dereferencing pointer to incomplete type

srinietrx · 12-12-2017, 05:17 AM

Hi,
I am writing simple c program using structure and passing structure to function

Code:

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

void FunctionCall(void* tag);
int main()
{
struct s_tag
{
        int a;
        int *ptr;
};
struct s_tag *tag;

        tag = malloc(sizeof(struct s_tag));

        tag->a= 1;
        int b = 2;
        tag->ptr = &b;

        FunctionCall((void*)&tag);

        return 0;
}

void FunctionCall(void* tag)
{
        struct tag *my_tag =  (struct tag*)tag;
        printf("tag->a = %d\ttag->ptr = %d\n", my_tag->a, my_tag->ptr);
}

Getting error as
error: dereferencing pointer to incomplete type

Thanks

NevemTeve · 12-12-2017, 05:39 AM

Indeed, 'struct tag' is undefined.
Guess you wanted something like this:

Code:

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

typedef struct s_tag
{
        int a;
        int *ptr;
} s_tag;

static void FunctionCall (const s_tag *tag);

int main()
{
	struct s_tag *tag;

        tag = malloc(sizeof(struct s_tag));

        tag->a= 1;
        int b = 2;
        tag->ptr = &b;

        FunctionCall(tag);

        return 0;
}

void FunctionCall (const s_tag *my_tag)
{
        printf("tag->a = %d\ttag->ptr = %p\n", my_tag->a, (void *)my_tag->ptr);
}

sundialsvcs · 12-12-2017, 08:27 AM

But also: since you know that it is, and always will be, "a pointer to s_struct," you should declare it that way, not as void *.

You should declare a "void pointer" only when you literally do not know what sort of thing this pointer points to, and/or when it must be unpredictable and/or in the occasional edge-case where you really don't care. You want the compiler to be able to do just as much compile-time error checking as it possibly can.

srinietrx · 01-12-2018, 07:25 AM

Hi,
I corrected the issue myself immediately after posting. Sorry I could not reply immediately.

Regarding the answer. I did same as NevemTeve it is working i.e.declared structure outside the main.

Thanks NevemTeve,

sundialsvcs:-
In my application I was passing this Structure argument to thread. In every example program it is written as void*. So I did the same.
But I will correct it.