Zero length array

Zero length array

1 Zero length array concept

GCC/GNU In standard C/C++ On the basis of it, it has made practical expansion , Zero length array is one of the well-known extensions . Most of the time , It is used in variable length arrays , Its definition is as follows ：

    typedef struct
    {
        uint32_t length;
        uint8_t payload[0];// there 0 Long structure provides very good support for variable length structure .
    } packet;

First of all, 0 Length array , Also called flexible array , Make an explanation ：

• purpose ： The length is 0 The main purpose of the array of is to meet the need for variable length structures
• usage ： At the end of a structure , Declare a length of 0 Array of , You can make the structure variable in length . For compilers , In this case, the length is 0 The array of does not take up space , Because the array name itself doesn't take up space , It's just an offset , The symbol array name itself represents an immutable address constant .

2 0 The purpose of the length array

Data buffer used in network communication , The buffer contains a len Fields and data Field , Identify the length of the data and the transmitted data respectively , There are several common design ideas ：

• Fixed length data buffer , Set a data buffer of sufficient size . But most of the time , Most of the buffer space is wasted .
• Set a pointer to the actual data , Every time I use it , Dynamically open up data buffer space according to the length of data . Need to open up extra space for data fields , When releasing, you also need to explicitly release the space of the data field .

So it's best to use zero length arrays .

3 example

#pragma pack(1)
    typedef struct
    {
        uint32_t age;  // 4
        gender gen;    // 4
        char name[32]; // 32
    } person;

#pragma pack(1)
    typedef struct
    {
        uint32_t length;
        uint8_t payload[0];
    } packet;

We want to person Structure as payload.

    person *p = new person{18, models::female, "zhuangjuan"};
    packet *pkt = (packet *)malloc(sizeof(packet) + sizeof(person));
    pkt->length = sizeof(*p);
    memcpy(pkt->payload, p, sizeof(*p));

    auto pp = (person *)pkt->payload;
    std::cout << "pp->age:" << (uint32_t)pp->age << endl;
    std::cout << "pp->gen:" << pp->gen << endl;
    std::cout << "pp->name:" << pp->name << endl;
    
    delete pkt;
    delete p;