nested structures with pointer in C

Question

i want to use nested structures with pointers in c. i write this code but i dont know why this code not working. actually how can i allocate memory for my second structure?

#include <stdio.h>
#include <stdlib.h>
struct address
{
    int code;
    char city[10];
};
struct student {
    char name[10];
    struct address *ads;
} *person1;

int main()
{

person1 = malloc(sizeof(struct student));
scanf("%s", person1->name);
scanf("%d", &person1->ads->code);
scanf("%s", person1->ads->city);

printf("%s", person1->name);
printf("%d", person1->ads->code);
printf("%s", person1->ads->city);
return 0;
}

NOTE: when i use "person1->ads = malloc(sizeof(struct address));" program ran to problem and stop working.

[Update from comment:]

I use DEV C++ v5.4.2

You have not allocated memory for the ads member. So you need to add something like person1->ads = malloc(sizeof(struct address)); — Lars Ljung
– Lars Ljung, Commented Jan 11, 2015 at 12:40
For this layout, I wouldn't make ads a pointer in the first place. Just declare it in struct student as struct address ads; and populate it with person1->ads. . — WhozCraig
– WhozCraig, Commented Jan 11, 2015 at 12:52
@WhozCraig you are right; but i want to test this way for myself. — Amir Mohammad Nasrollahi
– Amir Mohammad Nasrollahi, Commented Jan 11, 2015 at 12:56
@AmirMohammadNasrollahi The answers below are correct. You're not doing something right, and its too early/late for my code-clairvoyance to stab guesses. Amend your question with the attempt you currently have that incorporates either answer below. Don't replace your posted code; update the question with an EDIT content that shows exactly what you have now and how you're running it. — WhozCraig
– WhozCraig, Commented Jan 11, 2015 at 12:58

tumdum · Accepted Answer · 2015-01-11 12:41:41Z

4

You need to also allocate memory for members stored via pointer.

int main()
{  
    person1 = malloc(sizeof(struct student));
    person1->ads = malloc(sizeof(struct address));
    scanf("%s", person1->name);
    scanf("%d", &person1->ads->code);
    scanf("%s", person1->ads->city);

    printf("%s", person1->name);
    printf("%d", person1->ads->code);
    printf("%s", person1->ads->city);
    free(person1->ads);
    free(person1);
    return 0;
}

answered Jan 11, 2015 at 12:41

tumdum

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6 Comments

Spikatrix Over a year ago

@AmirMohammadNasrollahi , Change int name[10]; to char name[10]; and it will work.

Amir Mohammad Nasrollahi Over a year ago

@CoolGuy ohhh sorry!! a mistake from me! :)

Amir Mohammad Nasrollahi Over a year ago

@CoolGuy but this not the main issue of this problem. my problem is about allocating memory and still not working...

tumdum Over a year ago

@amir-mohammad-nasrollahi what exactly is not working? Please be more precise.

Spikatrix Over a year ago

@AmirMohammadNasrollahi , I tested the code in TomaszKlak's answer and also changed what I've said in my previous comment and it worked. Try checking the return value of malloc to see if it failed.

|

Iharob Al Asimi · Accepted Answer · 2015-01-11 12:58:40Z

3

You have a few poblems

You don't check the success of malloc.
You don't malloc for person1->ads member.
You don't check the success of scanf.

This is a fixed and anotated version of your code

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

struct address
{
    int code;
    char city[10];
};

struct student
{
    char name[10];
    struct address *ads;
};

int main()
{
    /* You don't need te struct to be global, and it's generally a bad idea, not always of course */
    struct student *person;

    /* you should check that malloc succeeded otherwise undefined behavior would happen */
    person = malloc(sizeof(*person));
    if (person == NULL)
    {
        printf("cannot allocate memory\n");
        return -1;
    }

    /* you should check that scanf succeeded too */
    if (scanf("%9s", person->name) != 1)
    /*          ^ prevent buffer overflow */
    {
        printf("Invalid, input\n");
        free(person);
        return -1;
    }

    person->ads = malloc(sizeof(*(person->ads)));
    if (person->ads == NULL)
    {
        printf("cannot allocate memory\n");
        /* on failure free successfuly allocated person */
        free(person);
        return -1;
    }

    /* you should check that scanf succeeded too */
    if (scanf("%d", &person->ads->code) != 1)
    {
        printf("Invalid, input\n");

        free(person->ads);
        free(person);

        return -1;
    }

    /* you should check that scanf succeeded too */
    if (scanf("%9s", person->ads->city) != 1)
    /*          ^ prevent buffer overflow */
    {
        printf("Invalid, input\n");

        free(person->ads);
        free(person);

        return -1;
    }

    printf("Name: %s\n", person->name);
    printf("Code: %d\n", person->ads->code);
    printf("City: %s\n", person->ads->city);

    free(person->ads);
    free(person);

    return 0;
}

answered Jan 11, 2015 at 12:58

Iharob Al Asimi

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7 Comments

alk Over a year ago

1+ for scaning with limits ... :-)

Iharob Al Asimi Over a year ago

@AmirMohammadNasrollahi عفوًا

Amir Mohammad Nasrollahi Over a year ago

@iharob my language is Persian not ARABIC! i think you try to say 'خواهش میکنم' :)))

Iharob Al Asimi Over a year ago

@alk there are more elegant ways of handling that, but just for ilustrative purposes I think it's ok.

alk Over a year ago

Yes, sure. Just as I was used to ... ;-) @iharob

|

alk · Accepted Answer · 2015-01-11 12:55:41Z

3

how can i allocate memory for my second structure?

For example the same way as you did for the first structure: allocate it from the heap

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

struct address
{
  int code;
  char city[10];
};

struct student 
{
  char name[10];
  struct address * ads;
};

int main(void)
{
  struct student * person1 = malloc(sizeof * person1);
  if (NULL == person1)
  {
    perror("malloc() failed for person1");
  }
  else
  {
    person1->ads = malloc(sizeof * person1->ads);
    if (NULL == person1->ads)
    {
      perror("malloc() failed for person1->ads");
    }
    else
    {
      /* scan and print */

      free(person1->ads);
    } 

    free(person1);
  }
}

edited Jan 11, 2015 at 12:55

answered Jan 11, 2015 at 12:42

alk

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6 Comments

Amir Mohammad Nasrollahi Over a year ago

okey i know!! but how? i test many ways and all of them not working! :D

Amir Mohammad Nasrollahi Over a year ago

good way and i understand it but still have problem with identifying "person1->ads" and stop working. i dont know why really your code not working in this compiler.

alk Over a year ago

@AmirMohammadNasrollahi: Does it work if you comment out the scaning/printing code?

Spikatrix Over a year ago

@AmirMohammadNasrollahi ,"...this compiler" Which compiler?

Amir Mohammad Nasrollahi Over a year ago

this version of C compiler which i use. i use DEV C++ v5.4.2

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