Question

I am using a variable-sized C struct, as follows:

typedef struct {  
  int num_elems;  
  int elem1;  
} mystruct;  
// say I have 5 elements I would like to hold.  
mystruct * ms = malloc(sizeof(mystruct) + (5-1) * sizeof(int));  
ms->num_elems = 5;  
// ... assign 5 elems and use struct  
free(ms);

Will this last free() free everything that was malloc’d, or only sizeof(mystruct)?

Answer 1

Yes. This will free the whole block that was allocated using malloc.

If you allocate a single block of memory using malloc (like you do in your example), then you need to call free exactly once to free that entire block.