CODEWITHSUNDEEP
c
user786
user786

Reputation: 4364

how to pass a pointer to struct to another function in C, in calling function if I have access to only pointer variable

I have a function

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

I like to know how to call this above function from main for example

I am doing it like this

 struct ads *data1=NULL;
 get_alloc_single_struct(&(data1));

But But I am getting a warning and "note:..."

:29: warning: passing argument 1 of ‘get_alloc_single_struct’ from incompatible pointer type [-Wincompatible-pointer-types]
   24 |     get_alloc_single_struct(&(data1));
      |                             ^~~~~~~~
      |                             |
      |                             struct ads **
data.c:14:37: note: expected ‘void **’ but argument is of type ‘struct ads **’
   14 | void get_alloc_single_struct(void **arr)

when compiled with -Wall -Wextra

What I am doing wrong

Upvotes: 0

Views: 66

Answers (3)

user786
user786

Reputation: 4364

thanks for the answers I think I should do it like this

struct ads{
    int id;
    char *title;
    char *name;
};

void* get_alloc_single(size_t bytes)
{
    return malloc(bytes);
}

int main()
{
    struct ads *data1=get_alloc_single(sizeof(struct ads));
    data1->title=get_alloc_single(10);

    strcpy(data1->title, "fawad khan")
    data1->id=102;

    printf("%s %d\n",data1->title,data1->id);
    return 0;

}

Upvotes: -1

4386427
4386427

Reputation: 44330

The function get_alloc_single_struct shall not have a parameter of type void **. The parameter must be struct ads **. So simply do:

void get_alloc_single_struct(void **arr) --> void get_alloc_single_struct(struct ads **arr)

The reason is sizeof...

With a void ** parameter, you end up doing sizeof(void) which is not what you want. You want sizeof(struct ads) so the parameter must be struct ads **.

That said... It doesn't make much sense to write the function like that. Passing a pointer to an actual object is not needed when all you want is to know the sizeof of an object type.

Upvotes: 1

unwind
unwind

Reputation: 399889

The use of types ("double void" i.e. void **) is a bit of a red flag here. If you want to wrap malloc(), the best idea is to keep the same interface.

You cannot both make the interface use void * like that, and have it compute the required size using sizeof, since you explicitly drop the type information when you go void.

So, if you want to drop the size from the call you have to encode it in the function itself, i.e. specialize it:

struct ads * get_alloc_ads(void)
{
  return malloc(sizeof (struct ads));
}

or be a 100% wrap and pass in the size:

void * get_alloc(size_t bytes)
{
  return malloc(bytes);
}

of course in the latter case you could also add logging, exit-on-failure, or other features specific to your application otherwise the wrapping becomes pointless.

Upvotes: 1

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