How to use flexible arrays?

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

C99 in , The last element in the structure is allowed to be an array of unknown size , This is called a flexible array member , But a flexible array member in a structure must be preceded by at least one other member . Flexible array members allow structures to contain an array of variable size .sizeof The size of the structure returned does not include the memory of the flexible array . Structures that contain flexible array members use malloc () Function to dynamically allocate memory , And the allocated memory should be larger than the size of the structure , To fit the expected size of the flexible array .

How to use flexible arrays ？ Here's an example ：

typedef struct st_type
{
    
	int i;
	int a[0];
}type_a;

Some compilers will report errors and cannot compile. They can be changed to ：

typedef struct st_type
{
    
	int i;
	int a[];
}type_a;

this sample I People Just can With set The righteous One individual can change Long Of junction structure body , use sizeof(type_a) have to To Of only Yes 4 , Just yes sizeof(i)=sizeof(int). that 0 An array of elements doesn't take up space , Then we can do the lengthening operation .

Allocate memory to the structure through the following expression ：

type_a *p = (type_a*)malloc(sizeof(type_a)+100*sizeof(int)); 

So we give the structure pointer p Allocated a block of memory . use p->item[n] You can simply access variable length elements . But then we use sizeof（*p） Test the size of the structure , It's still found that 4. Isn't it weird ？ We allocated space to this array ？

When defining this structure , The size of the module has determined the memory size that does not contain flexible arrays . Flexible arrays are just non staff , It does not occupy the compilation of the structure . Just say that when using a flexible array, you need to treat it as a member of the structure , That's it . And white points , Flexible arrays have nothing to do with structures , It's just “ Selling dog meat with sheep's head ” nothing more , Not a full member of the structure .

C89 I don't support this kind of thing ,C99 Add it to the standard as a special case . however ,C99 What is supported is that incomplete type, instead of zero array, It's the same int item[0]; This form is illegal ,C99 The form of support is in the form of int item[]; It's just that some compilers put int item[0]; As a non-standard extension to support , And in C99 There was this nonstandard extension before the release ,C99 After the release , Some compilers combine the two .

Of course , Since the top is used malloc Function allocates memory , Definitely need to use free Function to free memory ：

free(p);

Application scenarios ： Variable length arrays are often used in network communication to construct indefinite length packets , Don't waste space, waste network traffic .