free

Learn to release allocated memory with free.

free: releasing heap memory

Memory allocated with malloc must be released with free. Otherwise a memory leak occurs.

int *p = malloc(sizeof(int));
*p = 50;
printf("%d\n", *p);
free(p);  /* give the memory back */

Here the free(p) call returns the heap area to the operating system. p itself is still a valid pointer; but the place it points to is no longer yours. Using that pointer after free is a use after free error.

Avoiding memory leaks

Every malloc must have a matching free. Memory allocated and never released during the program is a leak.

for (int i = 0; i < 100; i++) {
    int *p = malloc(sizeof(int));
    /* use */
    free(p);  /* release each iteration */
}

Here each loop iteration has one malloc and one free; this balance prevents leaks. In long-running programs leaks accumulate and eventually memory runs out. So never forgetting free is very important.

Exercise malloc, use, free

Allocate an int with malloc; assign *p = 50; print with printf; finally release the memory with free. Include the <stdlib.h> header.

Show example solution

int *p = malloc(sizeof(int));
*p = 50;
printf("%d\n", *p);
free(p);

Test yourself

What is the purpose of free?
- Allocate memory
- Resize memory
- Reset a pointer
- Release allocated memory
Show answer

Release allocated memory — free gives back memory allocated with malloc.
Match function to task

malloc

Allocates memory
free

Releases memory
sizeof

Gives byte size
use after free

Error case

Fill in the blank: free call

int *p = malloc(sizeof(int));
*p = 50;
____(p);

Show answer

int *p = malloc(sizeof(int));
*p = 50;
free(p);

Next lesson: calloc and realloc

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