c - malloc typedef'd pointer

Question

Given a typedef'd ptr type, how can I access the base type?

Eg consider an "array" of doubles:

typedef double* vec_t;

If I try to initialise like this:

void init_vec(int n, vec_t v){
    v = (vec_t)malloc(n*sizeof(*vec_t));
    //...
}

gcc doesn't exactly like that

error: expected expression before 'vec_t'

v = (vec_t)malloc(n*sizeof(*vec_t));

  ^ 

What would be the proper way to do this?

I could, of course, simply write v = (vec_t)malloc(n*sizeof(double)); , but that's not what I'm looking for.

Answer 1

First, don't cast the result of malloc() in C. It returns a void pointer, which will be safely promoted to vec_t * .

Second, *vec_t is an attempt to dereference a type, which is semantically invalid. You need to dereference a variable of type vec_t . Conveniently, you have one.

v = malloc(n * sizeof *v);

Edit: As @chqrlie pointed out, the malloc() 'd memory is immediately lost on return. The allocated address is assigned to v , but the calling function's copy of v is not overwritten.

Answer 2

There is a problem with void init_vec(int n, vec_t v) : the function cannot return the allocated pointer to the caller. It should either take a pointer to the vec_t , or return a value of type vec_t .

A simple solution is:

vec_t init_vec(int n) {
    vec_t v = malloc(n * sizeof *v);
    //...
    return v;
}

Hiding pointers behind typedefs is not recommended, it tends to make the code less readable and more error prone. Your vec_t type should really be a structure that contains a size member for the size of the array and a pointer to double or a flexible array. You could also distinguish between the allocated length size and the used portion length :

typedef struct vec_t {
    size_t size, length;
    double a[];
} vec_t;

vec_t *init_vec(size_t n) {
    vec_t *v = malloc(sizeof *v + n * sizeof(*v->a));
    v->size = v->length = n;
    for (size_t i = 0; i < n; i++) {
        v->a[i] = 0;
    }
    return v;
}

Finally, since init_vec allocates the vector, it should be given to a more explicit name such as create_vec , new_vec or allocate_vec .