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Question

hey I am trying to create a program in which I am trying store elements from one array to another with the use of a pointer to pointer but the problem is that is caused undefined behavior I believe that the problem is that I do not pass the elements in members with a proper way I know it is a vague way of doing this but It is in only for practising reasons

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
 typedef struct student{
 
    char *name;
    int *number;    
 
 
 }T;
 
int main(void) {

char array[10][100]={"araaaa","bbgt","gffkghgh"};   

T arr[10][100];
T *p;
T **p1;
p=&arr[0][0];
p1=&p;


int i=0;

for(i = 0 ; i < 3 ; i++)
{   p=arr[i];
    strcpy((*p1)->name,array[i]);
    }

/*******print_elements*************/

for(i = 0 ; i < 3 ; i++)
{   p=arr[i];
    printf("\n the elements are %s",(*p1)-> name);
}


return 0;
}

Answer 1

When you do this:

strcpy ((*p1)->name, array[i]);

(*p1)->name is an uninitialised pointer. What happens, therefore, is in the lap of the gods.

The easiest fix is to modify your student structure such that name is a buffer, rather than a pointer. At the same time, change number to an int , rather than a pointer to an int ::

typedef struct student{
    char name [100];
    int number;    
} T;

---

If you want to keep name as a pointer then you have to allocate some memory before you store your string in it. This should work:

(*p1)->name = strdup (array[i]);

Don't forget to free the memory when done.

Answer 2

T is made of of two pointers, this first one points to a string of characters in memory.

arr is a 2D array that is allocated to store a total of 1000 T structures.

arr[i] would reference a 1D array of T structures within arr

*p1 would essentially be arr[i] , since dereferencing p1 gives you p , which was just set to arr[i] . So, that is not a pointer to a T structure, but to an array of T structures. Forcing the cast will likely give you a reference to the first T structure in that row, however.

->name This value is never set. You allocated an array, but "name" is a pointer to memory, not an array of characters, so '->name' is undefined.

I think you need to change arr to be a single dimension array. You aren't using 90% of it.

And, you need to initialize every T struct in that array. You can use malloc or strdup , and then remember to free them all. Or, set the struct to use an array instead.