Dynamic struct
- C course
- Intermediate
- Lesson 31/116
- 5 min
Learn to allocate memory for a struct with malloc.
Dynamic struct
To keep a struct on the heap for its lifetime, you use malloc. The call struct Person *p = malloc(sizeof(struct Person)); allocates enough bytes from the heap and returns a pointer.
sizeof(struct Person) gives the byte count the struct needs; you don't have to compute it by hand. The return type is void*; in C it's implicitly converted to struct Person*. On failure it returns NULL; that's why you must check before use. When done, call free(p) to release the memory.
free and NULL check
malloc isn't always successful; if memory is full it returns NULL. Accessing a NULL pointer is undefined behavior; that's why checking is required.
Example pattern: struct Person *p = malloc(sizeof(struct Person)); if (p == NULL) return 1; p->age = 25; ... free(p);. free(p) releases memory but doesn't set p to NULL; so writing p = NULL after free is a good habit. Every malloc needs a free; a missing free creates a memory leak.
Practice Create and print a dynamic struct
Define struct Person with char name[32]; and int age;. In main, allocate with struct Person *p = malloc(sizeof(struct Person));. Check with if (p == NULL) return 1;. Assign p->age = 25;, print with printf("%d\n", p->age);, then call free(p);.
Starter code
#include <stdio.h>
#include <stdlib.h>
// Define struct Person (name[32] and age)
// Allocate memory for p with malloc
// Set age to 25 via p, print, then call free
int main(void) {
return 0;
}Sample solution
#include <stdio.h>
#include <stdlib.h>
struct Person {
char name[32];
int age;
};
int main(void) {
struct Person *p = malloc(sizeof(struct Person));
if (p == NULL) return 1;
p->age = 25;
printf("%d\n", p->age);
free(p);
return 0;
}Expected output
25
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the malloc expressions
| Term | Meaning |
|---|---|
malloc(sizeof(struct Person)) |
Allocation |
p->age = 25 |
Field assignment |
free(p) |
Release |
p == NULL |
Null check |
2 What does malloc return on failure?
- NULL Correct answer
- 0
- -1
- Empty struct
Explanation: malloc returns NULL on failure; that's why checking is required.
3 Complete the malloc call
struct Person *p = ____(sizeof(struct Person));struct Person *p = malloc(sizeof(struct Person));Explanation: malloc allocates heap memory for the struct.