diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 0afbfc1..519dac2 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -7,5 +7,6 @@ jobs:
     runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v4
-      - run: make -j
-      - run: target/tests
+      - run: |
+          make -j
+          target/tests
diff --git a/Makefile b/Makefile
index 8fc9bd5..d09b778 100644
--- a/Makefile
+++ b/Makefile
@@ -1,6 +1,6 @@
 CPPFLAGS = -Isrc/include
 CFLAGS = -std=c11 -O2 -Wall -Wextra -flto
-CXXFLAGS = -std=c++11 -O2 -Wall -Wextra -flto
+CXXFLAGS = -std=c++17 -O2 -Wall -Wextra -flto
 LDLIBS = -lm
 
 Sources = src/sieve_of_eratosthenes.c src/sieve_of_atkin.c
@@ -16,5 +16,5 @@ target/sieve: $(Objects) src/main.c
 target/bench: $(Objects) bench/bench.cc
 	$(LINK.cc) $^ -o $@ $(LDLIBS)
 
-target/tests: $(Objects) tests/tests.c
-	$(LINK.c) $^ -o $@ $(LDLIBS)
+target/tests: $(Objects) tests/tests.cc
+	$(LINK.cc) $^ -o $@ $(LDLIBS)
diff --git a/autoformat.bash b/autoformat.bash
index 640d75a..0da128f 100755
--- a/autoformat.bash
+++ b/autoformat.bash
@@ -2,10 +2,13 @@
 
 shopt -s globstar
 
+# Switch to the directory containing the script so that relative paths may be
+# used.
+cd "${0%/*}"
 files=(**/*.[ch]*)
 if [ "$1" = check ]
 then
-    clang-format --dry-run -Werror ${files[@]}
+    clang-format --verbose --dry-run -Werror ${files[@]}
 else
-    clang-format -i ${files[@]}
+    clang-format --verbose -i ${files[@]}
 fi
diff --git a/tests/tests.c b/tests/tests.c
deleted file mode 100644
index 9fc036b..0000000
--- a/tests/tests.c
+++ /dev/null
@@ -1,27 +0,0 @@
-#include <assert.h>
-#include <stddef.h>
-
-#include "sieve_of_eratosthenes_atkin.h"
-
-int
-main(void)
-{
-    // Each count of prime numbers excludes 2, 3 and 5, since both sieves start
-    // finding primes numbers from 7 onwards.
-    size_t limits_primes[][2] = { { 10000000, 664576 }, { 100000000, 5761452 }, { 1000000000, 50847531 } };
-    for (size_t i = 0; i < sizeof limits_primes / sizeof *limits_primes; ++i)
-    {
-        size_t limit = limits_primes[i][0];
-        size_t primes = limits_primes[i][1];
-
-        struct SieveOfEratosthenes *erato = sieve_of_eratosthenes_new(limit);
-        size_t erato_count = sieve_of_eratosthenes_count(erato);
-        sieve_of_eratosthenes_delete(erato);
-        assert(erato_count == primes);
-
-        struct SieveOfAtkin *atkin = sieve_of_atkin_new(limit);
-        size_t atkin_count = sieve_of_atkin_count(atkin);
-        sieve_of_atkin_delete(atkin);
-        assert(atkin_count == primes);
-    }
-}
diff --git a/tests/tests.cc b/tests/tests.cc
new file mode 100644
index 0000000..178a8b1
--- /dev/null
+++ b/tests/tests.cc
@@ -0,0 +1,67 @@
+#include <cassert>
+#include <cstddef>
+#include <thread>
+
+#include "sieve_of_eratosthenes_atkin.h"
+
+// Each count of prime numbers excludes 2, 3 and 5, since both sieves start
+// finding primes numbers from 7 onwards.
+static std::size_t counts_limits[][2] = {
+    { 11145332, 201268277 },
+    { 14817689, 272068078 },
+    { 18739219, 348738693 },
+    { 20409369, 381656751 },
+    { 21423606, 401730367 },
+    { 21984274, 412847637 },
+    { 22879491, 430626488 },
+    { 28077424, 534558099 },
+    { 36669946, 708491638 },
+    { 40759133, 792066697 },
+    { 41876892, 814981682 },
+    { 43807217, 854635735 },
+    { 46429840, 908671296 },
+    { 46994786, 920313910 },
+    { 48028264, 941665889 },
+};
+
+/******************************************************************************
+ * Test the implementation of the sieve of Eratosthenes.
+ *****************************************************************************/
+void
+erato_work(void)
+{
+    for (auto const &[count, limit] : counts_limits)
+    {
+        SieveOfEratosthenes *erato = sieve_of_eratosthenes_new(limit);
+        std::size_t erato_count = sieve_of_eratosthenes_count(erato);
+        sieve_of_eratosthenes_delete(erato);
+        assert(erato_count == count);
+    }
+}
+
+/******************************************************************************
+ * Test the implementation of the sieve of Atkin.
+ *****************************************************************************/
+void
+atkin_work(void)
+{
+    for (auto const &[count, limit] : counts_limits)
+    {
+        SieveOfAtkin *atkin = sieve_of_atkin_new(limit);
+        std::size_t atkin_count = sieve_of_atkin_count(atkin);
+        sieve_of_atkin_delete(atkin);
+        assert(atkin_count == count);
+    }
+}
+
+/******************************************************************************
+ * Main function.
+ *****************************************************************************/
+int
+main(void)
+{
+    std::thread erato_worker(erato_work);
+    std::thread atkin_worker(atkin_work);
+    erato_worker.join();
+    atkin_worker.join();
+}