATE:
case AV_OPT_TYPE_RATIONAL:
ret = snprintf(buf, sizeof(buf), "%d/%d", ((AVRational *)dst)->num, ((AVRational *)dst)->den);
break;
case AV_OPT_TYPE_CONST:
ret = snprintf(buf, sizeof(buf), "%f", o->default_val.dbl);
break;
case AV_OPT_TYPE_STRING: // 字符串可以直接复制
if (*(uint8_t **)dst) {
*out_val = av_strdup(*(uint8_t **)dst);
} else if (search_flags & AV_OPT_ALLOW_NULL) {
*out_val = NULL;
return 0;
} else {
*out_val = av_strdup("");
}
return *out_val ? 0 : AVERROR(ENOMEM);
case AV_OPT_TYPE_BINARY: // 二进制文件需要特殊处理
if (!*(uint8_t **)dst && (search_flags & AV_OPT_ALLOW_NULL)) {
*out_val = NULL;
return 0;
}
len = *(int *)(((uint8_t *)dst) + sizeof(uint8_t *));
if ((uint64_t)len * 2 + 1 > INT_MAX)
return AVERROR(EINVAL);
if (!(*out_val = av_malloc(len * 2 + 1)))
return AVERROR(ENOMEM);
if (!len) {
*out_val[0] = '\0';
return 0;
}
bin = *(uint8_t **)dst;
for (i = 0; i < len; i++)
snprintf(*out_val + i * 2, 3, "%02X", bin[i]);
return 0;
case AV_OPT_TYPE_IMAGE_SIZE:
ret = snprintf(buf, sizeof(buf), "%dx%d", ((int *)dst)[0], ((int *)dst)[1]);
break;
case AV_OPT_TYPE_PIXEL_FMT:
ret = snprintf(buf, sizeof(buf), "%s", (char *)av_x_if_null(av_get_pix_fmt_name(*(enum AVPixelFormat *)dst), "none"));
break;
case AV_OPT_TYPE_SAMPLE_FMT:
ret = snprintf(buf, sizeof(buf), "%s", (char *)av_x_if_null(av_get_sample_fmt_name(*(enum AVSampleFormat *)dst), "none"));
break;
case AV_OPT_TYPE_DURATION:
i64 = *(int64_t *)dst;
format_duration(buf, sizeof(buf), i64);
ret = strlen(buf); // no overflow possible, checked by an assert
break;
case AV_OPT_TYPE_COLOR:
ret = snprintf(buf, sizeof(buf), "0x%02x%02x%02x%02x",
(int)((uint8_t *)dst)[0], (int)((uint8_t *)dst)[1],
(int)((uint8_t *)dst)[2], (int)((uint8_t *)dst)[3]);
break;
case AV_OPT_TYPE_CHANNEL_LAYOUT:
i64 = *(int64_t *)dst;
ret = snprintf(buf, sizeof(buf), "0x%"PRIx64, i64);
break;
default:
return AVERROR(EINVAL);
}
if (ret >= sizeof(buf))
return AVERROR(EINVAL);
*out_val = av_strdup(buf);
return *out_val ? 0 : AVERROR(ENOMEM);
}上面只有一个函数没有实现format_duration，其他代码如下：
static void format_duration(char *buf, size_t size, int64_t d)
{
char *e;
// 主要是为了时长的特殊的格式化输出
av_assert0(size >= 25);
if (d < 0 && d != INT64_MIN) {
*(buf++) = '-';
size--;
d = -d;
}
if (d == INT64_MAX)
snprintf(buf, size, "INT64_MAX");
else if (d == INT64_MIN)
snprintf(buf, size, "INT64_MIN");
else if (d > (int64_t)3600*1000000)
snprintf(buf, size, "%"PRId64":%02d:%02d.%06d", d / 3600000000,
(int)((d / 60000000) % 60),
(int)((d / 1000000) % 60),
(int)(d % 1000000));
else if (d > 60*1000000)
snprintf(buf, size, "%d:%02d.%06d",
(int)(d / 60000000),
(int)((d / 1000000) % 60),
(int)(d % 1000000));
else
snprintf(buf, size, "%d.%06d",
(int)(d / 1000000),
(int)(d % 1000000));
e = buf + strlen(buf);
while (e > buf && e[-1] == '0')
*(--e) = 0;
if (e > buf && e[-1] == '.')
