libdvbpsi 源代码学习（二）

        刚开始学习MPEG-2 TS流解析时，看ISO/IEC13818-1的文档上面的PAT，PMT表：program_association_section()和TS_program_map_section（）时，很容易就以为可以直接从188字节的TS packet中取数据填到各个字段中，网上也可以搜到这样类似的程序：

void ParsePat(psi_pat *p_pat, u8 *p_packet){ // declare variables and initial ... // get payload_unit_start_indicator b_pusi = p_packet[1] & 0x40; b_adaptation = p_packet[3] & 0x20; // calculate the skip to skip to payload field if(b_adaptation) { skip = 5 + p_packet[4]; } else { skip = 4; } // skip pointer_field if(b_pusi) { skip = skip + p_packet[skip] + 1; } // now skip to payload field p_payload = p_packet + skip; // now parse table p_pat->table_id = p_payload[0]; ...}

    

        这样在解析PAT和PMT表时往往也能得到正确的答案，这是因为PAT，PMT表数据量比较小的缘故。其实在TS流中还有一个重要的概念——section。文档中program_association_section()和TS_program_map_section（）指的就是PAT section和PMT section。它们中都有一个字段：section_length，文档告诉我们这个字段值的是此字段之后section的字节数，除了EIT section的最大字节数是4096外，其他section最大字节数是1024字节。 而TS packet最大只有188字节，因此这里需要考虑section到packet的映射问题，基本上存在三种情况：

1、section对应一个packet

2、section太长，在几个连续的packet中

3、在一个packet的负载中结束上一个section之后马上开始了一个新的section。

这三种情况可以在libdvbpsi源码的void dvbpsi_PushPacket(dvbpsi_handle h_dvbpsi, uint8_t* p_data)函数中找到。明白了这三种情况，再对照函数的注释，应该就不难看懂这个函数了。

 

