C Structures and Dynamic Memory

📌 C Structures and Dynamic Memory Allocation

C Structures and Dynamic Memory allocation can also be applied to structures, allowing flexible creation of objects at runtime.
This is useful when the number of elements is unknown during compile time.

✅ Basic Example: Allocating Memory for a Struct

#include <stdio.h>
#include <stdlib.h>struct Student {
int roll;
float marks;
};int main() {
struct Student *s;

// Allocate memory
s = (struct Student *)malloc(sizeof(struct Student));

if (s == NULL) {
printf("Memory allocation failed!\n");
return 1;
}

s->roll = 10;
s->marks = 88.5;

printf("Roll = %d\nMarks = %.2f\n", s->roll, s->marks);

free(s); // Free memory

return 0;
}

#include <stdio.h>

#include <stdlib.h>struct Student {

int roll;

float marks;

};int main() {

struct Student *s;

// Allocate memory

s = (struct Student *)malloc(sizeof(struct Student));

if (s == NULL) {

printf("Memory allocation failed!\n");

return 1;

}

s->roll = 10;

s->marks = 88.5;

printf("Roll = %d\nMarks = %.2f\n", s->roll, s->marks);

free(s); // Free memory

return 0;

}

🧠 Using `->` Operator

When accessing structure members via pointer:

Operator	Usage
`.`	Access using structure variable
`->`	Access using structure pointer

Example:

📌 Dynamically Allocating an Array of Structs

#include <stdio.h>
#include <stdlib.h>struct Employee {
char name[30];
float salary;
};int main() {
int n;
struct Employee *emp;

printf("Enter number of employees: ");
scanf("%d", &n);

emp = (struct Employee *)malloc(n * sizeof(struct Employee));

if (emp == NULL) {
printf("Memory allocation failed!\n");
return 1;
}

for (int i = 0; i < n; i++) {
printf("\nEnter name and salary for employee %d: ", i + 1);
scanf("%s %f", emp[i].name, &emp[i].salary);
}

printf("\nEmployee Details:\n");
for (int i = 0; i < n; i++) {
printf("%s - %.2f\n", emp[i].name, emp[i].salary);
}

free(emp);
return 0;
}

#include <stdio.h>

#include <stdlib.h>struct Employee {

char name[30];

float salary;

};int main() {

int n;

struct Employee *emp;

printf("Enter number of employees: ");

scanf("%d", &n);

emp = (struct Employee *)malloc(n * sizeof(struct Employee));

if (emp == NULL) {

printf("Memory allocation failed!\n");

return 1;

}

for (int i = 0; i < n; i++) {

printf("\nEnter name and salary for employee %d: ", i + 1);

scanf("%s %f", emp[i].name, &emp[i].salary);

}

printf("\nEmployee Details:\n");

for (int i = 0; i < n; i++) {

printf("%s - %.2f\n", emp[i].name, emp[i].salary);

}

free(emp);

return 0;

}

🔁 Using `realloc()` to Expand Struct Memory

Example:

🚨 Struct with Pointer Members (Deep Copy Required)

Sometimes a struct contains pointer members; then you must allocate memory inside the struct too.

struct Person {
char *name; // pointer inside struct
};int main() {
struct Person *p = malloc(sizeof(struct Person));p->name = malloc(20 * sizeof(char)); // Allocate memory for name string

strcpy(p->name, "John Doe");

printf("Name: %s\n", p->name);

free(p->name); // Free inner pointer
free(p); // Free struct memory

return 0;
}

struct Person {

char *name; // pointer inside struct

};int main() {

struct Person *p = malloc(sizeof(struct Person));p->name = malloc(20 * sizeof(char)); // Allocate memory for name string

strcpy(p->name, "John Doe");

printf("Name: %s\n", p->name);

free(p->name); // Free inner pointer

free(p); // Free struct memory

return 0;

}

🧹 Memory Cleanup Rule

If a struct contains dynamic memory pointers:

📝 Summary Table

Concept	Example
Allocate struct memory	`ptr = malloc(sizeof(struct MyStruct));`
Access data	`ptr->member`
Allocate array of structs	`malloc(n * sizeof(struct MyStruct))`
Resize struct array	`realloc(ptr, newSize)`
Free memory	`free(ptr);` (and internal pointers if any)

C Structures and Dynamic Memory