Use "%zu" to print a value of type size_t. A couple of ugly corner cases: const char str[] = "abcd\0efgh"; printf("length = %zu, size = %zu, value = \

Use "%zu" to print a value of type size_t. A couple of ugly corner cases: const char str[] = "abcd\0efgh"; printf("length = %zu, size = %zu, value = \"%s\"\n", strlen(str), sizeof str, str); output: length = 4, size = 10, value = "abcd" And: const char str[4] = "abcd"; printf("length = %zu, size = %zu, value = \"%s\"\n", strlen(str), sizeof str, str); This has undefined behavior. (Which is a good reason to let the compiler figure out how big the array has to be. Computers are better at counting things than you are. Let them.)