CODEWITHSUNDEEP
cpointersmalloc
user16464042
user16464042

Reputation:

Struct pointer and its pointer attribute issue

Has this code undefined behaviour which means for s is mandatory to allocate memory or is ok this way ? PS: what is the difference between

 struct X* x = (struct X*)malloc(sizeof(struct X));
      and
    struct X* x = (struct X*)malloc(sizeof(x)); 
    and 
    struct X* x = (struct X*)malloc(sizeof *x);

Thank you.

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

struct X 
{
    int x;
    char* s;
};
int main()
{  
    struct X* x = (struct X*)malloc(sizeof(struct X));
    x->x = 10;
    // x->s = (char*)malloc(10);
    // memcpy...
    x->s = "something";
    printf("is ok?");

    return 0;
}

Upvotes: 0

Views: 93

Answers (2)

Geoff
Geoff

Reputation: 11

Rather than throw my own interpretation at you i felt it would be more helpful to share a link that might clarify what you are aiming to achieve:

https://www.geeksforgeeks.org/new-vs-malloc-and-free-vs-delete-in-c/

When you create a pointer i see that you have added the pointer to your char* variable / struct, but when calling them the use of the ampersand & is used as a reference to the address in the memory.

But not applied quite right using the int variable when declaring it no '*' and then referencing the location using '&'.

Upvotes: 1

David Schwartz
David Schwartz

Reputation: 182761

This is fine. Since s is part of the struct, allocating memory for the struct allocates memory for s. I would strongly suggest changing the type of s to be a const pointer, since it points to a literal which, because it's a type of constant, cannot be modified.

You cannot do s[0]='n'; after this. You did not allocate any space to hold any string other than the unmodifiable literal "something".

Upvotes: 0

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