Extending the example.

src/flexible-array-member.c

@@ -1,5 +1,6 @@
-#include <stdlib.h>
+#include <stddef.h>
 #include <stdio.h>
+#include <stdlib.h>
 
 struct item {
 	int header;
@@ -10,7 +11,11 @@ struct item {
 static struct item *
 allocate(size_t payload_len)
 {
-	/* offsetof(struct item, payload) might be better to use here. */
+	/*
+	 * offsetof(struct item, payload) might be better to use here as
+	 * otherwise we might allocate more than necessary due to padding that
+	 * could otherwise be used for the flexible array member.
+	 */
 	struct item *p = malloc(sizeof (struct item) +
 	    payload_len * sizeof (p->payload[0]));
 	if (p == NULL)
@@ -29,8 +34,17 @@ main(void)
 {
 	struct item *p = allocate(30);
 
-	for (size_t i = 0; i < p->len; ++i)
+	/*
+	 * This is always the size without the flexible array member.  Note that
+	 * padding might be inserted by the compiler.
+	 */
+	printf("sizeof (item): %zu\n", sizeof (*p));
+	printf("offsetof (item, payload): %zu\n",
+	    offsetof(struct item, payload));
+
+	for (size_t i = 0; i < p->len; ++i) {
 		printf("%zu: %c\n", i, p->payload[i]);
+	}
 
 	free(p);
 }