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