add a small test for gettid()

test/unistd/tst-gettid.c

@@ -0,0 +1,39 @@
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <unistd.h>
+#include <pthread.h>
+#include <stdlib.h>
+
+void *thread_func(void *arg)
+{
+    printf("Thread %ld:\n", (long)arg);
+    printf("  PID        = %d\n", getpid());
+    printf("  TID        = %d (gettid)\n", gettid());
+    printf("  pthread ID = %lu\n\n", pthread_self());
+    return NULL;
+}
+
+int main(void)
+{
+    const int NUM_THREADS = 3;
+    pthread_t threads[NUM_THREADS];
+
+    printf("Main thread:\n");
+    printf("  PID        = %d\n", getpid());
+    printf("  TID        = %d (gettid)\n", gettid());
+    printf("  pthread ID = %lu\n\n", pthread_self());
+
+    for (long i = 0; i < NUM_THREADS; i++) {
+        if (pthread_create(&threads[i], NULL, thread_func, (void *)i) != 0) {
+            perror("pthread_create");
+            exit(EXIT_FAILURE);
+        }
+    }
+
+    for (int i = 0; i < NUM_THREADS; i++) {
+        pthread_join(threads[i], NULL);
+    }
+
+    return 0;
+}
+