How do you declare string constants in C?

One advantage (albeit very slight) of defining string constants is that you can concatenate them at compile time:

#define HELLO "hello"
#define WORLD "world"

puts( HELLO WORLD );

Not sure that's really an advantage, but it is a technique that cannot be used with const char *'s.

There's one more (at least) road to Rome:

static const char HELLO3[] = "Howdy";

(static — optional — is to prevent it from conflicting with other files). I'd prefer this one over const char*, because then you'll be able to use sizeof(HELLO3) and therefore you don't have to postpone till runtime what you can do at compile time.

The define has an advantage of compile-time concatenation, though (think HELLO ", World!") and you can sizeof(HELLO) as well.

But then you can also prefer const char* and use it across multiple files, which would save you a morsel of memory.

In short — it depends.

If you want a "const string" like your question says, I would really go for the version you stated in your question:

/* first version */
const char *HELLO2 = "Howdy";

Particularly, I would avoid:

/* second version */
const char HELLO2[] = "Howdy";

Reason: The problem with second version is that compiler will make a copy of the entire string "Howdy", PLUS that string is modifiable (so not really const).

On the other hand, first version is a const string accessible by pointer HELLO2, and it can not be modified.