*(--e) = 0;
}接下来是对应的不同类型的AVOption的获取，其基本逻辑是对av_opt_get的拆分，代码如下：
// 查找数字格式
static int get_number(void *obj, const char *name, const AVOption **o_out, double *num, int *den, int64_t *intnum,
int search_flags)
{
void *dst, *target_obj;
const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);
if (!o || !target_obj)
goto error;
dst = ((uint8_t *)target_obj) + o->offset;
if (o_out) *o_out= o;
return read_number(o, dst, num, den, intnum);
error:
*den =
*intnum = 0;
return -1;
}
int av_opt_get_int(void *obj, const char *name, int search_flags, int64_t *out_val)
{
int64_t intnum = 1;
double num = 1;
int ret, den = 1;
if ((ret = get_number(obj, name, NULL, &num, &den, &intnum, search_flags)) < 0)
return ret;
*out_val = num * intnum / den;
return 0;
}
int av_opt_get_double(void *obj, const char *name, int search_flags, double *out_val)
{
int64_t intnum = 1;
double num = 1;
int ret, den = 1;
if ((ret = get_number(obj, name, NULL, &num, &den, &intnum, search_flags)) < 0)
return ret;
*out_val = num * intnum / den;
return 0;
}
int av_opt_get_q(void *obj, const char *name, int search_flags, AVRational *out_val)
{
int64_t intnum = 1;
double num = 1;
int ret, den = 1;
if ((ret = get_number(obj, name, NULL, &num, &den, &intnum, search_flags)) < 0)
return ret;
if (num == 1.0 && (int)intnum == intnum)
*out_val = (AVRational){intnum, den};
else
*out_val = av_d2q(num*intnum/den, 1<<24);
return 0;
}
int av_opt_get_image_size(void *obj, const char *name, int search_flags, int *w_out, int *h_out)
{
void *dst, *target_obj;
const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);
if (!o || !target_obj)
return AVERROR_OPTION_NOT_FOUND;
if (o->type != AV_OPT_TYPE_IMAGE_SIZE) {
av_log(obj, AV_LOG_ERROR,
"The value for option '%s' is not an image size.\n", name);
return AVERROR(EINVAL);
}
dst = ((uint8_t*)target_obj) + o->offset;
if (w_out) *w_out = *(int *)dst;
if (h_out) *h_out = *((int *)dst+1);
return 0;
}
int av_opt_get_video_rate(void *obj, const char *name, int search_flags, AVRational *out_val)
{
int64_t intnum = 1;
double num = 1;
int ret, den = 1;
if ((ret = get_number(obj, name, NULL, &num, &den, &intnum, search_flags)) < 0)
return ret;
if (num == 1.0 && (int)intnum == intnum)
*out_val = (AVRational) { intnum, den };
else
*out_val = av_d2q(num * intnum / den, 1 << 24);
return 0;
}
static int get_format(void *obj, const char *name, int search_flags, int *out_fmt,
enum AVOptionType type, const char *desc)
{
void *dst, *target_obj;
const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);
if (!o || !target_obj)
return AVERROR_OPTION_NOT_FOUND;
if (o->type != type) {
av_log(obj, AV_LOG_ERROR,
"The value for option '%s' is not a %s format.\n", desc, name);
return AVERROR(EINVAL);
}
dst = ((uint8_t*)target_obj) + o->offset;
*out_fmt = *(int *)dst;
return 0;
}
int av_opt_get_pixel_fmt(void *obj, const char *name, int search_flags, enum AVPixelFormat *out_fmt)
{
return get_format(obj, name, search_flags, out_fmt, AV_OPT_TYPE_PIXEL_FMT, "pixel");
