[ffmpeg] av_opt_set 解析

背景

ffmpeg 创建编码器的时候，可以设置一些额外参数。可以通过两种方式，一是 av_opt_set，另一种是 avcodec_open2 最后一个参数传入。今天先分析一下第一种是什么设置进去的。

具体代码分析

av_opt_set 主要分成两部分，一个是在 obj 结构体中，找到 name 的地址。第二个是根据找到地址的类型， 解析 val，并将 val 赋值到该地址。

int av_opt_set(void *obj, const char *name, const char *val, int search_flags)
{int ret = 0;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;
FF_DISABLE_DEPRECATION_WARNINGSif (!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_DURATION && o->type != AV_OPT_TYPE_COLOR &&
#if FF_API_OLD_CHANNEL_LAYOUTo->type != AV_OPT_TYPE_CHANNEL_LAYOUT &&
#endifo->type != AV_OPT_TYPE_BOOL))return AVERROR(EINVAL);
FF_ENABLE_DEPRECATION_WARNINGSif (o->flags & AV_OPT_FLAG_READONLY)return AVERROR(EINVAL);if (o->flags & AV_OPT_FLAG_DEPRECATED)av_log(obj, AV_LOG_WARNING, "The \"%s\" option is deprecated: %s\n", name, o->help);dst = ((uint8_t *)target_obj) + o->offset;switch (o->type) {case AV_OPT_TYPE_BOOL:return set_string_bool(obj, o, val, dst);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_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:{int64_t usecs = 0;if (val) {if ((ret = av_parse_time(&usecs, val, 1)) < 0) {av_log(obj, AV_LOG_ERROR, "Unable to parse option value \"%s\" as duration\n", val);return ret;}}if (usecs < o->min || usecs > o->max) {av_log(obj, AV_LOG_ERROR, "Value %f for parameter '%s' out of range [%g - %g]\n",usecs / 1000000.0, o->name, o->min / 1000000.0, o->max / 1000000.0);return AVERROR(ERANGE);}*(int64_t *)dst = usecs;return 0;}case AV_OPT_TYPE_COLOR:return set_string_color(obj, o, val, dst);
#if FF_API_OLD_CHANNEL_LAYOUT
FF_DISABLE_DEPRECATION_WARNINGScase 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;
FF_ENABLE_DEPRECATION_WARNINGS
#endifcase AV_OPT_TYPE_CHLAYOUT:ret = set_string_channel_layout(obj, o, val, dst);if (ret < 0) {av_log(obj, AV_LOG_ERROR, "Unable to parse option value \"%s\" as channel layout\n", val);ret = AVERROR(EINVAL);}return ret;case AV_OPT_TYPE_DICT:return set_string_dict(obj, o, val, dst);}av_log(obj, AV_LOG_ERROR, "Invalid option type.\n");return AVERROR(EINVAL);
}

找到 name 的地址

主要关注 av_opt_next 函数，第一次进去返回 option 地址，之后进去每次往后移动一个 option。并将获得的 option name 与传入的 name 进行比较。

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;// 非必要，先不看if (search_flags & AV_OPT_SEARCH_CHILDREN) {if (search_flags & AV_OPT_SEARCH_FAKE_OBJ) {void *iter = NULL;const AVClass *child;while (child = av_opt_child_class_iterate(c, &iter))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;}}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;
}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;
}

