Dynamic strings
- C course
- Intermediate
- Lesson 18/116
- 5 min
Learn to create dynamic strings with malloc.
Dynamic strings
A dynamic string is a char array allocated at runtime with malloc. Unlike fixed-size char arrays, its size is decided at runtime.
Example: char *s = malloc(20);. This allocates 20 bytes and assigns the start address to s. To fill the string use strcpy(s, "Merhaba");. malloc needs <stdlib.h>, strcpy needs <string.h>. When done, free it with free(s);; otherwise a leak occurs.
Using a dynamic string
Once allocated, a dynamic string is used like a normal string. It's filled with strcpy, printed with printf. The key is not to exceed the allocated size.
Example: char *s = malloc(20); strcpy(s, "Merhaba"); printf("%s\n", s); free(s);. Here 20 bytes are allocated, 8 are used (Merhaba + null). The remaining 12 are unused. malloc doesn't initialize; so filling with strcpy is important. The free call prevents a leak.
Practice Create and print a dynamic string
Allocate a twenty-byte dynamic string with char *s = malloc(20);. Fill it with strcpy(s, "Merhaba");. Print it with printf("%s\n", s);. Finally free it with free(s);. Include <stdlib.h> and <string.h>.
Starter code
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
// Allocate 20 bytes with malloc
// Copy "Merhaba" with strcpy
// Print with printf, free with free
int main(void) {
return 0;
}Sample solution
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void) {
char *s = malloc(20);
strcpy(s, "Merhaba");
printf("%s\n", s);
free(s);
return 0;
}Expected output
Merhaba
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which function is used for a dynamic string?
- malloc Correct answer
- printf
- strcmp
- strlen
Explanation: malloc allocates heap memory for a dynamic string.
2 Allocate a dynamic string
char *s = ____(20);
strcpy(s, "Merhaba");char *s = malloc(20);
strcpy(s, "Merhaba");Explanation: malloc(20) allocates twenty bytes of dynamic memory.
3 Match the dynamic string parts
| Term | Meaning |
|---|---|
malloc(20) |
Allocation |
strcpy(s, ...) |
Filling |
printf("%s", s) |
Printing |
free(s) |
Release |