}
int av_opt_get_sample_fmt(void *obj, const char *name, int search_flags, enum AVSampleFormat *out_fmt)
{
return get_format(obj, name, search_flags, out_fmt, AV_OPT_TYPE_SAMPLE_FMT, "sample");
}
int av_opt_get_channel_layout(void *obj, const char *name, int search_flags, int64_t *cl)
{
void *dst, *target_obj;
const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);
if (!o || !target_obj)
return AVERROR_OPTION_NOT_FOUND;
if (o->type != AV_OPT_TYPE_CHANNEL_LAYOUT) {
av_log(obj, AV_LOG_ERROR,
"The value for option '%s' is not a channel layout.\n", name);
return AVERROR(EINVAL);
}
dst = ((uint8_t*)target_obj) + o->offset;
*cl = *(int64_t *)dst;
return 0;
}
int av_opt_get_dict_val(void *obj, const char *name, int search_flags, AVDictionary **out_val)
{
void *target_obj;
AVDictionary *src;
const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);
if (!o || !target_obj)
return AVERROR_OPTION_NOT_FOUND;
if (o->type != AV_OPT_TYPE_DICT)
return AVERROR(EINVAL);
src = *(AVDictionary **)(((uint8_t *)target_obj) + o->offset);
av_dict_copy(out_val, src, 0);
return 0;
}这几个函数逻辑个对应的av_opt_set_xxx类似，从代码可以直接看出来。
// 该函数返回AVOption的地址
void *av_opt_ptr(const AVClass *class, void *obj, const char *name)
{
const AVOption *opt= av_opt_find2(&class, name, NULL, 0, AV_OPT_SEARCH_FAKE_OBJ, NULL);
if(!opt)
return NULL;
return (uint8_t*)obj + opt->offset;
}杂项
此类函数可能就是部分工具函数。我们逐个分析下：
int av_opt_flag_is_set(void *obj, const char *field_name, const char *flag_name)
{
// 依次查找各部分，然后确定是否已经设置该flag
const AVOption *field = av_opt_find(obj, field_name, NULL, 0, 0);
const AVOption *flag = av_opt_find(obj, flag_name,
field ? field->unit : NULL, 0, 0);
int64_t res;
if (!field || !flag || flag->type != AV_OPT_TYPE_CONST ||
av_opt_get_int(obj, field_name, 0, &res) < 0)
return 0;
return res & flag->default_val.i64;
}下面几个函数是使用宏统一定义的：
int av_opt_eval_flags (void *obj, const AVOption *o, const char *val, int *flags_out);
int av_opt_eval_int (void *obj, const AVOption *o, const char *val, int *int_out);
int av_opt_eval_int64 (void *obj, const AVOption *o, const char *val, int64_t *int64_out);
int av_opt_eval_float (void *obj, const AVOption *o, const char *val, float *float_out);
int av_opt_eval_double(void *obj, const AVOption *o, const char *val, double *double_out);
int av_opt_eval_q (void *obj, const AVOption *o, const char *val, AVRational *q_out);
#define OPT_EVAL_NUMBER(name, opttype, vartype) \
int av_opt_eval_ ## name(void *obj, const AVOption *o, \
const char *val, vartype *name ## _out) \
{ \
if (!o || o->type != opttype || o->flags & AV_OPT_FLAG_READONLY) \
return AVERROR(EINVAL); \
return set_string_number(obj, obj, o, val, name ## _out); \
}
OPT_EVAL_NUMBER(flags, AV_OPT_TYPE_FLAGS, int)
OPT_EVAL_NUMBER(int, AV_OPT_TYPE_INT, int)
OPT_EVAL_NUMBER(int64, AV_OPT_TYPE_INT64, int64_t)
OPT_EVAL_NUMBER(float, AV_OPT_TYPE_FLOAT, float)
OPT_EVAL_NUMBER(double, AV_OPT_TYPE_DOUBLE, double)
OPT_EVAL_NUMBER(q, AV_OPT_TYPE_RATIONAL, AVRational)
