标准rtsp协议的基石是tcp，本节探索下ffmpeg的rtsp拉流协议tcp的socket创建及链接。

1.tcp创建及链接的流程图及对象图

tcp创建及链接的流程图，如下：

tcp创建及链接的对象图，如下：

2.解析

ffmpeg拉流，从avformat_open_input接口开始，去除与rtsp拉流无关的代码后，如下：

int avformat_open_input(AVFormatContext **ps, const char *filename,const AVInputFormat *fmt, AVDictionary **options)
{AVFormatContext *s = *ps;FFFormatContext *si;AVDictionary *tmp = NULL;int ret = 0;if (!s && !(s = avformat_alloc_context()))return AVERROR(ENOMEM);si = ffformatcontext(s);if (!s->av_class) {av_log(NULL, AV_LOG_ERROR, "Input context has not been properly allocated by avformat_alloc_context() and is not NULL either\n");return AVERROR(EINVAL);}if (options)av_dict_copy(&tmp, *options, 0);if ((ret = av_opt_set_dict(s, &tmp)) < 0)goto fail;if (!(s->url = av_strdup(filename ? filename : ""))) {ret = AVERROR(ENOMEM);goto fail;}if ((ret = init_input(s, filename, &tmp)) < 0)goto fail;s->probe_score = ret;if (s->format_whitelist && av_match_list(s->iformat->name, s->format_whitelist, ',') <= 0) {av_log(s, AV_LOG_ERROR, "Format not on whitelist \'%s\'\n", s->format_whitelist);ret = AVERROR(EINVAL);goto fail;}/* Check filename in case an image number is expected. */if (s->iformat->flags & AVFMT_NEEDNUMBER) {if (!av_filename_number_test(filename)) {ret = AVERROR(EINVAL);goto fail;}}s->duration = s->start_time = AV_NOPTS_VALUE;/* Allocate private data. */if (s->iformat->priv_data_size > 0) {if (!(s->priv_data = av_mallocz(s->iformat->priv_data_size))) {ret = AVERROR(ENOMEM);goto fail;}if (s->iformat->priv_class) {*(const AVClass **) s->priv_data = s->iformat->priv_class;av_opt_set_defaults(s->priv_data);if ((ret = av_opt_set_dict(s->priv_data, &tmp)) < 0)goto fail;}}if (s->iformat->read_header){if ((ret = s->iformat->read_header(s)) < 0) {if (s->iformat->flags_internal & FF_FMT_INIT_CLEANUP)goto close;goto fail;}}if ((ret = avformat_queue_attached_pictures(s)) < 0)goto close;si->raw_packet_buffer_size = 0;update_stream_avctx(s);if (options) {av_dict_free(options);*options = tmp;}*ps = s;return 0;close:if (s->iformat->read_close)s->iformat->read_close(s);
fail:av_dict_free(&tmp);avformat_free_context(s);*ps = NULL;return ret;
}

其中留下了字典参数配置流程，因为基本会有所设置，参数配置参见《ffmpeg面向对象——参数配置机制及其设计模式探索》。

流程图中，输入格式匹配参见《ffmpeg面向对象-rtsp拉流相关对象》的第2节。
协议匹配机制参见《ffmpeg面向对象——拉流协议匹配机制探索》。
输入格式类与协议类什么关系参见《ffmpeg面向对象——AVInputFormat与URLProtocol啥关系》