Question

I have the following code which simply prints out an introduction for a person’s name.

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

typedef struct {
    char* firstname;
    char* lastname;
}Person;

void intro(void *person){
    printf("The person you are looking for is %s %s\n", ((Person *)person)->firstname, ((Person *)person)->lastname);
}

int main()
{
    Person *a = NULL;
    a = (Person *)malloc(sizeof(Person));

    char *first = NULL, *last = NULL;
    first = (char *)malloc(sizeof(char)*20);
    strncpy(first,"Bob", 20);
    last = (char *)malloc(sizeof(char)*20);
    strncpy(last,"Newmonson", 20)
    a->firstname = first;
    a->lastname = last;

    intro(a);

    return 0;
}

Produces the output

The person you are looking for is Bob Newmonson

However changing intro(a) to intro(&a) produces

The person you are looking for is �@ Newmonson

When I open the first attempt in GDB and break on line 10 I find the address of person=0x601010. Both the first name and last name are stored where I would expect, 0x04006b9 and 0x4006bd since they where declared earlier in the stack.

What gets me is when I run GDB with the changes made to intro(&a). The address of person is now 0x7fffffffffdd38, with the first name pointing to 0x601010 and the last name pointing to 0x4006db.

Can anyone help explain to be what is going on and why I can still access the proper address of the last name in the second test.

EDIT :

As everyone seems to keep asking about it the void * was for a threading portion of this code that I did not include.

Answer 1

It’s because a is already pointer to a Person structure; therefore intro(&a) passes a pointer to that pointer, but intro() treats it’s argument as a pointer to Person.

Also, if intro() is intended to work on a Person, it should declare a Person * argument, not a void *.