以 aacenc.c 中的 aacenc_options 进行分析，可以注意到最后一个 option 是 NULL，和codec_list 一样的手法。

static const AVOption aacenc_options[] = {{"aac_coder", "Coding algorithm", offsetof(AACEncContext, options.coder), AV_OPT_TYPE_INT, {.i64 = AAC_CODER_TWOLOOP}, 0, AAC_CODER_NB-1, AACENC_FLAGS, "coder"},{"anmr",     "ANMR method",               0, AV_OPT_TYPE_CONST, {.i64 = AAC_CODER_ANMR},    INT_MIN, INT_MAX, AACENC_FLAGS, "coder"},{"twoloop",  "Two loop searching method", 0, AV_OPT_TYPE_CONST, {.i64 = AAC_CODER_TWOLOOP}, INT_MIN, INT_MAX, AACENC_FLAGS, "coder"},{"fast",     "Default fast search",       0, AV_OPT_TYPE_CONST, {.i64 = AAC_CODER_FAST},    INT_MIN, INT_MAX, AACENC_FLAGS, "coder"},{"aac_ms", "Force M/S stereo coding", offsetof(AACEncContext, options.mid_side), AV_OPT_TYPE_BOOL, {.i64 = -1}, -1, 1, AACENC_FLAGS},{"aac_is", "Intensity stereo coding", offsetof(AACEncContext, options.intensity_stereo), AV_OPT_TYPE_BOOL, {.i64 = 1}, -1, 1, AACENC_FLAGS},{"aac_pns", "Perceptual noise substitution", offsetof(AACEncContext, options.pns), AV_OPT_TYPE_BOOL, {.i64 = 1}, -1, 1, AACENC_FLAGS},{"aac_tns", "Temporal noise shaping", offsetof(AACEncContext, options.tns), AV_OPT_TYPE_BOOL, {.i64 = 1}, -1, 1, AACENC_FLAGS},{"aac_ltp", "Long term prediction", offsetof(AACEncContext, options.ltp), AV_OPT_TYPE_BOOL, {.i64 = 0}, -1, 1, AACENC_FLAGS},{"aac_pred", "AAC-Main prediction", offsetof(AACEncContext, options.pred), AV_OPT_TYPE_BOOL, {.i64 = 0}, -1, 1, AACENC_FLAGS},{"aac_pce", "Forces the use of PCEs", offsetof(AACEncContext, options.pce), AV_OPT_TYPE_BOOL, {.i64 = 0}, -1, 1, AACENC_FLAGS},FF_AAC_PROFILE_OPTS{NULL}
};typedef struct AVOption {const char *name;/*** short English help text* @todo What about other languages?*/const char *help;/*** The offset relative to the context structure where the option* value is stored. It should be 0 for named constants.*/int offset;enum AVOptionType type;/*** the default value for scalar options*/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 optiondouble max;                 ///< maximum valid value for the optionint flags;
#define AV_OPT_FLAG_ENCODING_PARAM  1   ///< a generic parameter which can be set by the user for muxing or encoding
#define AV_OPT_FLAG_DECODING_PARAM  2   ///< a generic parameter which can be set by the user for demuxing or decoding
#define AV_OPT_FLAG_AUDIO_PARAM     8
#define AV_OPT_FLAG_VIDEO_PARAM     16
#define AV_OPT_FLAG_SUBTITLE_PARAM  32
/*** The option is intended for exporting values to the caller.*/
#define AV_OPT_FLAG_EXPORT          64
/*** The option may not be set through the AVOptions API, only read.* This flag only makes sense when AV_OPT_FLAG_EXPORT is also set.*/
#define AV_OPT_FLAG_READONLY        128
#define AV_OPT_FLAG_BSF_PARAM       (1<<8) ///< a generic parameter which can be set by the user for bit stream filtering
#define AV_OPT_FLAG_RUNTIME_PARAM   (1<<15) ///< a generic parameter which can be set by the user at runtime
#define AV_OPT_FLAG_FILTERING_PARAM (1<<16) ///< a generic parameter which can be set by the user for filtering
#define AV_OPT_FLAG_DEPRECATED      (1<<17) ///< set if option is deprecated, users should refer to AVOption.help text for more information
#define AV_OPT_FLAG_CHILD_CONSTS    (1<<18) ///< set if option constants can also reside in child objects
//FIXME think about enc-audio, ... style flags/*** The logical unit to which the option belongs. Non-constant* options and corresponding named constants share the same* unit. May be NULL.*/const char *unit;
} AVOption;

根据找到地址的类型， 解析 val，并将 val 赋值到该地址

以 set_string_bool 举例。可以看见如果传入 auto，则会赋值 -1；true,y,yes,enable,enabled,on 赋值 1，false,n,no,disable,disabled,off 为0。
其他类型的赋值，大家可以去看具体的函数。

static int set_string_bool(void *obj, const AVOption *o, const char *val, int *dst)
{int n;if (!val)return 0;if (!strcmp(val, "auto")) {n = -1;} else if (av_match_name(val, "true,y,yes,enable,enabled,on")) {n = 1;} else if (av_match_name(val, "false,n,no,disable,disabled,off")) {n = 0;} else {char *end = NULL;n = strtol(val, &end, 10);if (val + strlen(val) != end)goto fail;}if (n < o->min || n > o->max)goto fail;*dst = n;return 0;fail:av_log(obj, AV_LOG_ERROR, "Unable to parse option value \"%s\" as boolean\n", val);return AVERROR(EINVAL);
}

寻找地址

target_obj 就是 av_opt_set 传入的地址， offset 是每个 option，在编码器结构体中的偏移量。 offsetof(AACEncContext, options.coder)

dst = ((uint8_t *)target_obj) + o->offset;

#define offsetof(s,m) ((size_t)&(((s*)0)->m))