标签：ffmpeg avoption avclass 源代码
本文继续上篇文章《FFmpeg源代码分析：结构体成员管理系统-AVClass》中的内容，记录FFmpeg中和AVOption相关的源代码。AVOption用于在FFmpeg中描述结构体中的成员变量。一个AVOption可以包含名称，简短的帮助信息，取值等等。
上篇文章中概括了AVClass，AVOption和目标结构体之间的关系。以AVFormatContext为例，可以表示为下图。
有关上篇文章的内容，这里不再重复。总体来说，上篇文章主要概括了AVClass，AVOption和目标结构体之间的从属关系，但是并没有分析有关AVOption的源代码。本文补充上一篇文章的内容，分析有关AVOption的源代码。
AVOption常用的API可以分成两类：用于设置参数的API和用于读取参数的API。其中最有代表性的用于设置参数的API就是av_opt_set()；而最有代表性的用于读取参数的API就是av_opt_get()。除了记录以上两个函数之外，本文再记录一个在FFmpeg的结构体初始化代码中最常用的用于设置默认值的函数av_opt_set_defaults()。
av_opt_set()的函数调用关系图如下所示。
av_opt_set_defaults()的函数调用关系图如下所示。
/**
* @defgroup opt_set_funcs Option setting functions
* @{
* Those functions set the field of obj with the given name to value.
*
* @param[in] obj A struct whose first element is a pointer to an AVClass.
* @param[in] name the name of the field to set
* @param[in] val The value to set. In case of av_opt_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.
* @param search_flags flags passed to av_opt_find2. I.e. if AV_OPT_SEARCH_CHILDREN
* is passed here, then the option may be set on a child of obj.
*
* @return 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
*/
int av_opt_set (void *obj, const char *name, const char *val, int search_flags);
int av_opt_set_int (void *obj, const char *name, int64_t val, int search_flags);
int av_opt_set_double (void *obj, const char *name, double val, int search_flags);
int av_opt_set_q (void *obj, const char *name, AVRational val, int search_flags);
int av_opt_set_bin (void *obj, const char *name, const uint8_t *val, int size, int search_flags);
int av_opt_set_image_size(void *obj, const char *name, int w, int h, int search_flags);
int av_opt_set_pixel_fmt (void *obj, const char *name, enum AVPixelFormat fmt, int search_flags);
int av_opt_set_sample_fmt(void *obj, const char *name, enum AVSampleFormat fmt, int search_flags);
int av_opt_set_video_rate(void *obj, const char *name, AVRational val, int search_flags);
int av_opt_set_channel_layout(void *obj, const char *name, int64_t ch_layout, int search_flags);int av_opt_set(void *obj, const char *name, const char *val, int search_flags)
{
int ret = 0;
void *dst, *target_obj;
//查找
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))
return AVERROR(EINVAL);
if (o->flags & AV_OPT_FLAG_READONLY)
return AVERROR(EINVAL);
//dst指向具体的变量
//注意：offset的作用
dst = ((uint8_t*)target_obj) + o->offset;
//根据AVOption不同的类型，调用不同的设置函数
switch (o->type) {
case AV_OPT_TYPE_STRING: return set_string(obj, o, val, dst);
case AV_OPT_TYPE_BINARY: return set_string_binary(obj, o, val, dst);
case AV_OPT_TYPE_FLAGS:
case AV_OPT_TYPE_INT:
case AV_OPT_TYPE_INT64:
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: return set_string_video_rate(obj, o, val, dst);
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 {
#if FF_API_GET_CHANNEL_LAYOUT_COMPAT
int64_t cl = ff_get_channel_layout(val, 0);
#else
int64_t cl = av_get_channel_layout(val);
#endif
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);
}
av_opt_find2()本身也是一个API函数，用于查找AVOption。它的声明位于libavutil\opt.h中，如下所示。
/**
* Look for an option in an object. Consider only options which
* have all the specified flags set.
*
* @param[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.
* @param[in] name The name of the option to look for.
