How can I make the code about structure and pointer

Question

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

typedf struct
{
    int a;
    int b;
} STR;

int main(void)
{
    STR *arr[4];
    STR **ptr=arr;
    for(int i=0; i<4; i++)
    {
        arr[i]=(STR*)malloc(sizeof(STR));
        (*arr[i]).a=i+7;
        (**(ptr+i)).b=i+3;
    }
    printf("%d\n", ptr[2]->a);
    printf("%d", (*(arr+3))->b);
    return 0;
}

How can I make the correct code and it can compile, also I want to know what is wrong at my code.

Answer 1

Apart from the misspelled typedef and that it leaks memory, it's not wrong. It's however doing the dereferencing in overly complex ways.

Suggested simplifications and notes:

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

#define Size(x) (sizeof (x) / sizeof *(x))  // to not have to hardcode `4` in the loops

typedef struct {
    int a;
    int b;
} STR;

int main(void) {
    STR *arr[4];
    STR **ptr = arr;                          // same as: &arr[0];
    
    for (size_t i = 0; i < Size(arr); i++) {  // use the helper macro
        arr[i] = malloc(sizeof *arr[i]);      // same as: sizeof(STR)
        arr[i]->a = i + 7;                    // simpler dereferencing
        ptr[i]->b = i + 3;                    // simpler dereferencing
    }
    
    printf("%d\n", ptr[2]->a);                // simpler dereferencing
    printf("%d\n", arr[3]->b);                // simpler dereferencing

    // and finally, free the allocated memory:
    for (size_t i = 0; i < Size(arr); i++) {  // use the helper macro again
        free(arr[i]);
    }
}

Output:

9
6

Answer 2

You misspelled typedef . Otherwise, it should work. Other criticisms:

• Why are you casting the return value of malloc() ?
• ptr + i == &arr[i]
• If your compiler doesn't support C99, you'll need to remove the initial declaration from the for loop.