gcc - C Struct array member without specific length

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?