C programming doubt

Question

I am getting segmentation fault for the following code.Please help to rectify.
 

#include <stdio.h>
#include <stdlib.h>
struct person {
   int age;
   float weight;
   char *name;
};

int main()
{
     struct person *ptr;
     int i, num;

       printf("Enter number of persons: ");
   scanf("%d", &num);
    
     ptr = (struct person*) malloc(num * sizeof(struct person));
   // Above statement allocates the memory for n structures with pointer personPtr pointing to base address */

      ptr->name=(char*)malloc(num*sizeof(char));

      for(i = 0; i < num; ++i)
   {
       printf("Enter name, age and weight of the person respectively:\n");
       scanf("%s%d%f",&ptr->name[i], &(ptr+i)->age, &(ptr+i)->weight);
   }

     printf("Displaying Infromation:\n");
      for(i = 0; i < num; ++i)
       printf("%s\t%d\t%.2f\n", ptr->name[i], (ptr+i)->age, (ptr+i)->weight);

   return 0;
}

Comments

it is not giving any error.

https://ideone.com/rXN67S

where u running the code?

---

Its giving error in runtime giving parameter.Run the code with parameters, output is not giving properly.

Answer 1

There are some conceptual problems. Let me explain your code only. 

#include <stdio.h>
#include <stdlib.h>
struct person {
   int age;
   float weight;
   char *name;
};
// You have defined struct correctly Now move further

int main()
{
    struct person *ptr;
    int i, num;

    printf("Enter number of persons: ");
    scanf("%d", &num);
    // Here you have taken the number of persons, whos record you are going to 
    // create. Cool upto this.
    
    ptr = (struct person*) malloc(num * sizeof(struct person));
    // Above statement allocates the memory for n structures with pointer personPtr pointing to base address
    // Abosolutely above line do that
    
    ptr->name=(char*)malloc(num*sizeof(char));
    // In the above line you are trying to create assign memory to the pointers
    // which are pointing to name "name" field. Here you have done a mistake. See 
    // your code is only assigning memory to the first structure variable.
    
    
    for(i = 0; i < num; ++i)
    {
       printf("Enter name, age and weight of the person respectively:\n");
       scanf("%s%d%f",&ptr->name[i], &(ptr+i)->age, &(ptr+i)->weight);
       // Here in scanf, you again done mistake..
   }

     printf("Displaying Infromation:\n");
      for(i = 0; i < num; ++i)
       printf("%s\t%d\t%.2f\n", ptr->name[i], (ptr+i)->age, (ptr+i)->weight);
        // Here again make some changes
   return 0;
}

Here, i have corrected this

#include <stdio.h>
#include <stdlib.h>
struct person {
   int age;
   float weight;
   char *name;
};

int main()
{
    struct person *ptr;
    int i, num;

    printf("Enter number of persons: ");
    scanf("%d", &num);

    ptr = (struct person*) malloc(num * sizeof(struct person));

    for(i = 0; i < num; i++){
           ptr[i].name = (char*)malloc(50*sizeof(char));
    }


    for(i = 0; i < num; ++i)
    {
       printf("Enter name, age and weight of the person respectively:\n");
       scanf("%s %d %f",ptr[i].name, &ptr[i].age, &ptr[i].weight);
   }
    printf("Displaying Information:\n");

    for(i = 0; i < num; ++i)
        printf("%s\t%d\t%.2f\n", ptr[i].name, ptr[i].age, ptr[i].weight);

   return 0;
}

Enjoy :) :) 

Comments

It is not giving actually in run time its terminating before printing anything.Please edit your code with this notation if you can.

---

@rude,can you please explain in simple words that what is happeing in this line?

ptr->name=(char*)malloc(num*sizeof(char));//code given by pranabhesh

and why do u  need to allocate memory in a loop to correct this??

please explain this part.:)

---

explained a little bit as a second answer. @akriti.

