RTMPdump（libRTMP） 源代码分析 9： 接收消息（Message）（接收视音频数据）(七)
Read_1_Packet()里面实现从网络中读取视音频数据的函数是RTMP_GetNextMediaPacket()。下面我们来看看该函数的源代码：
[cpp]
int
RTMP_GetNextMediaPacket(RTMP *r, RTMPPacket *packet)
{
int bHasMediaPacket = 0;
while (!bHasMediaPacket && RTMP_IsConnected(r)
&& RTMP_ReadPacket(r, packet))
{
if (!RTMPPacket_IsReady(packet))
{
continue;
}
bHasMediaPacket = RTMP_ClientPacket(r, packet);
if (!bHasMediaPacket)
{
RTMPPacket_Free(packet);
}
else if (r->m_pausing == 3)
{
if (packet->m_nTimeStamp <= r->m_mediaStamp)
{
bHasMediaPacket = 0;
#ifdef _DEBUG
RTMP_Log(RTMP_LOGDEBUG,
"Skipped type: %02X, size: %d, TS: %d ms, abs TS: %d, pause: %d ms",
packet->m_packetType, packet->m_nBodySize,
packet->m_nTimeStamp, packet->m_hasAbsTimestamp,
r->m_mediaStamp);
#endif
continue;
}
r->m_pausing = 0;
}
}
if (bHasMediaPacket)
r->m_bPlaying = TRUE;
else if (r->m_sb.sb_timedout && !r->m_pausing)
r->m_pauseStamp = r->m_channelTimestamp[r->m_mediaChannel];
return bHasMediaPacket;
}
这里有两个函数比较重要：RTMP_ReadPacket()以及RTMP_ClientPacket()。这两个函数中，前一个函数负责从网络上读取数据，后一个负责处理数据。这部分与建立RTMP连接的网络流（NetStream）的时候很相似，参考：RTMPdump（libRTMP） 源代码分析 6： 建立一个流媒体连接 （NetStream部分 1）
RTMP_ClientPacket()在前文中已经做过分析，在此不再重复叙述。在这里重点分析一下RTMP_ReadPacket()，来看看它的源代码。
[cpp]
//读取收下来的Chunk
int
RTMP_ReadPacket(RTMP *r, RTMPPacket *packet)
{
//packet 存读取完后的的数据
//Chunk Header最大值18
uint8_t hbuf[RTMP_MAX_HEADER_SIZE] = { 0 };
//header 指向的是从Socket中收下来的数据
char *header = (char *)hbuf;
int nSize, hSize, nToRead, nChunk;
int didAlloc = FALSE;
RTMP_Log(RTMP_LOGDEBUG2, "%s: fd=%d", __FUNCTION__, r->m_sb.sb_socket);
//收下来的数据存入hbuf
if (ReadN(r, (char *)hbuf, 1) == 0)
{
RTMP_Log(RTMP_LOGERROR, "%s, failed to read RTMP packet header", __FUNCTION__);
return FALSE;
}
//块类型fmt
packet->m_headerType = (hbuf[0] & 0xc0) >> 6;
//块流ID（2-63）
packet->m_nChannel = (hbuf[0] & 0x3f);
header++;
//块流ID第1字节为0时，块流ID占2个字节
if (packet->m_nChannel == 0)
{
if (ReadN(r, (char *)&hbuf[1], 1) != 1)
{
RTMP_Log(RTMP_LOGERROR, "%s, failed to read RTMP packet header 2nd byte",
__FUNCTION__);
return FALSE;
}
//计算块流ID（64-319）
packet->m_nChannel = hbuf[1];
packet->m_nChannel += 64;
header++;
}
//块流ID第1字节为0时，块流ID占3个字节
else if (packet->m_nChannel == 1)
{
int tmp;
if (ReadN(r, (char *)&hbuf[1], 2) != 2)
{
RTMP_Log(RTMP_LOGERROR, "%s, failed to read RTMP packet header 3nd byte",
__FUNCTION__);
return FALSE;
}
tmp = (hbuf[2] << 8) + hbuf[1];
//计算块流ID（64-65599）
packet->m_nChannel = tmp + 64;
RTMP_Log(RTMP_LOGDEBUG, "%s, m_nChannel: %0x", __FUNCTION__, packet->m_nChannel);
header += 2;
}
//ChunkHeader的大小（4种）
nSize = packetSize[packet->m_headerType];
if (nSize == RTMP_LARGE_HEADER_SIZE) /* if we get a full header the timestamp is absolute */
packet->m_hasAbsTimestamp = TRUE; //11字节的完整Chun