Freeing structure elements in C - c

Question

I have a structure:
struct student{
int roll_no;
char *name = malloc(25 * sizeof(char));;
char *phone_no = malloc(10 * sizeof(char));;
char *dob = malloc(10 * sizeof(char));;
}*s1;
int main(){
s1 = malloc(5 * sizeof(student)); //array of student
//.....
}
What is appropriate code for the complete loop for allocating an array of student of size 'n' and then de-allocating it afterwards?
Note: The question here deals with allocation and de-allocation of elements of the instance of a structure.

Answer 1

This...
typedef struct student{
int roll_no; // (the following illegal syntax commented out)
char *name; // = malloc(25 * sizeof(char));;
char *phone_no; // = malloc(10 * sizeof(char));;
char *dob; // = malloc(10 * sizeof(char));;
}*s1;
...from what is being described as the need, (minus the illegal assignment statements) could probably better be formed as:
typedef struct {
int roll_no;
char *name; //needs memory
char *phone; //needs memory
char *dob; //needs memory
}STUDENT;
Then, use the new variable type: STUDENT, to create the instances of the struct as needed. Your OP indicates you need 5:
STUDENT s[5]; //Although this array needs no memory, the
//members referenced by it do
//(as indicated above)
Now, all that is necessary is to create memory for the 3 members that require it, in each of the 5 instances.
for(i=0;i<5;i++)
{
s[i].name = calloc(80, 1); //calloc creates AND initializes memory.
s[i].phone = calloc(20, 1); //therefore safer than malloc IMO.
s[i].dob = calloc(20, 1); //Also, change values as needed to support actual
//length needs for name, phone and dob
}
// Use the string members of s[i] as you would any other string, But do not
// forget to free them when no longer needed.
...
for(i=0;i<5;i++)
{
free(s[i].name);
free(s[i].phone);
free(s[i].dob);
}
Note, because of the way the array s is created in this example, i.e. with memory on the stack instead of the heap, there is no need to free it.
One other note, the example code above focused on a method to create memory for the char * members of your struct array, but when actually coding for keeps, the return of [m][c][re]alloc should always be checked that memory was created before trying to use the variable. For example:
s[i].name = calloc(80, 1);
if(!s[i].name) //checking that memory was created
{
;//if failed, then handle error.
}
...

Answer 2

In addition to ryyker's answer, if you want to do it dynamically:
#include <stdlib.h>
struct student{
int roll_no;
char *name;
char *phone;
char *dob;
};
int main()
{
int i, student_count = 5;
struct student ** s = malloc(sizeof(struct student *) * student_count);
if (s)
{
for (i = 0; i < student_count; ++i)
{
s[i] = malloc(sizeof(struct student));
if (s[i])
{
//set up student's members
}
}
for (i = 0; i < student_count; ++i)
{
//free student's members before the next line.
free(s[i]);
}
free(s);
}
return 0;
}

Answer 3

You must free everything you malloc, and as mentioned in the comments you cannot malloc inside the struct.
#include <stdio.h>
#include <stdlib.h>
#define NUM_STUDENTS 5
struct student{
int roll_no;
char *name;
char *phone;
char *dob;
};
int main(void)
{
int i;
// if this was me, I would simply replace this with
// struct student s[NUM_STUDENTS];, but the goal here is to illustrate
// malloc and free
struct student* s = malloc(sizeof(struct student) * NUM_STUDENTS);
if (s == NULL) // handle error
for (i=0; i<NUM_STUDENTS; i++)
{
// sizeof(char) is guaranteed to be 1, so it can be left out
s[i].name = malloc(25);
if (s[i].name == NULL) // handle error
s[i].phone = malloc(10);
if (s[i].phone == NULL) // handle error
s[i].dob = malloc(10);
if (s[i].dob == NULL) // handle error
}
// do stuff with with the data
....
// time to clean up, free in the reverse order from malloc
for (i=0; i<NUM_STUDENTS; i++)
{
// the dob, phone, name order here isn't important, just make sure you
// free each struct member before freeing the struct
free(s[i].dob);
free(s[i].phone);
free(s[i].name);
}
// now that all the members are freed, we can safely free s
free(s);
return 0;
}

Answer 4

User Abhijit gave an answser that was in the right direction, but not complete. His answer should have been:
typedef struct STUDENT{
int roll_no;
char *name;
char *phone;
char *dob;
}student;
void example(int n_students)
{
student **s;
int i;
s= malloc(n_students * sizeof(student *));
for (i=0; i<n_students; i++)
{
s[i]= malloc(sizeof(student));
s[i]->name= malloc(25);
s[i]->phone= malloc(10);
s[i]->dob= malloc(10);
}
// now free it:
for (i=0; i<n_students; i++)
{
free(s[i]->name);
free(s[i]->phone);
free(s[i]->dob);
free(s[i]);
}
free(s);
}