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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;
}
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1
  • The first one is correct as that one allocates the correct number of bytes to hold the structure. The other two just allocates memory that is the size of a pointer: Either 4 or 8 bytes. You don't really need (struct X*) on the malloc as the result is compatible with all pointer types. Although it will work (and is a style issue), the conventional way to declare a pointer to a struct is struct X x*. Commented Jul 22, 2021 at 15:36

2 Answers 2

Reset to default
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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 '&'.

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2 Comments

Maybe the basic of pointers and strings would be more useful here, no ?
This did cross my mind too, so yes very likely.
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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".

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1 Comment

It seems to me that you should elaborate a little on static string versus dynamic allocation of the strings, as the provided code may not be intended to deal with const string. Explain for example that a char pointer is at first a pointer and can then point anything (static strings in the program text section, dynamic allocation, string address in a ROM, etc.). Thus, assign a static string to s is fine but implies there should be no more string manipulation from s except if the static assignment was meant to "allocate" static space, etc.

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