Episode 4 code updates: added barrier example code

high_performance_computing/hpc_openmp/code/examples/04-barriers.c

@@ -0,0 +1,42 @@
+#include <omp.h>
+#include <stdio.h>
+
+#define TABLE_SIZE 8
+
+/* Function to initialise the lookup table */
+void initialise_lookup_table(int thread_id, double lookup_table[TABLE_SIZE]) {
+    int num_threads = omp_get_num_threads();
+    for (int i = thread_id; i < TABLE_SIZE; i += num_threads) {
+        lookup_table[i] = thread_id * 2; /* Each thread initialises its own portion */
+        printf("Thread %d initializing lookup_table[%d] = %f\n", thread_id, i, lookup_table[i]);
+    }
+}
+
+/* Function to perform the main calculation */
+void do_main_calculation(int thread_id, double lookup_table[TABLE_SIZE]) {
+    int num_threads = omp_get_num_threads();
+    for (int i = thread_id; i < TABLE_SIZE; i += num_threads) {
+        printf("Thread %d processing lookup_table[%d] = %f\n", thread_id, i, lookup_table[i]);
+    }
+}
+
+int main() {
+    double lookup_table[TABLE_SIZE] = {0}; /* Initialise the lookup table to zeros */
+
+    #pragma omp parallel
+    {
+        int thread_id = omp_get_thread_num();
+
+        /* The initialisation of the lookup table is done in parallel */
+        initialise_lookup_table(thread_id, lookup_table);
+
+        #pragma omp barrier  /* As all threads depend on the table, we have to wait until all threads
+                            are done and have reached the barrier */
+
+
+        /* Each thread then proceeds to its main calculation */
+        do_main_calculation(thread_id, lookup_table);
+    }
+
+    return 0;
+}