Can someone explain this..??

Could not able to understand in line 13 that how a pointer function is assigned to a struct . And on line 15 *who is allocated with some memory space . normally a pointer is assigned to an address but how this way serves the purpose... By the way it is from the book Learn code the hard way...Please shed some light...

Code:

1  #include <stdio.h>
2  #include <assert.h>
3  #include <stdlib.h>
4  #include <string.h> 
5  
6  struct Person {
7      char *name;
8      int age;
9      int height;
10      int weight; 
11  };
12  
13  struct Person *Person_create(char *name, int age, int height, int weight) 
14  {
15      struct Person *who = malloc(sizeof(struct Person)); 
16      assert(who != NULL);
17  
18      who->name = strdup(name);
19      who->age = age; 
20      who->height = height;
21      who->weight = weight;
22  
23      return who;
24  } 
25  
26  void Person_destroy(struct Person *who)
27  {
28      assert(who != NULL);
29   
30      free(who->name);
31      free(who);
32  }
33  
34  void Person_print(struct Person *who) 
35  {
36      printf("Name: %s\n", who->name);
37      printf("\tAge: %d\n", who->age); 
38      printf("\tHeight: %d\n", who->height);
39      printf("\tWeight: %d\n", who->weight); 
40  }
41  
42  int main(int argc, char *argv[])
43  {
44      // make two people structures 
45      struct Person *joe = Person_create(
46              "Joe Alex", 32, 64, 140);
47   
48      struct Person *frank = Person_create(
49              "Frank Blank", 20, 72, 180); 
50  
51      // print them out and where they are in memory 
52      printf("Joe is at memory location %p:\n", joe);
53      Person_print(joe);
54   
55      printf("Frank is at memory location %p:\n", frank);
56      Person_print(frank); 
57  
58      // make everyone age 20 years and print them again 
59      joe->age += 20;
60      joe->height -= 2;
61      joe->weight += 40; 
62      Person_print(joe);
63  
64      frank->age += 20;
65      frank->weight += 20; 
66      Person_print(frank);
67  
68      // destroy them both so we clean up 
69      Person_destroy(joe);
70      Person_destroy(frank);
71  
72      return 0;
73  }

I tried to replace that function with the my understanding and the code works fine.....

Code:

int create_profile(char *name,int age,int height,int weight)
{
    struct Nani *item = malloc(sizeof(struct phani));
    assert(item != NULL);

    item->heissen = name;
    item->alt     = age;
    item->grosse  = height;
    item->gewicht = weight;



   return item;
}

Thanks n Regards
Max

The function Person_create creates a new struct person with the arguments passed to the function and returns a pointer to the new struct with all members filled in with the arguments. The malloc in line 15 allocates memory for the struct person that will be returned. What is it you don't understand exactly?

Edit: regarding your function, you are sharing *name with the struct and where ever that originated. The first code uses strdup() which creates a new unique string that is not depending on the argument string. If that string is local, or is deallocated then item->heissen will also disappear.

Line 13:

Code:

struct Person *Person_create(char *name, int age, int height, int weight)

says that the Person_create function returns a pointer to struct Person.

Hi Subsonics,
I dont understand how the function Person_create creates a new struct person with the arguments passed to the function and why malloc is used to allocate memory .... And i forgot to use strdup with my function and im aware about its usage ...

Normally, i have noticed something like passing an address
[code]
struct person profile;
struct Person *who = &profile

[\code]

Thank you
Max

Originally Posted by seemaxie

Normally, i have noticed something like passing an address

Code:

struct person profile;
struct Person *who = &profile

Yes, that will pass the address to "struct person profile". However "who" is just a second reference to "struct person profile" you do not get two instances of "struct person profile" by doing this. Also if this is declared inside a function then "struct person profile" is local.

If you want to create an arbitrary amount of "struct persons" you need to do it manually with that method. Let's say you want 1.000 "struct persons" from a file, then you can read the details from the file and call Person_create in a loop.

Im breaking my head to get this concept but if i want 1000 struct person i would do this
struct person profile[1000];

Originally Posted by seemaxie

Im breaking my head to get this concept but if i want 1000 struct person i would do this
struct person profile[1000];

What I really meant was "arbitrary amount", 1.000 was an example, consider a situation where you do not know how many struct persons you will need. Also, creating an array of struct person like that is local to the function that creates it, unless you create a global variable.

Originally Posted by seemaxie

Hi Subsonics,
I dont understand how the function Person_create creates a new struct person with the arguments passed to the function and why malloc is used to allocate memory

That's kind of the question and the answer, right there.

The 'how' is malloc, and the 'why' is to create a new 'person'.

There is no "Normally" with a pointer. They are frequently used to hold heap allocated objects, and they are frequently used to refer to non-owned objects such as an objects parent.
They are very seldom used to point to a local variable. Contrary to what you may have seen, that practice is usually limited to examples given to beginners.

Thanks all , special thanks to subsonics .. i lacked the understanding of "returning a pointer to an another pointer" for example

Code:

int m = 30,*j,*g;
m= &j;
g = m; (i have no idea that doing this points to variable j so the m)

[/code]

I have made few changes to program and could able to print the values in a struct . Someone please help me out..

Code:

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

struct profile{
    char *heissen;
    int alt;
    int grosse;
    int gewicht;
};

struct profile *create_profile(char *name,int age,int height,int weight)
{
    struct profile *who ; // Made changes replacing malloc
    who->heissen = name;
    who->alt     = age;
    who->grosse  = height;
    who->gewicht = weight;

    return who;
}


void print_profile(struct profile *who)
{
    printf("%s name \n= ",who->heissen);
    printf("%d alt\n",who->alt);
    printf("%d grosse\n",who->grosse);
    printf("%d gewicht\n\n",who->gewicht);

}

int main()
{
    struct profile *details = create_profile("Maxie", 24,6,68);
    print_profile(details);

return 0;
}

I just cant understand why is that code is not able to execute . I tried checking the behavior of program by removing the malloc but dont understand why is that it is not working..

Originally Posted by seemaxie

I just cant understand why is that code is not able to execute .

You simply never created a struct profile object. Look at this:

Code:

struct profile *who;
who->heissen = name;

So, you declared a pointer, named who, to a struct profile. This pointer was not initialised, so it does not actually point to a struct profile object. Next, you assign name to the heissen member of the struct profile object that who points to. But who does not point to a struct profile object!

Thank you laserlight ..!! Enlightened ..!!