Flexible Array Members in a structure in C

Flexible Array Member(FAM)

Flexible Array Member(FAM) is a feature introduced in the C99 standard of the C programming language. C99 标准里的。

1. For the structures in C programming language from C99 standard onwards, we can declare an array without a dimension and whose size is flexible in nature.
2. Such an array inside the structure should preferably be declared as the last member of structure and its size is variable(can be changed be at runtime). 这其实就是在 heap 中分配的内存。
3. The structure must contain at least one more named member in addition to the flexible array member. FAM 必须是结构体的最后一个成员。

举例

struct student
{
int stud_id;      // stack memory
int name_len;     // stack memory
int struct_size;  // stack memory，stud_name 长度 + sizeof(struct student *)。
char stud_name[]; // heap memory,其实就是个 char* 类型的指针。所以这里就是变长字段。这个不计入 sizeof(struct student)
};

如何使用？

// Memory allocation and initialisation of structure
struct student *createStudent(struct student *s, int id, char a[])               
{ // Allocating memory according to user provided    
  // array of characters   
  // student 指针大小 + stud_name 长度
	 s = malloc( sizeof(*s) + sizeof(char) * strlen(a));     
	 s->stud_id = id;    
	 s->name_len = strlen(a);    
	 strcpy(s->stud_name, a);      
	 // Assigning size according to size of stud_name    
	 // which is a copy of user provided array a[].    
	 s->struct_size = (sizeof(*s) + sizeof(char) * strlen(s->stud_name));      
	 return s;
 }

Reference

1. flexible-array-members-structure-c
2. http://redisbook.com/preview/sds/implementation.html