Answer 2

Here is one version of the corrected code:

#include <stdio.h>
#include <stdlib.h>
#define NAME_SIZE 30
struct person {
   int age;
   float weight;
   char *name;
};

int main() {
    struct person *ptr;
    int i, num;

    printf("Enter number of persons: ");
    scanf("%d", &num);
    
    // ptr is pointer 
    ptr = (struct person*) malloc(num * sizeof(struct person));


    // we must allocate memory for char* member of person struct
    // in case of  char *p = "hello" it is ok !
    // but in case of runtime scanf("%s",p) it will not work without memory allocation
    //
    // because there must be some valid place to write the std_input string
    // Here char *p = "hello" compiler takes care of string literal to 
    // store in a read only memory
    // but in runtime scanf("%s",p) : there is no role of compiler : we must have some 
    // memory to write into
    for(i=0;i<num;i++) {
        ptr[i].name = malloc(NAME_SIZE*sizeof(char));
    }

    printf("Enter name, age and weight of the person respectively:\n");
    
    // ptr[i]  = *(ptr+i) = is a person type varibale stored in (ptr+i) location

    for(i = 0; i < num; ++i) {
        scanf("%s%d%f",ptr[i].name, &(ptr[i].age), &(ptr[i].weight));
    }

    printf("Displaying Infromation:\n");

    for(i = 0; i < num; ++i)
        printf("%s\t%d\t%.2f\n", ptr[i].name, ptr[i].age, ptr[i].weight);

   return 0;
}

Output :

Enter number of persons: 2
Enter name, age and weight of the person respectively:
abc
12
20.5
xyz
13
21
Displaying Infromation:
abc	12	20.50
xyz	13	21.00

---

Another version :

#include <stdio.h>
#include <stdlib.h>
#define NAME_SIZE 30

struct person {
   int age;
   float weight;
   char *name;
};

int main() {

    struct person *ptr;
    int i, num;

    printf("Enter number of persons: ");
    scanf("%d", &num);
    
    struct person * db[num];

    printf("Enter name, age and weight of the person respectively:\n");

    i = 0;
    while(i < num) {
      db[i] = malloc(sizeof(struct person));
      db[i]->name = malloc(NAME_SIZE*sizeof(char));
      scanf("%s%d%f",db[i]->name, &(db[i]->age), &(db[i]->weight));
      i++;
    }

    printf("Displaying Infromation:\n");
    
    for(i = 0; i < num; ++i)
        printf("%s\t%d\t%.2f\n", db[i]->name, db[i]->age, db[i]->weight);

   return 0;
}
 
//output :
Enter number of persons: 2
Enter name, age and weight of the person respectively:
abc
12 
20.5
xyz
13
21
Displaying Information:
abc	12	20.50
xyz	13	21.00

---

But none of the above code is leak free:

So we need to de_allocate memory from heap:

for the first version :

    // add this piece of code
    for(i=0;i<num;i++) {
        free((ptr+i)->name);
        free(ptr+i);
    }

And for the second version :

    // add this piece of code
    for(i=0;i<num;i++) {
      free(db[i]->name);
      free(db[i]);
    }

We can use  valgrind ./a.out to check memory statistics of the program executable a.out

Comments

thanks debashish  for explaining.

i have some doubt now,in first version,you used '.' and in second you used  ' -> '.although both are pointers.

ptr is a pointer of type person in version 1.and db is pointer to an array of type person.

in below part of version 2,i want to ask that why do we needx to allocate memory for each index of array.cant we simple assign memory in one line to db??whats the difference it makes??

while(i < num) {
      db[i] = malloc(sizeof(struct person));

  db[i]->name = malloc(NAME_SIZE*sizeof(char));

---

Well, the purpose of giving the second version was to distinguish the difference which causes your first doubt.

struct person * db[4];

means db is an array of $4$ pointers of type person struct.

Therefore db[i] is a pointer so that we can assign malloc result to it directly.

---

However,

struct person * ptr;

ptr is just a pointer.

Similar to int * a.

So when we do 

ptr = malloc(SIZE * sizeof(struct person)) it is similar to a = malloc (SIZE*sizeof(int))

Now ptr[i] is actual content of the location ptr+i .... similarly a[i] is the actual integer value inside location a+i.

Therefore  ptr[i] is not a pointer so that we should use dot(.) operator , while in case of db[i] we can use arrow(->) operator (but not necessarily ).

Hopefully it clears your second doubt.

---

i got it..thanks debashish :)