valgrind
- C course
- Intermediate
- Lesson 97/116
- 5 min
Learn to find memory errors with valgrind.
Memory checks with valgrind
valgrind inspects a program's memory use at run time. Its most common use is finding leaks: valgrind --leak-check=full ./program.
valgrind runs the program slowly but checks that every malloc has a matching free. If there is a leak, the report shows a "definitely lost" line with the byte count. It also catches reads of uninitialized variables and out-of-bounds array access. These checks cannot be caught by the compiler; they appear only at run time. So valgrind should be run regularly on any C program that uses dynamic memory.
A leak-free program
For valgrind to report clean, every malloc must have a matching free. Free the memory as soon as you are done with it.
Example: int *arr = malloc(3 * sizeof(int)); allocates room for three integers. After filling the values you compute the sum and release the memory with free(arr);. sizeof(int) gives the type size so the code works across platforms. Forgetting free causes a leak; calling it too early causes a dangling pointer. The correct order is: allocate, use, free.
Practice Write a leak-free program
Allocate a three-element array with int *arr = malloc(3 * sizeof(int));. Assign arr[0] = 10; arr[1] = 20; arr[2] = 30;. Compute int sum = arr[0] + arr[1] + arr[2];. Print with printf("%d\n", sum);. Finally release the memory with free(arr);.
Starter code
#include <stdio.h>
#include <stdlib.h>
// Allocate a 3-element int array
// Assign 10, 20, 30 to the elements
// Print the sum
// Free the memory
int main(void) {
return 0;
}Sample solution
#include <stdio.h>
#include <stdlib.h>
int main(void) {
int *arr = malloc(3 * sizeof(int));
arr[0] = 10; arr[1] = 20; arr[2] = 30;
int sum = arr[0] + arr[1] + arr[2];
printf("%d\n", sum);
free(arr);
return 0;
}Expected output
60
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What kind of errors does valgrind catch?
- Syntax errors
- Type mismatches
- Compiler warnings
- Memory leaks Correct answer
Explanation: valgrind catches memory leaks and uninitialized reads.
2 Match the memory management
| Term | Meaning |
|---|---|
malloc |
Allocate memory |
free |
Release memory |
sizeof(int) |
Type size |
--leak-check |
Leak check |
3 Complete the malloc call
int *arr = malloc(3 * sizeof(____));int *arr = malloc(3 * sizeof(int));Explanation: sizeof(int) gives the byte size of the int type.