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 [];).

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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; }