TS码流解析-3-解析PAT表

任务三

解析PAT表，并且获取码流里面跟节目相关的PID

相关知识

我们首先来查看下PAT表的结构，如下图所示

在这个结构里面，我们需要关注的是program_number以及program_map_PID。
我们获取到PAT的表之后，对其中的section进行解析，把我们需要的信息取出来，然后存入数据结构，这个任务就算完成了。

解析过程

1 流程图

2 数据结构定义

typedef struct pat_info
{
    unsigned int program_number      :16;
    unsigned int reserved            :3;
    unsigned int program_number_pid  :13;
    struct pat_info *next;
}PAT_INFO;

3 代码

static PAT_INFO *insert_pat_info_to_link(PAT_INFO *pat_info_link_head, PAT_INFO additive_node)
{
    PAT_INFO *new_pat_info_node = NULL;
    new_pat_info_node = (PAT_INFO*)malloc(sizeof(PAT_INFO));
    
    if (NULL == new_pat_info_node)
    {
        log("malloc error!\n");
        return pat_info_link_head;
    }

    
    new_pat_info_node->program_number = additive_node.program_number;
    new_pat_info_node->program_number_pid = additive_node.program_number_pid;
    new_pat_info_node->reserved = additive_node.reserved;
    // head insert
    new_pat_info_node->next = pat_info_link_head;
    
    return new_pat_info_node;
}

PAT_INFO *create_pat_info_link(FILE *ts_file, unsigned int package_length, int first_sync_position)
{
    SECTION *pat_table = NULL;
    SECTION *current_handle_section = NULL;
    int current_read_position = 0;
    int end_read_position = 0;
    PAT_INFO swap_pat_node = {0};
    PAT_INFO *pat_info_link_head = NULL;

    if ((NULL==ts_file) || (first_sync_position<0) || (0==package_length))
    {
        log("function parse_pmt parameter error");
        return pat_info_link_head;
    }

    pat_table = create_section_link(ts_file, package_length, first_sync_position, PAT_TABLE_ID, PAT_PID, eit_check);
    if (NULL == pat_table)
    {
        log("get PAT table error!\n");
        return pat_info_link_head;
    }

    current_handle_section = pat_table;
    while (current_handle_section != NULL)
    {
        current_read_position = SECTION_HEAD_LENGTH;
        end_read_position = current_handle_section->section_length - SECTION_CRC_LENGTH;

        while (current_read_position < end_read_position)
        {
            swap_pat_node.program_number = (current_handle_section->section_buffer[current_read_position] << 8) | 
                current_handle_section->section_buffer[current_read_position + 1];
            swap_pat_node.program_number_pid = (current_handle_section->section_buffer[current_read_position + 2] & 0x1f) << 8 | 
                current_handle_section->section_buffer[current_read_position + 3];
            current_read_position += PROGRAM_INFO_LENGTH;

            if (NETWORK_PID == swap_pat_node.program_number)
            {
                continue;
            }
            pat_info_link_head = insert_pat_info_to_link(pat_info_link_head, swap_pat_node);
        }
        current_handle_section = current_handle_section->next;
    }
    log("-----------------------------------PAT-----------------------------------\n");

    show_pat_info(pat_info_link_head);
    destroy_section_link(pat_table);
    pat_table = NULL;

    return pat_info_link_head;
}

相对来说，解析PAT表的难度不大，需要注意的是读取字节的位置一定要准确，是读取当前位置呢，还是读取下一个位置一定要清楚，不然整个读取过程都会乱套的。