rots: add list of subcommands with descriptions and implement usage

source/main.c

@@ -8,27 +8,47 @@
 #include <unpack.h>
 #include <verify.h>
 
-struct entry_point {
+struct entry {
 	const char *cmd;
 	int (* main)(int argc, char *argv[]);
+	char *desc;
 };
 
-struct entry_point entries[] = {
-	{ "get-offset", do_get_offset },
-	{ "get-size", do_get_size },
-	{ "pack", do_pack },
-	{ "sign", do_sign },
-	{ "unpack", do_unpack },
-	{ "verify", do_verify },
-	{ NULL, NULL },
+struct entry entries[] = {
+	{ "get-offset", do_get_offset,
+		"print the offset of the payload within an image" },
+	{ "get-size", do_get_size,
+		"print the size of the payload within an image" },
+	{ "pack", do_pack,
+		"pack the payload within an image" },
+	{ "sign", do_sign,
+		"sign an image" },
+	{ "unpack", do_unpack,
+		"unpack the payload of an image" },
+	{ "verify", do_verify,
+		"verify the signatures of an image" },
+	{ NULL, NULL, NULL },
 };
 
+static void show_usage(const char *prog_name)
+{
+	struct entry *entry;
+
+	fprintf(stderr, "usage: %s <command> ...\n\n"
+		"ROTS utilities to work with signed images\n\n"
+		"Available commands:\n", prog_name);
+
+	for (entry = entries; entry->cmd; ++entry) {
+		fprintf(stderr, "\t%-16s %-80s\n", entry->cmd, entry->desc);
+	}
+}
+
 int main(int argc, char *argv[])
 {
-	struct entry_point *entry;
+	struct entry *entry;
 
 	if (argc < 2) {
-		fprintf(stderr, "usage: %s <command>\n", argv[0]);
+		show_usage(argv[0]);
 		return -1;
 	}
 
@@ -37,6 +57,7 @@ int main(int argc, char *argv[])
 			return entry->main(argc, argv);
 	}
 
-	fprintf(stderr, "usage: %s <command>\n", argv[0]);
+	show_usage(argv[0]);
+
 	return -1;
 }