我有一些 mpeg ts 位域,例如传输流包:
struct ts_package_header_s {
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;
};
struct ts_package_s {
struct ts_package_header_s ts_header;
unsigned char ts_body[TS_BODY];
};
union ts_package_u {
struct ts_package_s ts_package;
unsigned char bytes[TS_PACKAGE];
};
在我的源代码中,我初始化了头结构:
pat_package_header.sync_byte = 0x47;
pat_package_header.transport_error_indicator = 0;
pat_package_header.payload_unit_start_indicator = 1;
pat_package_header.transport_priority = 0;
pat_package_header.PID = PAT_PID;
pat_package_header.transport_scrambling_control = 0;
pat_package_header.adaptation_field_control = 1;
pat_package_header.continuity_counter = 0;
然后我创建 ts_packag union
union ts_package_u package;
package.ts_package.ts_header = pat_package_header;
比我填充 ts_body 数组。 当我将这个包写入文件时。我得到向后数组:
10 00 40 47 XX XX XX.. instead of 47 40 10 00 XX XX XX..
我尝试将我的结构转换为 char* 而不是使用 union ,但得到了相同的结果。
错在哪里?谢谢。
最佳答案
像这样直接使用尝试和序列化结构到磁盘是很危险的,因为格式必须在不同的体系结构中是已知的,或者使用不同的编译器。
编译器在存储位域的方式上有所不同,您的体系结构的底层字节序也会改变数据的存储方式
例如,编译器可能选择在字节、字或其他边界上对齐位域。这是编译器的决定。它还可以选择以任何顺序保存位,这通常取决于您机器的字节顺序。
为了安全地将此 header 写入磁盘,您需要自己序列化数据。根据 Wikipedia., header 是 32 位和 Big Endian
例如:
#include <stdio.h>
#define TS_BODY 1024
#define PAT_PID 0x40
struct ts_package_header_s {
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;
};
struct ts_package_s {
struct ts_package_header_s ts_header;
unsigned char ts_body[TS_BODY];
};
static void write_ts( struct ts_package_s pat_package )
{
FILE* f = fopen( "test.ts", "wb+" );
unsigned int header = 0;
if( f == NULL )
return;
header = pat_package.ts_header.sync_byte << 24;
header |= ( pat_package.ts_header.transport_error_indicator << 23 );
header |= ( pat_package.ts_header.payload_unit_start_indicator << 22 );
header |= ( pat_package.ts_header.transport_priority << 21 );
header |= ( pat_package.ts_header.PID << 8 );
header |= ( pat_package.ts_header.transport_scrambling_control << 6 );
header |= ( pat_package.ts_header.adaptation_field_control << 4 );
header |= ( pat_package.ts_header.continuity_counter );
/* Write the 32-bit header as big-endian */
unsigned char byte = header >> 24;
fwrite( &byte, 1, 1, f );
byte = ( header >> 16 ) & 0xFF;
fwrite( &byte, 1, 1, f );
byte = ( header >> 8 ) & 0xFF;
fwrite( &byte, 1, 1, f );
byte = header & 0xFF;
fwrite( &byte, 1, 1, f );
fclose( f );
}
int main( int argc, char* argv[] )
{
struct ts_package_s pat_package;
pat_package.ts_header.sync_byte = 0x47;
pat_package.ts_header.transport_error_indicator = 0;
pat_package.ts_header.payload_unit_start_indicator = 1;
pat_package.ts_header.transport_priority = 0;
pat_package.ts_header.PID = PAT_PID;
pat_package.ts_header.transport_scrambling_control = 0;
pat_package.ts_header.adaptation_field_control = 1;
pat_package.ts_header.continuity_counter = 0;
write_ts( pat_package );
return 0;
}
写入具有以下 header 的文件:
0x47 0x40 0x01 0x10
根据您使用的值,这似乎是正确的。
关于将位域转换为字节数组,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/28739622/