lang/c: Add an example on flexible array checks

Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>

1 files changed, 67 insertions, 0 deletions

diff --git a/lang/c/counted_by.c b/lang/c/counted_by.c
new file mode 100644
index 0000000..09d4699
--- /dev/null
+++ b/lang/c/counted_by.c
@@ -0,0 +1,67 @@
+#define _GNU_SOURCE
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+
+struct foo {
+	uint64_t a;
+	uint64_t b;
+};
+
+struct bounded_flex_t {
+	uint64_t count;
+
+	// NOTE: try to move this member somewhere else and you will get:
+	//   counted_by.c:13:20: error: flexible array member not at end of struct
+	struct foo flex_array[] __attribute__((counted_by(count)));
+};
+
+void free_flex(struct bounded_flex_t **flex)
+{
+	struct bounded_flex_t *ptr = *flex;
+
+	printf("Freeing a flexible array of %ld members.\n", ptr->count);
+	free(*flex);
+}
+
+__attribute__((noreturn)) void die(const char *const str)
+{
+	fprintf(stderr, str);
+	exit(EXIT_FAILURE);
+}
+
+int main(int argc, char *argv[])
+{
+	uint64_t len = 1;
+
+	if (argc == 2) {
+		len = (uint64_t) strtol(argv[1], NULL, 10);
+		if (!len) {
+			die("error: bad integer value!\n");
+		}
+	} else if (argc > 2) {
+		die("error: only pass one argument maximum!\n");
+	}
+
+	// That's a mouthful :P
+	struct bounded_flex_t *flex __attribute__((cleanup(free_flex))) = malloc(sizeof(struct bounded_flex_t) + (len * sizeof(struct foo)));
+	if (!flex) {
+		die("error: could not allocate memory for it");
+	}
+
+	// NOTE: if this is not the first thing that we do, and we use 'len' for the
+	// for loop below, then we will get a runtime error from the array bound
+	// checker. Hence, pretty much like in Rust :)
+	flex->count = len;
+
+	uint64_t accumulator = 0;
+	for (uint64_t i = 0; i < flex->count; i++) {
+		flex->flex_array[i] = (struct foo){ .a = accumulator, .b = accumulator + 1};
+		accumulator++;
+	}
+
+	for (uint64_t i = 0; i < flex->count; i++) {
+		printf("%ld: .a = %ld, .b = %ld\n", i, flex->flex_array[i].a, flex->flex_array[i].b);
+	}
+}