Thread: dynamic memory allocation

I have allocated dynamic memory for array and pointer in following example

dynamic memory allocation for array

Code:

#include<stdio.h>

int main()
{
    int size = 10 ;
	
    int * array = malloc(size * sizeof(int));
	
    return 0;
}

dynamic memory allocation for pointer

Code:

#include<stdio.h>

int main()
{
    int size = 20 ;
	
    int * ponter = malloc(size * sizeof(int));
	
    return 0;
}

Is it correct way to allocate dynamic memory ?

Other than the variable name there doesn't appear to be anything different between the two snippets.

Originally Posted by jimblumberg

Other than the variable name there doesn't appear to be anything different between the two snippets.

how to allocate dynamic memory for this structure

Code:

#include <stdio.h>

struct student{
    char *name;
    int id;
    int age;
}Data;
int main()
{
 .............
............
     return 0;
}

First a couple of questions.

1. Why do you have the global variable Data?

2. Do you know how to make a "regular" array of that structure?

3. How are you going to allocate memory for that pointer member variable?

We can declare structure as global variable Data

Yes you can, but why are you? You're not using that horrible global variable so why define it?

Yes Let's take an example:

So show me how you would create a normal (no dynamic memory involved) array of that structure.

I wanted to learn about dynamic memory allocation for array pointer and structure

Do you realize that a dynamically allocated array starts out as a pointer? Your first post had two snippets that only differed in the name of the variable, but not in the concept.


Okay, so what did you try? And remember you're first snippet had a pointer member variable that will need memory allocated for it as well.

Okay, good first try. But you forgot to define the variable "record", you should probably be using "Records" instead.

You should get rid of the cast of the return value from malloc(), that cast can hide some mistakes. And by the way you're making one of those mistakes in your code. You need to #include the proper header file for the malloc() function.

Lastly for now, you should never use a function to retrieve a C-string that doesn't limit the number of characters it will try to retrieve to help avoid possible buffer overflow errors. With scanf() you should be using the proper width specifier (9 in this program).

Originally Posted by jimblumberg

Okay, good first try. But you forgot to define the variable "record", you should probably be using "Records" instead.

You should get rid of the cast of the return value from malloc(), that cast can hide some mistakes. And by the way you're making one of those mistakes in your code. You need to #include the proper header file for the malloc() function.

I did mistake while copy paste and editing code

Lastly for now, you should never use a function to retrieve a C-string that doesn't limit the number of characters it will try to retrieve to help avoid possible buffer overflow errors. With scanf() you should be using the proper width specifier (9 in this program).

I didn't understand this paragraph. can you give me more detail

I didn't understand this paragraph. can you give me more detail

What don't you understand?

Did you fix this?

You should get rid of the cast of the return value from malloc(), that cast can hide some mistakes. And by the way you're making one of those mistakes in your code. You need to #include the proper header file for the malloc() function.

Originally Posted by jimblumberg

What don't you understand?

Did you fix this?

I have fixed it

Code:

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


struct student{
  int marks;
  char name[10];
};
int main()
{
    struct student *ptr;
    int i, record = 3;
   
    ptr = (struct student*) malloc (record * sizeof(struct student));
   
 
    for(i = 0; i < record; ++i)
   {
       printf("Enter name of the student and marks respectively:\n");
       scanf("%s %d", &(ptr+i)->name, &(ptr+i)->marks);
   }
   
     printf("Displaying Information:\n");


   for(i = 0; i < record ; ++i)
       printf("%s\t%d\n", (ptr+i)->name, (ptr+i)->marks);
       
    return 0;
}

Originally Posted by Salem

> ptr = (struct student*) malloc (record * sizeof(struct student));
Remove the cast, compile it, and tell us what the compiler says (if anything).

Code:

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


struct student{
  int marks;
  char name[10];
};
int main()
{
    struct student *ptr;
    int i, record = 3;
   
    ptr =  malloc (record * sizeof(struct student));
   
 
    for(i = 0; i < record; ++i)
   {
       printf("Enter name of the student and marks respectively:\n");
       scanf("%s %d", ptr[i].name, &ptr[i].marks);
   }
   
     printf("Displaying Information:\n");


   for(i = 0; i < record ; ++i)
       
       printf("%s\t%d\n", ptr[i].name, ptr[i].marks);
    return 0;
}