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cpointersdynamic-memory-allocation

What is the right way to initialize double pointer in c

As title, I want to know how to initialize double pointer with sizeof a pointer.

For instance

int **p=malloc(sizeof *p * rows);

for(size_t i = 0; i < rows; i++){
    p[i]=malloc(sizeof ? * cols);
}

What should I fill in ?.

Any help would be appreciated.

Solution

  • What should I fill in ?.

    In general when you have

    X = malloc(sizeof ? * NUMBER);

    the ? is to be replaced with the type that X points to. That can simply written as *X.

    So the line:

    p[i]=malloc(sizeof ? * cols);

    is to be:

    p[i]=malloc(sizeof *p[i] * cols);

    Notice that a 2D array can be created much simpler. All you need is

    int (*p)[cols] = malloc(sizeof *p * rows);

    Here p is a pointer to an array of cols int. Consequently sizeof *p will be the size of an array of cols int.

    Using this VLA based technic means that you can allocate the 2D array using a single malloc. Besides making the code more simple (i.e. only 1 malloc) it also ensures that the whole 2D array is in consecutive memory which may give you better cache performance.