C语言的位域赋值问题
系统：ubuntu 11.04 little-end
描述：这个问题是在研究TS包头解析时遇到的，用两种方式取TS包头，但结果不同
耗时：2天
程序：
1. 使用正常顺序方式定义TS包头结构体，然后将TS包头数据拷贝到包头结构体中。
void bob_get_packet_header_mem_cpy_1(unsigned char *ts_package)
{
printf(" [%s]:\n", __func__);
printf(" Package Source Data: ");
unsigned char *p = ts_package;
while(p < (ts_package + sizeof(TS_packet_header))){
printf("%02x ", *p);
p++;
}
printf(" (0x%04x)\n", *(unsigned int *)ts_package);
struct bob_TS_packet_header
{
unsigned int sync_byte :8;
unsigned int transport_error_indicator :1;
unsigned int payload_unit_start_indicator :1;
unsigned int transport_priority :1;
unsigned int PID :13;
unsigned int transport_scrambling_control :2;
unsigned int adaptation_field_control :2;
unsigned int continuity_counter :4;
}header;
memcpy((void *)&header, ts_package, sizeof(header));
printf(" sync_byte(8b): 0x%02X (%u) %s\n", header.sync_byte, header.sync_byte, (header.sync_byte == 0x47 ? "" : "(sync error)"));
printf(" transport_error_indicator(1b): 0x%02X (%u) %s\n", header.transport_error_indicator, header.transport_error_indicator, (header.transport_error_indicator == 0x00 ? "" : "(packet error)"));
printf(" payload_unit_start_indicator(1b): 0x%02X (%u)\n", header.payload_unit_start_indicator, header.payload_unit_start_indicator);
printf(" transport_priority(1b): 0x%02X (%u)\n", header.transport_priority, header.transport_priority);
printf(" PID(13b): 0x%04X (%u)\n", header.PID, header.PID);
printf(" transport_scrambling_control(2b): 0x%02X (%u) %s\n", header.transport_scrambling_control, header.transport_scrambling_control, (header.transport_scrambling_control == 0x00 ? "(not scrambled)" : "(scrambled)"));
printf(" adaptation_field_control(2b): 0x%02X (%u) %s\n", header.adaptation_field_control, header.adaptation_field_control, (header.adaptation_field_control == 0x00 ? "(no adap_field, no payload)" : header.adaptation_field_control == 0x01 ? "(no adap_field, payload only)" : header.adaptation_field_control == 0x10 ? "(adap_field only, no payload)" : "(adap_field followed by payload)"));
printf(" continuity_counter(4b): 0x%02X (%u)\n", header.continuity_counter, header.continuity_counter);
printf("\n");
}2. 使用正常顺序方式定义TS包头结构体，使用buffer存储TS包头，然后通过位移过滤方式将值赋给TS包头结构体的成员。
void bob_get_packet_header_bit_shift(unsigned char *ts_package)
{
printf(" [%s]:\n", __func__);
printf(" Package Source Data: ");
unsigned char *p = ts_package;
while(p < (ts_package + sizeof(TS_packet_header))){
printf("%02x ", *p);
p++;
}
printf(" (0x%04x)\n", *(unsigned int *)ts_package);
struct bob_TS_packet_header
{
unsigned int sync_byte :8;
unsigned int transport_error_indicator :1;
unsigned int payload_unit_start_indicator :1;
unsigned int transport_priority :1;
unsigned int PID :13;
unsigned int transport_scrambling_control :2;
unsigned int adaptation_field_control :2;
unsigned int continuity_counter :4;
}header;
unsigned char buf[4];
memcpy(buf, ts_package, sizeof(header));
int i;
for(i=0; i<4; i++){
printf("%x[%d] ", buf[i], i);
}
printf("\n");
/* *
* buf:
* Hex: 0x47 0x40 0x00 0x10
* Bin: 0100 0111 0100 0000 0000 0000 0001 0000
* buf[0]: 0100 0111|
* buf[1]: 0|1|0|0
* buf[2]: 0000 0000
* buf[3]: 0000|
* buf[4]: 00|01| 0000
*/
header.sync_byte = buf[0];
header.transport_error_indicator = buf[1] >> 7;
header.payload_unit_start_indicator = buf[1] >> 6 & 0x01;
header.transport_priority = buf[1] >> 5 & 0x01;
header.PID = (buf[1] & 0x1F) << 8 | buf[2];
header.transport_scrambling_control = buf[3] >> 6;
header.adaptation_field_control = buf[3] >> 4 & 0x03;
header.continuity_counter = buf[3] & 0x0F;
printf(" sync_byte(8b): 0x%02X (%u) %s\n", header.sync_byte, header.sync_byte, (header.sync_byte == 0x47 ? "" : "(sync error)"));
printf(" transport_error_indicator(1b): 0x%02X (%u) %s\n", header.transport_error_indicator, header.transport_error_indicator, (header.transport_error_indicator == 0x00 ? "" : "(packet error)"));
