Update

openmp/1-intro-critical.cpp

@@ -16,5 +16,6 @@ int main(int argc, char** argv)
 #pragma omp parallel num_threads(2)
 #pragma omp critical
   std::cout << "Parallel region 3: tid:" << omp_get_thread_num() << std::endl;
-}
 
+  return 0;
+}

openmp/1-intro-mutex.cpp

@@ -28,5 +28,6 @@ int main(int argc, char** argv)
     std::cout << "Parallel region 3: tid:" << omp_get_thread_num() << std::endl;
     omp_unset_lock(&writelock);
   }
-}
 
+  return 0;
+}

openmp/1-intro.cpp

@@ -13,5 +13,6 @@ int main(int argc, char** argv)
 
 #pragma omp parallel num_threads(2)
   std::cout << "Parallel region 3: tid:" << omp_get_thread_num() << std::endl;
-}
 
+  return 0;
+}

openmp/2-parallel-region-nested.cpp

+#include <iostream>
+#include <omp.h>
+
+int main(int argc, char** argv)
+{
+#pragma omp parallel num_threads(3)
+  {
+    int tid0 = omp_get_thread_num();
+    #pragma omp parallel num_threads(2) firstprivate(tid0)
+    {
+      int tid1 = omp_get_thread_num();
+      #pragma omp critical
+      std::cout << "Parallel region : " << tid0 << "/" << tid1 << std::endl;
+    }
+  }
+  return 0;
+}