malloc of array in struct passed as argument

Question

I would like to allocate memory for arrays that are members of a struct I need to use, inside a function that takes the struct as an argument.

arg->A.size=(int*) malloc(N*sizeof(int));

will not compile (request for member 'size' is something not a structure.

arg->A->size=(int*) malloc(N*sizeof(int));

will throw a segmentation fault error

Any help will be appreciated. Here is the code, thanks:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// struct A 
struct A {
    int dim;                // dimensions
    int* size;              // points per dim array
    double* val;            // values stored in array
    int total;              // pow(size,dim) 
};

// struct B that uses A
struct B {
    int tag;
    struct A* A;
};

int function_AB(struct B* B);

int main(void){
    struct B B;
    function_AB(&B);

    return 0;
}

int function_AB(struct B* arg){
    int N=10;
    arg->tag=99;
    printf("tag assigned = %d \n", arg->tag);
    arg->A->size=(int*) malloc(N*sizeof(int));

    return 0;
}

Answer 1

You simply haven't allocated memory for struct A *A. Before assigning anything to A->size you would first need to do something like

B->A = malloc(sizeof(struct A));

Answer 2

The second case is correct, but crashes because the A inside the B declared in main has not been assigned a value. You probably want something like

struct A A;
struct B B;
B.A = &A;

function_AB(&B);

Answer 3

When you have structure pointer in another structure, first you need to allocate memory for that. then allocate the memory for structure members!

struct B {
    int tag;
    struct A* A;
};

Here A is a pointer to a structure called A. First allocate memory for this, then allocate memory for the elements of struct A

arg->A = malloc(sizeof(struct A));

then do-

arg->A->size = malloc(N*sizeof(int));

Try the following changes in your int function_AB(struct B* arg)-

int function_AB(struct B* arg){
int N=10;
arg->tag=99;
printf("tag assigned = %d \n", arg->tag);

arg->A = malloc(sizeof(struct A)); // First allocate the memory for struct A* A; 

arg->A->size = malloc(N*sizeof(int)); // Allocate the memory for struct A members
arg->A->val = malloc(N*sizeof(double));

// do your stuff

// free the allocated memories here
free(arg->A->size);
free(arg->A->val);
free(arg->A);

return 0;
}

And don't cast the result of malloc()!