printf(" payload_unit_start_indicator(1b): 0x%02X (%u)\n", header.payload_unit_start_indicator, header.payload_unit_start_indicator);
printf(" transport_priority(1b): 0x%02X (%u)\n", header.transport_priority, header.transport_priority);
printf(" PID(13b): 0x%04X (%u)\n", header.PID, header.PID);
printf(" transport_scrambling_control(2b): 0x%02X (%u) %s\n", header.transport_scrambling_control, header.transport_scrambling_control, (header.transport_scrambling_control == 0x00 ? "(not scrambled)" : "(scrambled)"));
printf(" adaptation_field_control(2b): 0x%02X (%u) %s\n", header.adaptation_field_control, header.adaptation_field_control, (header.adaptation_field_control == 0x00 ? "(no adap_field, no payload)" : header.adaptation_field_control == 0x01 ? "(no adap_field, payload only)" : header.adaptation_field_control == 0x10 ? "(adap_field only, no payload)" : "(adap_field followed by payload)"));
printf(" continuity_counter(4b): 0x%02X (%u)\n", header.continuity_counter, header.continuity_counter);
printf("\n");
}3. 使用倒序方式定义TS包头结构体，并将原始的TS包头数据颠倒4->1, 3->2, 2->3, 1->4，然后将颠倒后的TS包头数据拷贝到包头结构体中。
struct _TS_packet_header//defined in reverse order
{
unsigned int continuity_counter :4 ;
unsigned int adaptation_field_control :2 ;
unsigned int transport_scrambling_control :2 ;
unsigned int PID :13 ;
unsigned int transport_priority :1 ;
unsigned int payload_unit_start_indicator :1 ;
unsigned int transport_error_indicator :1 ;
unsigned int sync_byte :8 ;
};
typedef struct _TS_packet_header TS_packet_header;
void TS_header_decode(TS_packet_header *p_header,byte* ts_package)
{
*(byte *)p_header = *(ts_package + 3);
*((byte *)(p_header) + 1) = *(ts_package + 2);
*((byte *)(p_header) + 2) = *(ts_package + 1);
*((byte *)(p_header) + 3) = *(ts_package);
printf(" Package Source Data: ");
unsigned char *p = (unsigned char *)p_header;
while(p < ((unsigned char *)p_header + sizeof(TS_packet_header))){
printf("%02x ", *p);
p++;
}
printf(" (0x%04x)\n", *(unsigned int *)p_header);
printf("PACKET HEADER:\n");
printf(" sync_byte(8b): 0x%02X (%u) %s\n", p_header->sync_byte, p_header->sync_byte, (p_header->sync_byte == 0x47 ? "" : "(sync error)"));
printf(" transport_error_indicator(1b): 0x%02X (%u) %s\n", p_header->transport_error_indicator, p_header->transport_error_indicator, (p_header->transport_error_indicator == 0x00 ? "" : "(packet error)"));
printf(" payload_unit_start_indicator(1b): 0x%02X (%u)\n", p_header->payload_unit_start_indicator, p_header->payload_unit_start_indicator);
printf(" transport_priority(1b): 0x%02X (%u)\n", p_header->transport_priority, p_header->transport_priority);
printf(" PID(13b): 0x%04X (%u)\n", p_header->PID, p_header->PID);
printf(" transport_scrambling_control(2b): 0x%02X (%u) %s\n", p_header->transport_scrambling_control, p_header->transport_scrambling_control, (p_header->transport_scrambling_control == 0x00 ? "(not scrambled)" : "(scrambled)"));
printf(" adaptation_field_control(2b): 0x%02X (%u) %s\n", p_header->adaptation_field_control, p_header->adaptation_field_control, (p_header->adaptation_field_control == 0x00 ? "(no adap_field, no payload)" : p_header->adaptation_field_control == 0x01 ? "(no adap_field, payload only)" : p_header->adaptation_field_control == 0x10 ? "(adap_field only, no payload)" : "(adap_field followed by payload)"));
printf(" continuity_counter(4b): 0x%02X (%u)\n", p_header->continuity_counter, p_header->continuity_counter);
}
结果：程序1：错误，程序2和3：正确。
原因分析：
主要分析点是程序1和程序3的区别，而程序2是一种繁琐但能保证正确性的实现方式，所以没有仔细分析程序2。
1.首先，考虑到可能是由于大小端的造成的，但是仔细分析后发现这是明显的单字节内数据错误问题，而大小端影响的是多字节的排列顺序。
2.其次，仔细分析了两种数据的结构的数据内存布局，发现是由于处理器定义字节的顺序是从右向左的，也就是说低位在最右边。如下图所示：
程序1的内存布局：当结构体位域成员不是字节对齐的时候，就出现了错误。
程序3的内存布局：使用倒序，巧妙地解决了处理器从右向左的访问顺序对程序造成的影响
总结：通过这次的问题，搞明白了3个概念，C语言的位域，大小端，处理器数据访问顺序。也掌握了一种更快速地解析数据包的方法
posted on
2012-06-27 10:23 SunBo 阅读(
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