Memory management
- C course
- Beginner
- Lesson 115/116
- 6 min
Add dynamic memory management with malloc and free.
Dynamic Book with malloc
Instead of a fixed array, you can allocate a Book at runtime with malloc. This lets the catalog size be set dynamically.
struct Book *b = malloc(sizeof(struct Book));
strcpy(b->title, "Suç ve Ceza");
b->year = 1866;
free(b);Here b is a pointer; it points to the Book on the heap. You access members with the -> operator (not dot). When done, you give the memory back with free. This is the basis of dynamic catalogs.
Assigning the title with strcpy
You cannot assign directly to a char array; you copy characters with strcpy. The <string.h> header is required.
strcpy(b->title, "Suç ve Ceza");
b->year = 1866;Here strcpy copies the second string into the first. year is an int so it can be assigned directly with =. This difference comes from C treating string and integer assignments differently.
Practice Allocate a Book with malloc and print
Add the <stdlib.h> and <string.h> headers. Allocate a Book with malloc(sizeof(struct Book)); set the title to "War and Peace" with strcpy; assign b->year = 1869. Print with print_book(*b); release with free(b).
Starter code
#include <stdio.h>
struct Book {
char title[32];
int year;
};
void print_book(struct Book b) {
printf("%s %d\n", b.title, b.year);
}
// Allocate a Book with malloc; assign title with strcpy; print with print_book; release with free
int main(void) {
return 0;
}Sample solution
#include <stdlib.h>
#include <string.h>
struct Book *b = malloc(sizeof(struct Book));
strcpy(b->title, "War and Peace");
b->year = 1869;
print_book(*b);
free(b);Expected output
War and Peace 1869
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Fill in the blank: pointer member access
b____ = 1866;b->year = 1866;Explanation: b->year accesses the pointer's member.
2 Match expression to meaning
| Term | Meaning |
|---|---|
b->title |
Title member |
b->year |
Year member |
malloc |
Allocates on heap |
free |
Releases heap |
3 Which function is called to release a Book allocated with malloc?
- delete
- release
- free Correct answer
- destroy
Explanation: free gives back heap memory allocated with malloc.