mull-demo

A minimal demo of Mull mutation testing, based on the Quick Primer.

What this project does

range_tests.c implements a (buggy) in_range function that checks whether a value falls within a closed interval [min, max], along with three assertions that serve as its test suite:

int in_range(int value, int min, int max) {
  return value >= min && value < max;
}

The tests cover below-min, mid-range, and above-max cases — achieving 100% line coverage. Despite this, Mull reveals that two mutations survive:

• >= changed to > (the boundary at min is not tested)
• < changed to <= (the boundary at max is not tested)

This demonstrates a core limitation of coverage-based testing: coverage does not guarantee that the tests actually validate the semantics of the code.

Running locally

Install mull (see installation docs), then:

clang-22 \
  -fpass-plugin=/usr/lib/mull-ir-frontend-22 \
  -g -grecord-command-line \
  range_tests.c -o range_tests

mull-runner-22 range_tests