* @param[in] unit When searching for named constants, name of the unit
* it belongs to.
* @param opt_flags Find only options with all the specified flags set (AV_OPT_FLAG).
* @param search_flags A combination of AV_OPT_SEARCH_*.
* @param[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.
*
* @return A pointer to the option found, or NULL if no option
* was found.
*/
const AVOption *av_opt_find2(void *obj, const char *name, const char *unit,
int opt_flags, int search_flags, void **target_obj);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;
//查找范围包含子节点的时候
//递归调用av_opt_find2()
if (search_flags & AV_OPT_SEARCH_CHILDREN) {
if (search_flags & AV_OPT_SEARCH_FAKE_OBJ) {
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
while (o = av_opt_next(obj, o)) {
//比较名称
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;
}
av_opt_next()也是一个FFmpeg的API函数。使用它可以循环遍历目标结构体的所有AVOption，它的声明如下。
/** * Iterate over all AVOptions belonging to obj. * * @param obj an AVOptions-enabled struct or a double pointer to an * AVClass describing it. * @param prev result of the previous call to av_opt_next() on this object * or NULL * @return next AVOption or NULL */ const AVOption *av_opt_next(void *obj, const AVOption *prev);av_opt_next()的定义如下所示。
const AVOption *av_opt_next(void *obj, const AVOption *last)
{
AVClass *class = *(AVClass**)obj;
if (!last && class && class->option && class->option[0].name)
return class->option;
if (last && last[1].name)
return ++last;
return NULL;
} 从av_opt_next()的代码可以看出，输入的AVOption类型的last变量为空的时候，会返回该AVClass的option数组的第一个元素，否则会返回数组的下一个元素。
1. AV_OPT_TYPE_STRING
当AVOption的type为AV_OPT_TYPE_STRING的时候，调用set_string()方法设置相应的值。set_string()的定义如下：
static int set_string(void *obj, const AVOption *o, const char *val, uint8_t **dst)
{
av_freep(dst);
*dst = av_strdup(val);
return 0;
} 其中又调用了一个函数av_strdup()，这是一个FFmpeg的API函数，用于拷贝字符串。它的代码如下所示，其中调用了memcpy()。char *av_strdup(const char *s)
{
char *ptr = NULL;
if (s) {
int len = strlen(s) + 1;
ptr = av_realloc(NULL, len);
if (ptr)
memcpy(ptr, s, len);
}
return ptr;
}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_parse_video_size()是一个FFmpeg的API函数，用于解析出输入的分辨率字符串的宽高信息。例如，输入的字符串为“1920x1080”或者“1920*1080”，经过av_parse_video_size()的处理之后，可以得到宽度为1920，高度为1080；此外，输入一个“特定分辨率”字符串例如“vga”，也可以得到宽度为640，高度为480。该函数不属于AVOption这部分的内容，而是整个FFmpeg通用的一个字符串解析函数。声明位于libavutil\parseutils.h中，如下所示。
/** * Parse str and put in width_ptr and height_ptr the detected values. * * @param[in,out] width_ptr pointer to the variable which will contain the detected * width value * @param[in,out] height_ptr pointer to the variable which will contain the detected * height value * @param[in] str the string to parse: it has to be a string in the format * width x height or a valid video size abbreviation. * @return >= 0 on success, a negative error code otherwise */ int av_parse_video_size(int *width_ptr, int *height_ptr, const char *str);
//解析分辨率
int av_parse_video_size(int *width_ptr, int *height_ptr, const char *str)
{
int i;
int n = FF_ARRAY_ELEMS(video_size_abbrs);
const char *p;
int width = 0, height = 0;
//先看看有没有“分辨率简称”相同的（例如vga，qcif等）
for (i = 0; i < n; i++) {