/***************************************************************************** * dvbpsi_PushPacket ***************************************************************************** * Injection of a TS packet into a PSI decoder. *****************************************************************************/void dvbpsi_PushPacket(dvbpsi_handle h_dvbpsi, uint8_t* p_data){ uint8_t i_expected_counter; /* Expected continuity counter */ dvbpsi_psi_section_t* p_section; /* Current section */ uint8_t* p_payload_pos; /* Where in the TS packet */ uint8_t* p_new_pos = NULL; /* Beginning of the new section, updated to NULL when the new section is handled */ int i_available; /* Byte count available in the packet */ /* 本次放进来的packet的负载字节数 */ /* TS start code */ if(p_data[0] != 0x47) { DVBPSI_ERROR("PSI decoder", "not a TS packet"); return; } /* Continuity check */ i_expected_counter = (h_dvbpsi->i_continuity_counter + 1) & 0xf; h_dvbpsi->i_continuity_counter = p_data[3] & 0xf; if(i_expected_counter == ((h_dvbpsi->i_continuity_counter + 1) & 0xf) && !h_dvbpsi->b_discontinuity) { DVBPSI_ERROR_ARG("PSI decoder", "TS duplicate (received %d, expected %d) for PID %d", h_dvbpsi->i_continuity_counter, i_expected_counter, ((uint16_t)(p_data[1] & 0x1f) << 8) | p_data[2]); return; } if(i_expected_counter != h_dvbpsi->i_continuity_counter) { DVBPSI_ERROR_ARG("PSI decoder", "TS discontinuity (received %d, expected %d) for PID %d", h_dvbpsi->i_continuity_counter, i_expected_counter, ((uint16_t)(p_data[1] & 0x1f) << 8) | p_data[2]); h_dvbpsi->b_discontinuity = 1; if(h_dvbpsi->p_current_section) { dvbpsi_DeletePSISections(h_dvbpsi->p_current_section); h_dvbpsi->p_current_section = NULL; } } /* Return if no payload in the TS packet */ if(!(p_data[3] & 0x10)) { return; } /* Skip the adaptation_field if present */ if(p_data[3] & 0x20) p_payload_pos = p_data + 5 + p_data[4]; else p_payload_pos = p_data + 4; /* Unit start -> skip the pointer_field and a new section begins */ /* payload_unit_start_indicator 为1时不仅有一个pointer_field指针，更意味着一个新的分段section开始 */ if(p_data[1] & 0x40) { p_new_pos = p_payload_pos + *p_payload_pos + 1; p_payload_pos += 1; } /* 用来判断上个分段是否结束，为NULL表示上个分段结束了并已被收集 */ p_section = h_dvbpsi->p_current_section; /* If the psi decoder needs a begginning of section and a new section begins in the packet then initialize the dvbpsi_psi_section_t structure */ /* 上个分段结束，一般就开始新的分段(payload_unit_start_indicator为1),否则错误直接返回 */ if(p_section == NULL) { if(p_new_pos) { /* Allocation of the structure */ h_dvbpsi->p_current_section = p_section = dvbpsi_NewPSISection(h_dvbpsi->i_section_max_size); /* Update the position in the packet */ p_payload_pos = p_new_pos; /* New section is being handled */ p_new_pos = NULL; /* Just need the header to know how long is the section */ /* 开始新的分段，先读分段的header，到section_length刚好3字节，主要是要先读section_length用来确定此分段有多少字节。 */ h_dvbpsi->i_need = 3; h_dvbpsi->b_complete_header = 0; } else { /* No new section => return */ return; } } /* Remaining bytes in the payload */ /* p_payload_pos = p_data + offset, 其中offset是section包头的长度 */ /* 所以，i_available = 188 - offset */ i_available = 188 + p_data - p_payload_pos; while(i_available > 0) { /* h_dvbpsi->i_need 用来记录该分段还差多少个字节才结束分段 */ /* 此包的负载足够填充当前分段，需要考虑： * 1、如果已经填充过分段的header，则将此包的负载追加到分段数据的末尾即结束此分段，并调用h_dvbpsi->pf_callback收集当前section以完成一个子表 * 这个时候还要判断，如果此包的负载还有剩余，并且不是填充字段（0xff）则意味着直接开始了一个新的分段。 * 2、如果还没有填充过分段的header，则分析段的header，主要是section_length字段用来更新h_dvbpsi->i_need */ if(i_available >= h_dvbpsi->i_need) { /* There are enough bytes in this packet to complete the header/section */ memcpy(p_section->p_payload_end, p_payload_pos, h_dvbpsi->i_need); p_payload_pos += h_dvbpsi->i_need; p_section->p_payload_end += h_dvbpsi->i_need; i_available -= h_dvbpsi->i_need; if(!h_dvbpsi->b_complete_header) { /* Header is complete */ h_dvbpsi->b_complete_header = 1; /* Compute p_section->i_length and update h_dvbpsi->i_need */ /* 分段已经读了3个字节也即读到section_length字段，恰恰还需要section_length个字节 */ h_dvbpsi->i_need = p_section->i_length = ((uint16_t)(p_section->p_data[1] & 0xf)) << 8 | p_section->p_data[2]; /* Check that the section isn't too long */ if(h_dvbpsi->i_need > h_dvbpsi->i_section_max_size - 3) { DVBPSI_ERROR("PSI decoder", "PSI section too long"); dvbpsi_DeletePSISections(p_section); h_dvbpsi->p_current_section = NULL; /* If there is a new section not being handled then go forward in the packet */ if(p_new_pos) { h_dvbpsi->p_current_section = p_section = dvbpsi_NewPSISection(h_dvbpsi->i_section_max_size); p_payload_pos = p_new_pos; p_new_pos = NULL; h_dvbpsi->i_need = 3; h_dvbpsi->b_complete_header = 0; i_available = 188 + p_data - p_payload_pos; } else { i_available = 0; } } } else /* 分段准备结束并调用回调函数收集section */ { /* PSI section is complete */ p_section->b_syntax_indicator = p_section->p_data[1] & 0x80; p_section->b_private_indicator = p_section->p_data[1] & 0x40; /* Update the end of the payload if CRC_32 is present */ if(p_section->b_syntax_indicator) p_section->p_payload_end -= 4; /* 主要是CRC校验 */ if(p_section->p_data[0] != 0x72 && dvbpsi_ValidPSISection(p_section)) { /* PSI section is valid */ p_section->i_table_id = p_section->p_data[0]; if(p_section->b_syntax_indicator) { p_section->i_extension = (p_section->p_data[3] << 8) | p_section->p_data[4]; p_section->i_version = (p_section->p_data[5] & 0x3e) >> 1; p_section->b_current_next = p_section->p_data[5] & 0x1; p_section->i_number = p_section->p_data[6]; p_section->i_last_number = p_section->p_data[7]; p_section->p_payload_start = p_section->p_data + 8; } else { p_section->i_extension = 0; p_section->i_version = 0; p_section->b_current_next = 1; p_section->i_number = 0; p_section->i_last_number = 0; p_section->p_payload_start = p_section->p_data + 3; } /* 收集当前section */ h_dvbpsi->pf_callback(h_dvbpsi, p_section); /* 结束当前section */ h_dvbpsi->p_current_section = NULL; } else { /* PSI section isn't valid => trash it */ dvbpsi_DeletePSISections(p_section); h_dvbpsi->p_current_section = NULL; } /* A TS packet may contain any number of sections, only the first * new one is flagged by the pointer_field. If the next payload * byte isn't 0xff then a new section starts. */ /* 负载还有剩余，并且不是填充字段，则新的分段开始了，这种情况就是一个packet中有2个分段。 */ if(p_new_pos == NULL && i_available && *p_payload_pos != 0xff) p_new_pos = p_payload_pos; /* If there is a new section not being handled then go forward in the packet */ if(p_new_pos) { h_dvbpsi->p_current_section = p_section = dvbpsi_NewPSISection(h_dvbpsi->i_section_max_size); p_payload_pos = p_new_pos; p_new_pos = NULL; h_dvbpsi->i_need = 3; h_dvbpsi->b_complete_header = 0; i_available = 188 + p_data - p_payload_pos; } else { i_available = 0; } } } /* 此包的负载不够填充当前分段，则直接把此包的负载追加到当前分段的分段数据末尾上。 * 这种情况就是一个分段在多个packet中。 */ else { /* There aren't enough bytes in this packet to complete the header/section */ memcpy(p_section->p_payload_end, p_payload_pos, i_available); p_section->p_payload_end += i_available; h_dvbpsi->i_need -= i_available; i_available = 0; } }}