// 最后直接调用set_string_number函数，这个函数在上文中有提及av_opt_copy函数完成AVOption的复制工作。下面看看代码，原则上应该跟初始化函数类似。
int av_opt_copy(void *dst, const void *src)
{
const AVOption *o = NULL;
const AVClass *c;
int ret = 0;
if (!src)
return AVERROR(EINVAL);
c = *(AVClass **)src;
if (!c || c != *(AVClass **)dst)
return AVERROR(EINVAL);
// 遍历+按照类型复制
while ((o = av_opt_next(src, o))) {
void *field_dst = (uint8_t *)dst + o->offset;
void *field_src = (uint8_t *)src + o->offset;
uint8_t **field_dst8 = (uint8_t **)field_dst;
uint8_t **field_src8 = (uint8_t **)field_src;
if (o->type == AV_OPT_TYPE_STRING) {
if (*field_dst8 != *field_src8)
av_freep(field_dst8);
*field_dst8 = av_strdup(*field_src8);
if (*field_src8 && !*field_dst8)
ret = AVERROR(ENOMEM);
} else if (o->type == AV_OPT_TYPE_BINARY) {
int len = *(int *)(field_src8 + 1);
if (*field_dst8 != *field_src8)
av_freep(field_dst8);
*field_dst8 = av_memdup(*field_src8, len);
if (len && !*field_dst8) {
ret = AVERROR(ENOMEM);
len = 0;
}
*(int *)(field_dst8 + 1) = len;
} else if (o->type == AV_OPT_TYPE_CONST) {
// do nothing
} else if (o->type == AV_OPT_TYPE_DICT) {
AVDictionary **sdict = (AVDictionary **) field_src;
AVDictionary **ddict = (AVDictionary **) field_dst;
if (*sdict != *ddict)
av_dict_free(ddict);
*ddict = NULL;
av_dict_copy(ddict, *sdict, 0);
if (av_dict_count(*sdict) != av_dict_count(*ddict))
ret = AVERROR(ENOMEM);
} else {
int size = opt_size(o->type); // 返回特定类型的长度
if (size < 0)
ret = size;
else
memcpy(field_dst, field_src, size);
}
}
return ret;
}
static int opt_size(enum AVOptionType type)
{
switch(type) {
case AV_OPT_TYPE_BOOL:
case AV_OPT_TYPE_INT:
case AV_OPT_TYPE_FLAGS:
return sizeof(int);
case AV_OPT_TYPE_DURATION:
case AV_OPT_TYPE_CHANNEL_LAYOUT:
case AV_OPT_TYPE_INT64:
case AV_OPT_TYPE_UINT64:
return sizeof(int64_t);
case AV_OPT_TYPE_DOUBLE:
return sizeof(double);
case AV_OPT_TYPE_FLOAT:
return sizeof(float);
case AV_OPT_TYPE_STRING:
return sizeof(uint8_t*);
case AV_OPT_TYPE_VIDEO_RATE:
case AV_OPT_TYPE_RATIONAL:
return sizeof(AVRational);
case AV_OPT_TYPE_BINARY:
return sizeof(uint8_t*) + sizeof(int);
case AV_OPT_TYPE_IMAGE_SIZE:
return sizeof(int[2]);
case AV_OPT_TYPE_PIXEL_FMT:
return sizeof(enum AVPixelFormat);
case AV_OPT_TYPE_SAMPLE_FMT:
return sizeof(enum AVSampleFormat);
case AV_OPT_TYPE_COLOR:
return 4;
}
return AVERROR(EINVAL);
}最后三个值是判断是否是默认值、以及序列化输出。
int av_opt_is_set_to_default(void *obj, const AVOption *o)
{
int64_t i64;
double d, d2;
float f;
AVRational q;
int ret, w, h;
char *str;
void *dst;
if (!o || !obj)
return AVERROR(EINVAL);
dst = ((uint8_t*)obj) + o->offset;
// 基本思路是根据类型，判断当前属性值和默认值是否一致。
switch (o->type) {
case AV_OPT_TYPE_CONST:
return 1;
case AV_OPT_TYPE_BOOL:
case AV_OPT_TYPE_FLAGS:
case AV_OPT_TYPE_PIXEL_FMT:
case AV_OPT_TYPE_SAMPLE_FMT:
case AV_OPT_TYPE_INT:
case AV_OPT_TYPE_CHANNEL_LAYOUT:
case AV_OPT_TYPE_DURATION:
case AV_OPT_TYPE_INT64:
case AV_OPT_TYPE_UINT64:
read_number(o, dst, NULL, NULL, &i64); // 数值类型直接读取
return o->default_val.i64 == i64;// 判断默认值是否一致
