[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_WARNINGS
    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_DURATION && o->type != AV_OPT_TYPE_COLOR &&
#if FF_API_OLD_CHANNEL_LAYOUT
                 o->type != AV_OPT_TYPE_CHANNEL_LAYOUT &&
#endif
                 o->type != AV_OPT_TYPE_BOOL))
        return AVERROR(EINVAL);
FF_ENABLE_DEPRECATION_WARNINGS

    if (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_WARNINGS
    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;
FF_ENABLE_DEPRECATION_WARNINGS
#endif
    case 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 option
    double max;                 ///< maximum valid value for the option

    int 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))