Allocating pointers inside an array of structs

Question

OK, so the problem is basically like the title. Couldn't find a question that got all bits of it, so I figured I'd ask.

Say I want an array of structs shaped like the following

typedef struct s_woo{
    char** n;
    char* x;
} t_woo;

So I believe I should do

t_woo* woos = malloc(num_woos * sizeof(*woos));

Seems simple enough (and should deter people from yelling at me for my habit of casting malloc).

Then I want to initialize the things in each of those structs.

So intuitively I do:

for(i = 0; i < num_woos; i++){

    num_ns = randomint1 / randomint2; //let's say num_ns is big, like 250-ish, average, and changes every round of the loop

    woos[i].n = malloc(num_ns * sizeof(char*));
    woos[i].x = malloc(num_ns * sizeof(char));
    for(j = 0; j < num_ns; j++){
        woos[i].n[j] = malloc(16 * sizeof(char)); // I just want 16 characters per char*
    }
}

This is the boiled down version of what I have in my code. I want to know what can possibly go wrong with what I've written - like any possible thing. I'm not looking for anything in particular, just general problems with the above, like memory/heap issues, pointer mistakes etc.

Leave out "Virtual Memory Exhausted". My code error checks for that using a wrapper function on malloc, so I'm very sure that's not it.

Looks fine from here. It would help avoid coding errors to use a define/constant instead of 16. I don't know if you intend to store strings in these arrays but if so, don't forget about the null terminator. — M.M
– M.M, Commented Mar 31, 2014 at 5:14
If you're going for robust design you could use a bit more encapsulation, i.e. turn t_woo into a self-contained object. This would involve putting num_ns insde t_woo, and having a function which does all the construction for a single t_woo (and another one that does the destruction later). Then you loop through your list of t_woo's calling the construction function on each one, as soon as possible after you allocate the memory for them. — M.M
– M.M, Commented Mar 31, 2014 at 5:17
and changes every round of the loop.. Are you deallocating memory before allocating in each round of loop? Maybe your outside loop runs so many times, that it gives a memory error. — HelloWorld123456789
– HelloWorld123456789, Commented Mar 31, 2014 at 5:35
@MattMcNabb Thanks, I may do just that with the encapsulation. This works in C? — user3475234
– user3475234, Commented Mar 31, 2014 at 6:39

Gábor Buella · Accepted Answer · 2014-03-31 06:35:56Z

1

Even better:

 static const size_t woo_n_size = 16; 
  /* To make sure you use 16 everywhere,
   * also easier to change it
   */

 struct woo_item {
     char n[woo_n_size];
     char x;
 };

 struct s_woo {
     struct woo_item *items;
     size_t size; / * optinal, to keep track of item count */ 
 }

With the woo_item struct you can make sure there is no x without n[woo_n_size] allocated, and vice versa. You can remember the count of woo_items by having a designated null element to close each of your lists, or just store a size member in s_woo

answered Mar 31, 2014 at 6:35

Gábor Buella

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