Question

#include<stdio.h>

struct a
{
    void *ptr;
    unsigned long val;
};

void main()
{
    unsigned char errno;

    struct a *id;
    id = malloc(sizeof(*id));
    func2(id);
    printf("After changing %d\n", id->val);
}


void func2(struct a *id)
{
    unsigned char errno;
    func(id,&errno);
}

void func(struct a *id,void *ptr)
{   
    memset(id, 0, sizeof(*id));

    id->ptr = ptr;
    if (sizeof(id->val) >= sizeof(id->ptr))
    {   
        id->val = (unsigned long)id->ptr;
        return;
    }

}

when i am printing id->val in main function it is printing -1075050593
. But i am trying to access a invalid address. Please explain. I am very new to c programming.

Answer 1

The following statement is not attempting to access an invalid address, it is just printing the address:

printf("After changing %d\n", id->val);

what value it will hold is unknown as id is being incorrectly passed to func2().

Some problems with the code (some already mentioned in the comments):

• incorrectly taking address of id when passing to func2() as id is already a struct a*
• implicit declarations of memset() and malloc() as missing include directives
• implicit declarations of func() and func2()
• return type of main() should be int