if (!strcmp(video_size_abbrs[i].abbr, str)) {
width = video_size_abbrs[i].width;
height = video_size_abbrs[i].height;
break;
}
}
//如果没有使用“分辨率简称”，而是使用具体的数值（例如“1920x1080”），则执行下面的步骤
if (i == n) {
//strtol()：字符串转换成整型，遇到非数字则停止
width = strtol(str, (void*)&p, 10);
if (*p)
p++;
height = strtol(p, (void*)&p, 10);
/* trailing extraneous data detected, like in 123x345foobar */
if (*p)
return AVERROR(EINVAL);
}
//检查一下正确性
if (width <= 0 || height <= 0)
return AVERROR(EINVAL);
*width_ptr = width;
*height_ptr = height;
return 0;
}
typedef struct {
const char *abbr;
int width, height;
} VideoSizeAbbr;
static const VideoSizeAbbr video_size_abbrs[] = {
{ "ntsc", 720, 480 },
{ "pal", 720, 576 },
{ "qntsc", 352, 240 }, /* VCD compliant NTSC */
{ "qpal", 352, 288 }, /* VCD compliant PAL */
{ "sntsc", 640, 480 }, /* square pixel NTSC */
{ "spal", 768, 576 }, /* square pixel PAL */
{ "film", 352, 240 },
{ "ntsc-film", 352, 240 },
{ "sqcif", 128, 96 },
{ "qcif", 176, 144 },
{ "cif", 352, 288 },
{ "4cif", 704, 576 },
{ "16cif", 1408,1152 },
{ "qqvga", 160, 120 },
{ "qvga", 320, 240 },
{ "vga", 640, 480 },
{ "svga", 800, 600 },
{ "xga", 1024, 768 },
{ "uxga", 1600,1200 },
{ "qxga", 2048,1536 },
{ "sxga", 1280,1024 },
{ "qsxga", 2560,2048 },
{ "hsxga", 5120,4096 },
{ "wvga", 852, 480 },
{ "wxga", 1366, 768 },
{ "wsxga", 1600,1024 },
{ "wuxga", 1920,1200 },
{ "woxga", 2560,1600 },
{ "wqsxga", 3200,2048 },
{ "wquxga", 3840,2400 },
{ "whsxga", 6400,4096 },
{ "whuxga", 7680,4800 },
{ "cga", 320, 200 },
{ "ega", 640, 350 },
{ "hd480", 852, 480 },
{ "hd720", 1280, 720 },
{ "hd1080", 1920,1080 },
{ "2k", 2048,1080 }, /* Digital Cinema System Specification */
{ "2kflat", 1998,1080 },
{ "2kscope", 2048, 858 },
{ "4k", 4096,2160 }, /* Digital Cinema System Specification */
{ "4kflat", 3996,2160 },
{ "4kscope", 4096,1716 },
{ "nhd", 640,360 },
{ "hqvga", 240,160 },
{ "wqvga", 400,240 },
{ "fwqvga", 432,240 },
{ "hvga", 480,320 },
{ "qhd", 960,540 },
};
av_opt_set_defaults()是一个FFmpeg的API，作用是给一个结构体的成员变量设定默认值。在FFmpeg初始化其各种结构体（AVFormatContext，AVCodecContext等）的时候，通常会调用该函数设置结构体中的默认值。av_opt_set_defaults()的声明如下所示。
/** * Set the values of all AVOption fields to their default values. * * @param s an AVOption-enabled struct (its first member must be a pointer to AVClass) */ void av_opt_set_defaults(void *s);
void av_opt_set_defaults(void *s)
{
//奇怪的#if...#endif
#if FF_API_OLD_AVOPTIONS
av_opt_set_defaults2(s, 0, 0);
}
void av_opt_set_defaults2(void *s, int mask, int flags)
{
#endif
const AVOption *opt = NULL;
//遍历所有的AVOption
while ((opt = av_opt_next(s, opt))) {
//注意：offset的使用
void *dst = ((uint8_t*)s) + opt->offset;
#if FF_API_OLD_AVOPTIONS
if ((opt->flags & mask) != flags)
continue;
#endif
if (opt->flags & AV_OPT_FLAG_READONLY)
continue;
//读取各种default_val
switch (opt->type) {
case AV_OPT_TYPE_CONST:
/* Nothing to be done here */
break;
case AV_OPT_TYPE_FLAGS:
case AV_OPT_TYPE_INT:
case AV_OPT_TYPE_INT64:
case AV_OPT_TYPE_DURATION:
case AV_OPT_TYPE_CHANNEL_LAYOUT:
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_PIXEL_FMT:
write_number(s, opt, dst, 1, 1, opt->default_val.i64);
break;
case AV_OPT_TYPE_SAMPLE_FMT:
