array pointers to structures

Question

I wrote this program:

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

struct inventory{
    int ID;
    char name[20];
    int value;
};


int main()
{
    struct inventory *inv;

    inv = malloc(sizeof(1));

    inv[0].ID = 10;
    strcpy(inv[0].name,"hello charlie old mate");
    inv[0].value = 20;

    inv[1].ID = 20;

    printf("%d", inv[1].ID);

    return 0;
   }

Can you tell me how can inv[1].ID be set up to 20. When I allocated just 1 byte of memory for inv . How can it carry data of multiple structures?

Answer 1

There is undefined behaviour in your code -

inv = malloc(sizeof(1));      // you allocate size of int
inv[0].ID = 10;
strcpy(inv[0].name,"hello charlie old mate");    //copying more than your buffer can hold

You allocate size equal to that of int and is not enough for structure and access unauthorized memory.And then the strcpy , you try to store more contents than the space available. name can contain 19 characters and '\\0' at end. And the string you copy is larger then 19 characters.

Answer 2

Can you tell me how can inv[1].ID be set up to 20. When I allocated just 1 byte of memory for inv. How can it carry data of multiple structures?

TL;DR It cannot.

What you're seeing is invalid access of memory which invokes undefined behavior . There is nothing in C standard that prevents you from from writing a code accessing invalid memory, but as soon as you do that, voila!!

The memory allocated to inv (by calling malloc(1) ) is way less then it should be. Thus, basically, you're trying to access memory that does not belong to you (your process) and hence that memory is invalid . Any attempt to access invalid memory results in UB.

Following the same trail, even after you have allocated proper memory for inv , then also

 strcpy(inv[0].name,"hello charlie old mate");

will be UB, as you're trying to copy more than 20 elements into the destination having only size of 20 (which can hold 19 valid chars + 1 null terminator, if you want name to be used as a string ). Count your memory requirement properly and stay within bounds.

That said, always check for the return value of malloc() for success before using the returned pointer.