Exercise: The Program That Is Right and Still Broken

Tools: GCC, Make, Valgrind

Goal

Find and fix a memory leak in a program whose output is entirely correct, using Valgrind.

Background

main.c reads words from standard input and reports how many there were and which one was the longest. The answer it prints is correct, it never crashes, and -Wall -Wextra says nothing.

The program keeps exactly one string on the heap — the longest word seen so far — and main() frees it at the end. That sounds like it should balance.

Build & Run

make
./longest < input.txt
valgrind --leak-check=full ./longest < input.txt

Valgrind reports bytes definitely lost, which means that at exit no pointer to that memory existed anywhere in the program.

Your Task

1. Find the leak. Count how many words in input.txt are longer than every word before them: that is how many heap copies the program makes. A pointer variable holds one address, so ask what happens to the address that was in it when a new one is assigned on top.
2. Fix it, keeping the output identical. The struct owns the string it points to, so the old string has to be released before the pointer is overwritten.

Check Your Work

valgrind --leak-check=full ./longest < input.txt
printf 'aaa\n' | ./longest
printf '' | ./longest
printf 'zz\naaa\nbbbb\naaaaa\n' | ./longest

Every run must end at "All heap blocks were freed" with 0 errors, and the reported longest word must still be right in each case.

The last two are the interesting ones. free(NULL) is legal and does nothing, so empty input needs no special case — but freeing the same pointer twice is undefined behaviour, and Valgrind will report it as an invalid free. A fix that trades a leak for a double free is not a fix; check both.