case AV_OPT_TYPE_STRING:
str = *(char **)dst;
if (str == o->default_val.str) //2 NULLs
return 1;
if (!str || !o->default_val.str) //1 NULL
return 0;
return !strcmp(str, o->default_val.str);
case AV_OPT_TYPE_DOUBLE:
read_number(o, dst, &d, NULL, NULL);
return o->default_val.dbl == d;
case AV_OPT_TYPE_FLOAT:
read_number(o, dst, &d, NULL, NULL);
f = o->default_val.dbl;
d2 = f;
return d2 == d;
case AV_OPT_TYPE_RATIONAL:
q = av_d2q(o->default_val.dbl, INT_MAX);
return !av_cmp_q(*(AVRational*)dst, q);
case AV_OPT_TYPE_BINARY: {
struct {
uint8_t *data;
int size;
} tmp = {0};
int opt_size = *(int *)((void **)dst + 1);
void *opt_ptr = *(void **)dst;
if (!opt_size && (!o->default_val.str || !strlen(o->default_val.str)))
return 1;
if (!opt_size || !o->default_val.str || !strlen(o->default_val.str ))
return 0;
if (opt_size != strlen(o->default_val.str) / 2)
return 0;
ret = set_string_binary(NULL, NULL, o->default_val.str, &tmp.data);
if (!ret)
ret = !memcmp(opt_ptr, tmp.data, tmp.size);
av_free(tmp.data);
return ret;
}
case AV_OPT_TYPE_DICT:
/* Binary and dict have not default support yet. Any pointer is not default. */
return !!(*(void **)dst);
case AV_OPT_TYPE_IMAGE_SIZE:
if (!o->default_val.str || !strcmp(o->default_val.str, "none"))
w = h = 0;
else if ((ret = av_parse_video_size(&w, &h, o->default_val.str)) < 0)
return ret;
return (w == *(int *)dst) && (h == *((int *)dst+1));
case AV_OPT_TYPE_VIDEO_RATE:
q = (AVRational){0, 0};
if (o->default_val.str) {
if ((ret = av_parse_video_rate(&q, o->default_val.str)) < 0)
return ret;
}
return !av_cmp_q(*(AVRational*)dst, q);
case AV_OPT_TYPE_COLOR: {
uint8_t color[4] = {0, 0, 0, 0};
if (o->default_val.str) {
if ((ret = av_parse_color(color, o->default_val.str, -1, NULL)) < 0)
return ret;
}
return !memcmp(color, dst, sizeof(color));
}
default:
av_log(obj, AV_LOG_WARNING, "Not supported option type: %d, option name: %s\n", o->type, o->name);
break;
}
return AVERROR_PATCHWELCOME;
}
int av_opt_is_set_to_default_by_name(void *obj, const char *name, int search_flags)
{
const AVOption *o;
void *target;
if (!obj)
return AVERROR(EINVAL);
// 先查找，然后对比是否是默认值
o = av_opt_find2(obj, name, NULL, 0, search_flags, &target);
if (!o)
return AVERROR_OPTION_NOT_FOUND;
return av_opt_is_set_to_default(target, o);
}
int av_opt_serialize(void *obj, int opt_flags, int flags, char **buffer,
const char key_val_sep, const char pairs_sep)
{
const AVOption *o = NULL;
uint8_t *buf;
AVBPrint bprint;
int ret, cnt = 0;
const char special_chars[] = {pairs_sep, key_val_sep, '\0'};
if (pairs_sep == '\0' || key_val_sep == '\0' || pairs_sep == key_val_sep ||
pairs_sep == '\\' || key_val_sep == '\\') {
av_log(obj, AV_LOG_ERROR, "Invalid separator(s) found.");
return AVERROR(EINVAL);
}
if (!obj || !buffer)
return AVERROR(EINVAL);
*buffer = NULL;
av_bprint_init(&bprint, 64, AV_BPRINT_SIZE_UNLIMITED);
// 序列化的基本思路是遍历
while (o = av_opt_next(obj, o)) {
if (o->type == AV_OPT_TYPE_CONST)
continue;
