当前位置：网站首页>Detailed explanation of zero length array in C language (1) [information at the end of the article]

Detailed explanation of zero length array in C language (1) [information at the end of the article]

2022-07-25 00:51:00 【an520_】

Zero length array concept ：

as everyone knows , GNU/GCC In standard C/C++ On this basis, it has made practical expansion , Zero length array （Arrays of Length Zero） Is one of the well-known extensions .

Most of the time , It is used in variable length arrays , Its definition is as follows ：

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

purpose : The length is 0 The main purpose of the array 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 array name itself represents an immutable address constant

（ Be careful : The array name will never be a pointer !）, But for the size of this array , We can dynamically allocate .

Be careful ： If the structure is through calloc、malloc or person new And other dynamic allocation methods , Free up space when you don't need it .

advantage ： Instead of declaring a pointer variable in a structure 、 Then carry out dynamic analysis Matching method , This method is more efficient . Because when accessing the contents of an array , No indirect access is required , Two memory accesses are avoided .

shortcoming ： In the structure , The array is 0 Must be declared at the end of , There are certain restrictions on use .

For compilers , The array name is just a symbol , It doesn't take up any space , It's in the structure , It just represents an offset , Represents an immutable address constant ！

0 The purpose of the length array ：

Let's imagine a scene like this , The data buffer we use 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 sufficient size MAX_LENGTH Data buffer for
Set a pointer to the actual data , Every time I use it , Dynamically open up data buffer space according to the length of data

We consider their advantages and disadvantages from the design applied in the actual scene . The main considerations are , Development of buffer space 、 Release and access .

1、 Fixed length bag ( Open up space , Release , visit )：

For example, I want to send 1024 Bytes of data , If you use a fixed length bag , Suppose the length of a fixed length packet MAX_LENGTH by 2048, It will be wasted 1024 Bytes of space , It also causes unnecessary waste of traffic ：

Data structure definition ：

Data structure size ： Consider alignment , So the size of the data structure >= sizeof(int) + sizeof(char) * MAX_LENGTH

Considering the data overflow , In variable length packets data The array length is usually set to be long enough to hold the largest amount of data , therefore max_buffer Medium data In many cases, arrays are not filled with data , So there is waste .

The construction of packets ： If we want to send CURR_LENGTH = 1024 Bytes , How do we construct this packet ; Generally speaking , We will return a data structure pointing to the buffer max_buffer The pointer to ：

visit ： This memory is divided into two parts ; The first part 4 Bytes p->len, As a Baotou ( That's the extra part ), This header is used to describe the length of the data section immediately after the header , Here is 1024, So the first four bytes are assigned to 1024 ( Since we are going to construct indefinite length packets , So how long is this bag , therefore , We have to use a variable to indicate the length of the packet , This is it. len The role of ); And the memory immediately after that is the real data part , adopt p->data, Last , Carry out a memcpy() Memory copy , Put the data to be sent into this memory
Release ： When the data space is released after use , Just release it

2、 Summary :

Use fixed length arrays , As a data buffer , To avoid buffer overflow , The size of the array is usually set to enough space MAX_LENGTH, But in actual use , achieve MAX_LENGTH There's very little data on length , So most of the time , Most of the buffer space is wasted
But the process is simple , It's easy to open up and release data space , No need for programmers to think about extra operations

3、 Pointer packet ( Open up space , Release , visit ):

If you set the length of the top to MAX_LENGTH Replace the fixed length array with a pointer , Dynamic development at each use CURR_LENGTH Size space , Then avoid causing MAX_LENGTH - CURR_LENGTH Waste of space , Only the space of a pointer field is wasted ：

Packet definition ：

Data structure size ： Consider alignment , So the size of the data structure >= sizeof(int) + sizeof(char *)
Space allocation ： But it also causes the use of memory allocation , There are two steps

First , You need to allocate a memory space for the structure ; Secondly, allocate memory space for member variables in the structure .

In this way, the memory allocated twice is not continuous , They need to be managed separately . When using arrays of length , The principle of one-time distribution is adopted , Allocate all the required memory to it at one time .

Release ： contrary , It's the same when it's released ：

Summary ：
- Use pointer results as buffers , Only one more pointer size space is used , No need to use MAX_LENGTH An array of lengths , It won't cause a lot of waste of space .
But that's when opening up space , Need to open up extra space for data fields , When casting, you also need to display the space to release the data field , But in actual use , Often open up space in a function , And then back to the user pointing to struct point_buffer The pointer to , At this time, we can't assume that users know the details of our development , And release space according to the agreed operation , So it's inconvenient to use , Even cause memory leaks .

4、 Variable length data buffer ( Open up space , Release , visit )

Fixed length array is easy to use , But it's a waste of space , The pointer form uses only one more pointer space , It won't waste a lot of space , But it needs to be allocated many times , Multiple releases , So is there a way to implement it without wasting space , And easy to use ?

GNU C Of 0 Length array , Also called variable length arrays , A flexible array is such an extension . about 0 This feature of long arrays , It's easy to construct into a structure , Such as buffer , Packets and so on ：

Data structure definition ：

Data structure size ： Such variable length arrays are often used to construct indefinite length packets in network communication , Don't waste space, waste network traffic , because char data[0]; It's just an array name , It doesn't take up storage space ：

Open up space ： So when we use it , Just open up a space once

Release space ： The same goes for freeing up space , Release once

summary ：

The length is 0 The array does not occupy memory space , The pointer mode needs to occupy memory space .
For length is 0 Array , When applying for memory space , Adopt the principle of one-time distribution ; For structures that contain pointers , It needs to be done separately when applying for space , It also needs to be released separately .
For length is 0 The array of can be accessed in the form of array

【 Learning exchange group 607439754】

【 Free information package of online disk collected by myself , If you need it, you can collect it yourself 】：

Embedded Internet of things 22 individual STM32 project 、 Competition works ,【 Huaqing vision issued the information package 】http://makerschool.mikecrm.com/f4wjYBB【 Share some tutorial materials below 】：