Usage of Structures in C

Question

I am reading the script on the implementation of malloc (first-fit), and I am a little confused about the value assignment of metadata structure. Could anyone give some explanations why the malloc returns flag_block->ptr (as a pointer to the allocated memory)? As far as I can see, there is no specific assignment to it.

typedef struct _metadata {
    size_t size;
    char free;
    struct _metadata* next;
    struct _metadata* prev;
    char ptr[];
} metadata;

metadata* flag_block = NULL; 

void *malloc(size_t size)
{
    if (size==0) {
        return NULL;
    }

    if (flag_block == NULL) {
        flag_block = sbrk(size);
        sbrk(sizeof(metadata));
        if (flag_block == (void *)-1) {
            return NULL;
        }
        flag_block->free = 0;
        flag_block->next=NULL;
        flag_block->prev=NULL;
        flag_block->size = size;
        return flag_block->ptr;
    } else {

    /* 
        ....
    */

    }
}

Answer 1

The ptr is called a flexible array member; it's an array without a size, and can only appear at the end of a struct.

So basically this:

return flag_block->ptr;

is equivalent to

return &flag_block->ptr[0];

So it's returning the address of the first byte after the rest of the members in the struct.