write_number(s, opt, dst, 1, 1, opt->default_val.i64);
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);
}
}
}
const AVOption *av_opt_next(void *obj, const AVOption *last)
{
AVClass *class = *(AVClass**)obj;
if (!last && class && class->option && class->option[0].name)
return class->option;
if (last && last[1].name)
return ++last;
return NULL;
} 从av_opt_next()的代码可以看出，输入的AVOption类型的last为空的时候，会返回该AVClass的option数组的第一个元素，否则会返回下一个元素。/**
* @defgroup opt_get_funcs Option getting functions
* @{
* Those functions get a value of the option with the given name from an object.
*
* @param[in] obj a struct whose first element is a pointer to an AVClass.
* @param[in] name name of the option to get.
* @param[in] search_flags flags passed to av_opt_find2. I.e. if AV_OPT_SEARCH_CHILDREN
* is passed here, then the option may be found in a child of obj.
* @param[out] out_val value of the option will be written here
* @return >=0 on success, a negative error code otherwise
*/
/**
* @note the returned string will be av_malloc()ed and must be av_free()ed by the caller
*/
int av_opt_get (void *obj, const char *name, int search_flags, uint8_t **out_val);
int av_opt_get_int (void *obj, const char *name, int search_flags, int64_t *out_val);
int av_opt_get_double (void *obj, const char *name, int search_flags, double *out_val);
int av_opt_get_q (void *obj, const char *name, int search_flags, AVRational *out_val);
int av_opt_get_image_size(void *obj, const char *name, int search_flags, int *w_out, int *h_out);
int av_opt_get_pixel_fmt (void *obj, const char *name, int search_flags, enum AVPixelFormat *out_fmt);
int av_opt_get_sample_fmt(void *obj, const char *name, int search_flags, enum AVSampleFormat *out_fmt);
int av_opt_get_video_rate(void *obj, const char *name, int search_flags, AVRational *out_val);
int av_opt_get_channel_layout(void *obj, const char *name, int search_flags, int64_t *ch_layout);int av_opt_get(void *obj, const char *name, int search_flags, uint8_t **out_val)
{
void *dst, *target_obj;
//查找
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;
//注意：offset的使用
dst = (uint8_t*)target_obj + o->offset;
//使用sprintf()转换成字符串，存入buf
buf[0] = 0;
switch (o->type) {
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_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:
if (*(uint8_t**)dst)
*out_val = av_strdup(*(uint8_t**)dst);
else
*out_val = av_strdup("");
return 0;
case AV_OPT_TYPE_BINARY:
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:
//分辨率
ret = snprintf(buf, sizeof(buf), "%dx%d", ((int *)dst)[0], ((int *)dst)[1]);
break;
case AV_OPT_TYPE_PIXEL_FMT:
//像素格式
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:
//采样格式
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:
//时长
i64 = *(int64_t *)dst;
ret = snprintf(buf, sizeof(buf), "%"PRIi64":%02d:%02d.%06d",
i64 / 3600000000, (int)((i64 / 60000000) % 60),
(int)((i64 / 1000000) % 60), (int)(i64 % 1000000));
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);
//拷贝
*out_val = av_strdup(buf);
return 0;
}
FFmpeg源代码分析：结构体成员管理系统-AVOption
标签：ffmpeg avoption avclass 源代码
原文地址：http://blog.csdn.net/leixiaohua1020/article/details/44279329
