Character array without a null terminator in C

Question

Sometimes I am using sequences of characters (strings) except the null terminator is not needed or wanted, for example if I am using memcpy() and the length is already known. A such, I prefer to omit the null terminator. A cumbersome way to do this would be declaring an array:

char no_term[5] = {'h', 'e', 'l', 'l', 'o'};

However, I would prefer to use quoted strings, as these are much more efficient to program with. However quoted strings automatically include a null terminator at the end. But would specifying the array size to exclude the null terminator invoke undefined behavior? Is the following valid C, as long as I do not use these where a null terminated string is required (e.g., passing them to strlen())?

char no_term[5] = "hello";
char no_term_array[3][3] = {"foo", "bar", "baz"};

Code such as char no_term[5] = "hello"; will get you whatever "You broke the build!" award/hat/dunce cap often enough your org will engrave your name on it. — Andrew Henle
– Andrew Henle, Commented Sep 4, 2022 at 22:41
One drawback to this is that humans are fallible, and counting characters is tedious and error prone. What is the size (without '\0') of "Supercalifragilisticexpialidotious" ?(sp??) — user17592432
– user17592432, Commented Sep 4, 2022 at 23:32
@Fe2O3 How about a macro function? #define NO_NULL_TERM(string_literal) (const char[sizeof(string_literal)-1]){string_literal} — CPlus
– CPlus, Commented Sep 4, 2022 at 23:33
@user16217248 Or a VLA perhaps... (Can't try it... Old compiler...) :-) — user17592432
– user17592432, Commented Sep 4, 2022 at 23:35
@Fe2O3 How would a zero length array compound literal behave? I do not know — CPlus
– CPlus, Commented Sep 4, 2022 at 23:42

Andreas Wenzel · Accepted Answer · 2022-09-13 22:58:26Z

6

According to §6.7.9 ¶14 of the ISO C11 standard, arrays of character type may be initialized from a string literal, even if there is no room for the terminating null character.

So yes, your posted code is valid and will not invoke undefined behavior.

Note however that this is only legal in C, not C++. In C++, there must be room for a terminating null character.

edited Sep 13, 2022 at 22:58

answered Sep 4, 2022 at 22:40

Andreas Wenzel

27k4 gold badges33 silver badges52 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Weather Vane Over a year ago

But the resultant variable is not a C string, because it has no NUL terminator. It's just an array, and it is undefined behaviour to handle it as if it is a string.

CPlus Over a year ago

@WeatherVane That is why I said 'character array' in the title instead of 'string'

Weather Vane Over a year ago

Yes, it is strictly true that the posted code will not be undefined behaviour but is slightly misleading, hence the comment.

M.M Over a year ago

@WeatherVane the question specifically asks for an array that will not be handled like a null-terminated string

supercat Over a year ago

It would have been nice if the Standard had provided a syntax to indicate whether the contents of a string literal were expected to completely fill a space, be exactly one byte shorter than a space, be at least one byte shorter than a space, or have an arbitrary size relationship with a space. In many cases, a programmer will expect one of those things, and it would be useful to have a diagnostic any time the expectation is violated.

Collectives™ on Stack Overflow

Character array without a null terminator in C

1 Answer 1

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related