C Struct数组成员没有特定长度

Question

I have encountered this piece of code:

struct test                   
{                                        
 uint32       num_fields;            
 char array_field [];               
}; 

How can I understand array_field ? Is this a gcc extension for the C language?

Answer 1

It's a C99 feature, called flexible array member which is typically used to create a variable length array.

It can only be specified as the last member of a struct without specifying the size (as in array_field []; ).

---

For example, you can do the following and the member arr will have 5 bytes allocated for it:

struct flexi_example
{
int data;
char arr[];
};


struct flexi_example *obj;

obj = malloc(sizeof (struct flexi_example) + 5);

Its pros/cons discussed here:

Flexible array members in C - bad?

Answer 2

Such structures are usually allocated on the heap with a calculated size, with code such as the following:

#include <stddef.h>

struct test * test_new(uint32 num_fields)
{
    size_t sizeBeforeArray = offsetof(struct test, array_field);
    size_t sizeOfArray = num_fields * sizeof(char);
    struct test * ret = malloc(sizeBeforeArray + sizeOfArray);
    if(NULL != ret)
    {
        ret->num_fields = num_fields;
    }
    return ret;
}