if ((flags & AV_OPT_SERIALIZE_OPT_FLAGS_EXACT) && o->flags != opt_flags)
continue;
else if (((o->flags & opt_flags) != opt_flags))
continue;
if (flags & AV_OPT_SERIALIZE_SKIP_DEFAULTS && av_opt_is_set_to_default(obj, o) > 0)
continue;
if ((ret = av_opt_get(obj, o->name, 0, &buf)) < 0) {
av_bprint_finalize(&bprint, NULL);
return ret;
}
if (buf) {
if (cnt++)
av_bprint_append_data(&bprint, &pairs_sep, 1);
av_bprint_escape(&bprint, o->name, special_chars, AV_ESCAPE_MODE_BACKSLASH, 0);
av_bprint_append_data(&bprint, &key_val_sep, 1);
av_bprint_escape(&bprint, buf, special_chars, AV_ESCAPE_MODE_BACKSLASH, 0);
av_freep(&buf);
}
}
av_bprint_finalize(&bprint, buffer);
return 0;
}2 AVOption的用法
只要是支持AVOption的对象都可以使用上面提供的接口。支持AVOption的结构体或对象的第一个数据域必须是一个指向AVClass对象的指针，其中该AVClass的对象的option域必须设置为一个以NULL结束的AVOption数组。
通常可以使用av_opt_next()编译一个对象的全部选项，也可以使用av_opt_find()通过指定选项名称查找特定选项。
也可以通过av_opt_child_class_next()和av_opt_child_next()两种不同的遍历子对象属性的方式。
对于读取和设置AVOption可以通过av_opt_set()和av_opt_get，也可以通过更详细的区分的av_opt_set_xxx和av_opt_get_xxx。
3 如何实现支持AVOption的对象
按照之前的AVOption的接口要求，下面给出一个简单的支持AVOption属性的结构体，首先给出一个示例结构体的定义，代码如下：
struct zyj_object {
const AVClass *class;
int int_opt;
char *str_opt;
uint8_t *bin_opt;
int bin_len;
struct child_object * child;
};
static const AVOption zyj_object_options[] = {
{ "example_int", "This is a test option of int type.", offsetof(zyj_object, int_opt),
AV_OPT_TYPE_INT, { .i64 = -1 }, INT_MIN, INT_MAX },
{ "example_str", "This is a test option of string type.", offsetof(zyj_object, str_opt),
AV_OPT_TYPE_STRING },
{ "example_bin", "This is a test option of binary type.", offsetof(zyj_object, bin_opt),
AV_OPT_TYPE_BINARY },
{ NULL },
};
static const AVClass zyj_object_class = {
.class_name = "zyj class",
.item_name = av_default_item_name,
.option = zyj_object_options,
.version = LIBAVUTIL_VERSION_INT,
};这样一个结构体可以使用av_opt_set_defaults()将AVOption所有成员初始化为默认值。下面我们看一下如何创建和销毁zyj_object，代码如下：
zyj_object *alloc_zyj_object(void)
{
zyj_object *ret = av_mallocz(sizeof(*ret));
ret->class = &zyj_object_class;
av_opt_set_defaults(ret);
return ret;
}
void free_zyj_object(zyj_object **foo)
{
av_opt_free(*foo);
av_freep(foo);
}特例一：嵌套AVOption实现
像AVCodecContext一样，其中包含了libavcodec导出的通用选项，同时它的priv_data成员包含了codec相关的选项。在这种情况下，需要设置父结构体来导出子选项。也就是实现在父结构体的AVClass中实现AVClass.child_next()和AVClass.child_class_next()。这里给出一个简单的嵌套示例：
typedef struct child_object {
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)
{
zyj_object *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;
}
// 这个是更新
static const AVClass zyj_object_class = {
.class_name = "zyj class",
.item_name = av_default_item_name,
.option = zyj_object_options,
.child_next = child_next,
.child_class_next = child_class_next,
.version = LIBAVUTIL_VERSION_INT,
};命名常量
AVOption支持命名常量。使用AV_OPT_TYPE_CONST可以声明常量，但是必须给定默认的初始值。示例代码如下：
{ "flag1", "This is a flag with value 16", 0, AV_OPT_TYPE_CONST, { .i64 = 16 }, 0, 0, "test_unit" },4 小结
本文可能涉及比较多的源代码，是对libavutil/opt.c的整理及摘取，通过梳理相关实现代码，让我基本理解AVOption的实现逻辑，值得后续更深入的使用作为参考。