RTMPdump source code analysis: RTMP_Connect establishes a network connection

This article analyzes the function calls in the process of establishing a network connection in RTMPdump.

The function call process is as follows :
RTMP_Connect
　　-> add_addr_info: fill the struct sockaddr_in structure for subsequent socket communication
　　-> RTMP_Connect0: mainly used to establish a Socket connection, but did not start to establish a real RTMP connection
　　-> RTMP_Connect1: this is a real RTMP connection Function
　　　　-> HandShake: handshake
　　　　-> SendConnectPacket: the function to establish a connection

RTMP_Connect：

bool  RTMP_Connect(RTMP *r, RTMPPacket *cp)
{
	struct sockaddr_in service;
	if (!r->Link.hostname.av_len)
		return false;

	memset(&service, 0, sizeof(struct sockaddr_in));
	service.sin_family = AF_INET;

	if (r->Link.socksport)
	{
		/* Connect via SOCKS */
		if (!add_addr_info(&service, &r->Link.sockshost, r->Link.socksport))
			return false;
	}
	else
	{
		/* zx 填充struct sockaddr_in结构体用于之后的socket通信 */
		if (!add_addr_info(&service, &r->Link.hostname, r->Link.port))
			return false;
	}

	/* zx 一般先把sockaddr_in变量赋值后，强制类型转换后传入用sockaddr做参数的函数 */
	if (!RTMP_Connect0(r, (struct sockaddr *)&service))
		return false;

	r->m_bSendCounter = true;

	return RTMP_Connect1(r, cp);
}

add_addr_info function:　fill the struct sockaddr_in structure for subsequent socket communication

static bool
add_addr_info(struct sockaddr_in *service, AVal *host, int port)
{
	char *hostname;
	bool ret = true;
	if (host->av_val[host->av_len])
	{
			hostname = malloc(host->av_len+1);
			memcpy(hostname, host->av_val, host->av_len);
			hostname[host->av_len] = '\0';
	}
	else
	{
		hostname = host->av_val;
	}

	/* zx inet_addr函数将一个IP字符串转化为一个网络字节序的整数值 */
	service->sin_addr.s_addr = inet_addr(hostname);
	if (service->sin_addr.s_addr == INADDR_NONE)		/* zx 传入的字符串不是一个合法的Internet地址就返回INADDR_NONE */
	{
		struct hostent *host = gethostbyname(hostname);		/* zx gethostbyname函数将主机名转化为网络字节序的整数ip值 */
		if (host == NULL || host->h_addr == NULL)
		{
			RTMP_Log(RTMP_LOGERROR, "Problem accessing the DNS. (addr: %s)", hostname);
			ret = false;
			goto finish;
		}
		service->sin_addr = *(struct in_addr *)host->h_addr;
	}

	/* zx 端口号的主机字节序转化为网络字节序 */
	service->sin_port = htons(port);
finish:
	if (hostname != host->av_val)
		free(hostname);
	return ret;
}

RTMP_Connect0() is mainly used to establish a Socket connection, but does not actually establish an RTMP connection:

bool  RTMP_Connect0(RTMP *r, struct sockaddr * service)
{
	int on = 1;
	r->m_sb.sb_timedout = false;
	r->m_pausing = 0;
	r->m_fDuration = 0.0;

	r->m_sb.sb_socket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
	if (r->m_sb.sb_socket != -1)
	{
		/* zx 客户端调用connect与服务端创建连接 */
		if (connect(r->m_sb.sb_socket, service, sizeof(struct sockaddr)) < 0)
		{
			int err = GetSockError();
			RTMP_Log(RTMP_LOGERROR, "%s, failed to connect socket. %d (%s)", __FUNCTION__, err, strerror(err));
			RTMP_Close(r);
			return false;
		}

		if (r->Link.socksport)
		{
			RTMP_Log(RTMP_LOGDEBUG, "%s ... SOCKS negotiation", __FUNCTION__);
			if (!SocksNegotiate(r))
			{
				RTMP_Log(RTMP_LOGERROR, "%s, SOCKS negotiation failed.", __FUNCTION__);
				RTMP_Close(r);
				return false;
			}
		}
	}
	else
	{
		RTMP_Log(RTMP_LOGERROR, "%s, failed to create socket. Error: %d", __FUNCTION__,
		GetSockError());
		return false;
	}

	/* set timeout */
	{
		SET_RCVTIMEO(tv, r->Link.timeout);
		/* zx 设置接收超时时间 */
		if (setsockopt(r->m_sb.sb_socket, SOL_SOCKET, SO_RCVTIMEO, (char *)&tv, sizeof(tv)))
		{
			RTMP_Log(RTMP_LOGERROR, "%s, Setting socket timeout to %ds failed!", __FUNCTION__, r->Link.timeout);
		}
	}

	/* zx 禁止nagle算法 */
	setsockopt(r->m_sb.sb_socket, IPPROTO_TCP, TCP_NODELAY, (char *) &on, sizeof(on));

	return true;
}

RTMP_Connect1() , this is the function that really establishes RTMP connection:

bool  RTMP_Connect1(RTMP *r, RTMPPacket *cp)
{
	if (r->Link.protocol & RTMP_FEATURE_SSL)
	{
#if defined(CRYPTO) && !defined(NO_SSL)
		TLS_client(RTMP_TLS_ctx, r->m_sb.sb_ssl);
		TLS_setfd(r->m_sb.sb_ssl, r->m_sb.sb_socket);
		if (TLS_connect(r->m_sb.sb_ssl) < 0)
		{
			RTMP_Log(RTMP_LOGERROR, "%s, TLS_Connect failed", __FUNCTION__);
			RTMP_Close(r);
			return false;
		}
#else
		RTMP_Log(RTMP_LOGERROR, "%s, no SSL/TLS support", __FUNCTION__);
		RTMP_Close(r);
		return false;

#endif
	}
	if (r->Link.protocol & RTMP_FEATURE_HTTP)
	{
		r->m_msgCounter = 1;
		r->m_clientID.av_val = NULL;
		r->m_clientID.av_len = 0;
		HTTP_Post(r, RTMPT_OPEN, "", 1);
		HTTP_read(r, 1);
		r->m_msgCounter = 0;
	}
	RTMP_Log(RTMP_LOGDEBUG, "%s, ... connected, handshaking", __FUNCTION__);
	if (!HandShake(r, true))	/* zx 握手 */
	{
		RTMP_Log(RTMP_LOGERROR, "%s, handshake failed.", __FUNCTION__);
		RTMP_Close(r);
		return false;
	}
	RTMP_Log(RTMP_LOGDEBUG, "%s, handshaked", __FUNCTION__);

	if (!SendConnectPacket(r, cp))		/* zx 真正建立网络连接 */
	{
		RTMP_Log(RTMP_LOGERROR, "%s, RTMP connect failed.", __FUNCTION__);
		RTMP_Close(r);
		return false;
	}
	return true;
}