freeswitch系列7 IO

freeswitch的大部分媒體邏輯在switch_ivr_*.c中實現，但是這些是功能邏輯，最後會調用switch_core_io.c中的讀寫幀函數進行io操作。本章不分析功能，只對讀寫幀這些底層IO的API進行分析。

讀寫幀代碼比較長，這裏對關鍵流程進行註解。

對一些參數進行判斷

從端點讀幀

if (session->endpoint_interface->io_routines->read_frame) {
switch_mutex_unlock(session->read_codec->mutex);
switch_mutex_unlock(session->codec_read_mutex);
if ((status = session->endpoint_interface->io_routines->read_frame(session, frame, flags, stream_id)) == SWITCH_STATUS_SUCCESS) {
for (ptr = session->event_hooks.read_frame; ptr; ptr = ptr->next) {
if ((status = ptr->read_frame(session, frame, flags, stream_id)) != SWITCH_STATUS_SUCCESS) {
break;
}
}
}

調用端點的io接口去讀，io層不具體去讀，而是調用端點的接口，說明它支持很多種讀法，如果是mod_sofia，從網絡讀RTP，在mod_portaudio則從本地聲卡讀。另外，除了端點可以讀，還可以從事件回調接口讀，session->event_hooks.read_frame。

傳給監控媒體流

if (bp->ready) {
if (switch_test_flag(bp, SMBF_TAP_NATIVE_READ)) {
if ((*frame)->codec && (*frame)->codec->implementation &&
(*frame)->codec->implementation->encoded_bytes_per_packet &&
(*frame)->datalen != (*frame)->codec->implementation->encoded_bytes_per_packet) {
switch_set_flag((*frame), SFF_CNG);
break;
}
if (bp->callback) {
bp->native_read_frame = *frame;
ok = bp->callback(bp, bp->user_data, SWITCH_ABC_TYPE_TAP_NATIVE_READ);
bp->native_read_frame = NULL;
}
}
}

freeswitch支持通過bug這種機制進行監控媒體流，這裏會把讀到的幀傳給監控bug。

解碼幀

status = switch_core_codec_decode(codec,
session->read_codec,
read_frame->data,
read_frame->datalen,
session->read_impl.actual_samples_per_second,
session->raw_read_frame.data, &session->raw_read_frame.datalen, &session->raw_read_frame.rate,
&read_frame->flags);

從網絡讀到的RTP包是編碼的，這裏如果code不一樣，或者直接播放，要進行解碼。

重採樣

除了支持轉碼，還支持重採樣。

再編碼

status = switch_core_codec_encode(session->read_codec,
enc_frame->codec,
enc_frame->data,
enc_frame->datalen,
session->read_impl.actual_samples_per_second,
session->enc_read_frame.data, &session->enc_read_frame.datalen, &session->enc_read_frame.rate, &flag);

所謂轉碼，就是兩端協商使用的codec不一樣，A的媒體數據，freeswitch先解碼成中間格式L16，再編碼成B支持的codec。

快速發

有些情況，比如這個是代理包標誌還是啥，就要調用perform_write直接發。

轉碼

if (frame->codec) {
session->raw_write_frame.datalen = session->raw_write_frame.buflen;
frame->codec->cur_frame = frame;
session->write_codec->cur_frame = frame;
status = switch_core_codec_decode(frame->codec,
session->write_codec,
frame->data,
frame->datalen,
session->write_impl.actual_samples_per_second,
session->raw_write_frame.data, &session->raw_write_frame.datalen, &session->raw_write_frame.rate, &frame->flags);

重採樣

switch (status) {
case SWITCH_STATUS_RESAMPLE:
resample++;
write_frame = &session->raw_write_frame;
write_frame->rate = frame->codec->implementation->actual_samples_per_second;
if (!session->write_resampler) {
switch_mutex_lock(session->resample_mutex);
status = switch_resample_create(&session->write_resampler,
frame->codec->implementation->actual_samples_per_second,
session->write_impl.actual_samples_per_second,
session->write_impl.decoded_bytes_per_packet, SWITCH_RESAMPLE_QUALITY, session->write_impl.number_of_channels);
switch_resample_process(session->write_resampler, data, write_frame->datalen / 2 / session->write_resampler->channels);

再編碼

status = switch_core_codec_encode(session->write_codec,
frame->codec,
enc_frame->data,
enc_frame->datalen,
session->write_impl.actual_samples_per_second,
session->enc_write_frame.data, &session->enc_write_frame.datalen, &session->enc_write_frame.rate, &flag);

都處理完後，最後調用perform_write把數據發出去。

perform_write實際調用端點的IO函數。

if (session->endpoint_interface->io_routines->write_frame) {
if ((status = session->endpoint_interface->io_routines->write_frame(session, frame, flags, stream_id)) == SWITCH_STATUS_SUCCESS) {
for (ptr = session->event_hooks.write_frame; ptr; ptr = ptr->next) {
if ((status = ptr->write_frame(session, frame, flags, stream_id)) != SWITCH_STATUS_SUCCESS) {
break;
}
}
}
}