creating an array containing pointers - c

Question

I'm trying to create an array of pointers in C. Each value of the array should be a pointer to a struct (let's call it struct Type*).
Should i do
struct Type* myVariable= malloc(sizeof(struct Type*)*MY_SIZE);
or
struct Type** myVariable= malloc(sizeof(struct Type*)*MY_SIZE);
The second one looks like what i should do when i want to create a two dimensional array, which are an array of pointer, and those pointers are used to create arrays of the wanted type.
EDIT : But in my case the second dimension size would be only one
The first one looks like a regular array with int* as the contained values type.
How can i pass the good solution to a function (by pointer, not by value because the array may be fairly large) and use it in the fonction ?

Answer 1

The second one the right solution. However, you'll need to allocate memory for the objects too. Also, make sure to check the value returned by malloc.
// Allocate memory for the array of pointers.
struct Type** myVariable = malloc(sizeof(struct Type*)*MY_SIZE);
if ( myVariable == NULL )
{
// Deal with error
exit(1);
}
for (int i = 0; i < MY_SIZE; ++i )
{
// Allocate memory for the array of objects.
myVariable[i] = malloc(sizeof(struct Type)*THE_SIZE_IN_THE_OTHER_DIMENSION);
if ( myVariable[i] == NULL )
{
// Free everything that was allocated so far
for (int j = 0; j < i-1; ++j )
{
free(myVariable[j]);
}
free(myVariable);
// Exit the program.
exit(1);
}
}
However, if THE_SIZE_IN_THE_OTHER_DIMENSION is going to be 1, you are better off using your first approach.
struct Type* myVariable = malloc(sizeof(struct Type)*MY_SIZE);
// ^^^^^^^^^^^ Drop the *
if ( myVariable == NULL )
{
// Deal with error
exit(1);
}

Answer 2

Neither!
Use an idiom that reduces work and errors
pointer = malloc(sizeof *pointer * Number_of_elements);
Or in OP's case "to create an array of pointers in C"
#define ARRAY_N 100
struct Type **myVariable = malloc(sizeof *myVariable * N);
if (myVariable == NULL) Handle_OutOfMemmory();
Now set those pointers to some value
#define M 50
size_t i;
for (i=0; i<N; i++) {
myVariable[i] = malloc(sizeof *(myVariable[i]) * M);
if (myVariable[i] == NULL) Handle_OutOfMemmory();
for (size_t m = 0; m<M; m++) {
// Initialize the fields of
myVariable[i][m].start = 0;
myVariable[i][m].value = 0.0;
myVariable[i][m].set = NULL;
}
}