Using NULL with C's char* strings

Question

As we all know, strings in C are null-terminated. Does that mean that according to the standard it is legal to use the NULL constant as the terminator? Or is the similarity of the name of NULL pointer and null-terminator for a string only a happy coincidence?

Consider the code:

char str1[] = "abc";
char str2[] = "abc";
str1[3] = NULL;
str2[3] = '\0';

Here, we change the terminator of str1 to NULL. Is this legal and well-formed C code and str1 adheres to C's definition of null-terminated string? Will it be the same in case of C++?

In practice, I have always used NULL instead of '\0' in my code for strings and everything worked - but is such practice 100% legal?

EDIT: I understand that it's very bad style and refrain from endorsing it and now understand the difference between 0, NULL and '\0' (as in a duplicate What is the difference between NULL, '\0' and 0). I'm still quite curious as for the legality of this code - and voices here seem to be mixed - and the duplicate does not give an authoritative answer to that in my opinion.

Answer 1

Does that mean that according to the standard it is legal to use the NULL constant as the terminator? (OP)

str1[3] = NULL;

Sometimes. Further: does it always properly cause a character array to form a string without concerns?

First, it looks wrong. Akin to int z = 0.0;. Yes it is legal well defined code, but unnecessarily draws attention to itself.

In practice, I have always used NULL instead of '\0' (OP)

I doubt you will find any modern style guide or group of coders endorsing that. NULL is best reserved for pointer contexts.¹

These are 2 common and well understood alternatives.

str1[3] = '\0';
str1[3] = 0;

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strings in C are null-terminated (OP)

The C spec consistently uses null character, not just null.

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The macros are NULL which expands to an implementation-defined null pointer constant; and ... C11 §7.19 3

OK, now what is a null pointer constant?

An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant. §6.3.2.3 5

If the null pointer constant is a void* then we have something like

str1[3] = (void*) 0;

The above can warn about converting a pointer to a char. This is something best avoided.

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Will it be the same in case of C++? (OP)

Yes, the above applies. (Aside: str1[3] = 0 may warn.) Further, NULL is less preferred than nullptr. So NULL is rarely the best to use in C++ in any context.

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¹Note: @Joshua reports a style that matches OP's in 1995 Turbo C 4.5

Answer 2

The bottom line is that in C/C++, NULL is for pointers and is not the same as the null character, despite the fact that both are defined as zero. You might use NULL as the null character and get away with it depending on the context and platform, but to be correct, use '\0'. This is described in both standards:

• C specifies that the macro NULL is defined as a macro in <stddef.h> which "expands to an implementation-defined null pointer constant" (Section 7.17.3), which is itself defined as "an integer constant expression with the value 0, or such an expression cast to type void *" (Section 6.3.2.3.3).

  The null character is defined in section 5.2.1.2: "A byte with all bits set to 0, called the null character, shall exist in the basic execution character set; it is used to terminate a character string." That same section explains that \0 will be the representation of this null character.

• C++ makes the same distinctions. From section 4.10.1 of the C++ standard: "A null pointer constant is an integer literal (2.13.2) with value zero or a prvalue of type std::nullptr_t." In section 2.3.3, it describes the as "null character (respectively, null wide character), whose value is 0". Section C.5.2 further confirms that C++ respects NULL as a standard macro imported from the C Standard Library.

Answer 3

No, I don't think it's strictly legal.

NULL is specified to be either:

• an integer constant expression with the value ​0​
• an integer constant expression with the value 0 cast to the type void*

In an implementation that uses the first format, using it as the string terminator will work.

But in an implementation that uses the second format, it's not guaranteed to work. You're converting a pointer type to an integer type, and the result of this is implementation-dependent. It happens to do what you want in common implementations, but nothing requires it.

If you have the second type of implementation, you're likely to get a warning like:

warning: incompatible pointer to integer conversion assigning to 'char' from 'void *' [-Wint-conversion]

If you want to use a macro, you can define:

#define NUL '\0'

and then use NUL instead of NULL. This matches the official name of the ASCII null character.