How can I make the code about structure and pointer-CodePudding

#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.

CodePudding user response：

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

CodePudding user response：

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.