struct pointer can contain member string of size known at compile time plus function can be changed to accomodate any type of struct usng void pointer

Question

I have created a simple program. basically I created struct pointer in main and two functions. One function is simply allocating space for ads->title which is char array. Assume at compile time I don't know the sizeof title, the second function is allocating memory for struct so this function can take any type of struct and not just single struct type. But when I compile the program with -Wall Wextra -Werror I get plenty of errors like this

warning: assignment to ‘char’ from ‘void *’ makes integer from pointer without a cast [-Wint-conversion]
11 |     *arr= malloc(sizeof(char)*n);

and many more.

All I am trying to do this with pointers and void pointers

#include <stdio.h>
#include <malloc.h>
struct ads{
    int id;
    char *title;
    char *name;
};

void get_alloc_string(char *arr,int n)
{
    *arr= malloc(sizeof(char)*n);
}

void get_alloc_single_struct(void **arr)
{
    arr=malloc(sizeof(struct ads));
}

int main()
{

    struct ads *data1;
    //data1->id=102;
    get_alloc_single_struct(data1);
    get_alloc_string(data1->title,10);
    data1->title="fawad khan";
    data1->id=102;
    printf("%s %d\n",data1->title,data1->id);


    //get_alloc_string(data1->title);
    return 0;

}

@AbhilekhGautam No [*dont't* cast `malloc`](https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc). — Some programmer dude, Sep 09 '21 at 06:02
Your `get_alloc_string` makes no sense. If you're trying to emulate pass-by-reference (instead of the more natural and simple `return`) you need to pass a pointer to the pointer using the pointer-to operator `&`. When you do `*arr = malloc(...)` you try to assign the pointer returned by `malloc` to `arr[0]` which makes no sense. — Some programmer dude, Sep 09 '21 at 06:03
The warning means: "this isn't valid C, pointers must be assigned to pointers". See ["Pointer from integer/integer from pointer without a cast" issues](https://stackoverflow.com/questions/52186834/pointer-from-integer-integer-from-pointer-without-a-cast-issues) and related: [What must a C compiler do when it finds an error?](https://software.codidact.com/posts/277340), [What compiler options are recommended for beginners learning C?](https://software.codidact.com/posts/282565) — Lundin, Sep 09 '21 at 06:15
It seems like you are attempting some sort of polymorphism. In C, one must be vary specific with that sort of thing. What are you trying to do, exactly? — Neil, Sep 09 '21 at 07:16
@Neil u mean what the type of application. It will keep simple classifieds ad info data program I like to save info. It could be any data. Because I like this idea that I can use in number of other things I like to do — user786, Sep 09 '21 at 08:17
Key-Value string pairs like XML? Will you modify these strings, or just replace them? Is it going to be a map, or multimap? It would be more efficient if the entries were in one place in memory. — Neil, Sep 09 '21 at 19:26

score 3 · Accepted Answer · answered Sep 09 '21 at 06:07

Both allocation functions are wrong, but for different reasons:

void get_alloc_string(char *arr,int n)
{
    *arr= malloc(sizeof(char)*n);
}

That should be:

void get_alloc_string(char **arr,int n)
                        /* ^ extra indirection */
{
    *arr= malloc(sizeof(char)*n);
}

As you want to return the pointer.

void get_alloc_single_struct(void **arr)
{
    arr=malloc(sizeof(struct ads));
}

That should be:

void get_alloc_single_struct(void **arr)
{
    *arr=malloc(sizeof(struct ads));
 /* ^ extra indirection */
}

As you want again to return the pointer.

Personally, I'd change the function to something like this:

char *get_alloc_string(size_t n)
{
    return malloc(sizeof(char)*n);
}

And similar for get_alloc_single_struct().

What if I don't know the type of struct ads at compile time in get_alloc_single_struct function? — user786, Sep 09 '21 at 06:23

struct pointer can contain member string of size known at compile time plus function can be changed to accomodate any type of struct usng void pointer

1 Answers1