如何在 MySQL 数据库中使用 __attribute__((packed)) 存储 c++ 结构？

Question

How can I easily store and retrieve c++ structures of any size with attribute ((packed)) in MySQL DB?

Structure example:

typedef struct __attribute__((packed))
{
    signed char result;
    unsigned char errorCode;
} Response; 

typedef struct __attribute__((packed))
{
    unsigned char   rackId;
    unsigned char   subRackId;
    unsigned char   cardId;
    unsigned char   cardType;
    unsigned char   cardSubType;
    unsigned char   portId;
    Response        ret; 
}Configure;

Answer 1

Since everything is char the packed attribute is pointless, just static_assert(sizeof(Configure) == 8); .

You can store it as CHAR(8).

If you have a struct with different types try arranging them so no padding occurs (static_assert the size to make sure). It's even better to add manual padding than to use packed .

But if you must use packed you should use an unpacked structure to work with and only copy the data to a packed structure to write to mysql and copy out of the packed structure after reading from mysql. Never access a packed structure more than once, the access will be horrible slow on many architectures.