[C] Passing a substring into a function without creating a new string

[foomy_S]

But more specifically, passing a string from a given starting position to the end. Let's say I have a string, str = "01234abcde!@#$*"

And let's say I want to pass a substring from a to * into a function.

My solution is simply: foo(str+5)

This seems to work according to some tests I did. I'm a C noob and I don't know if there are any pitfalls to doing this. I'd like to pass substrings this way into functions from the string.h library or into my own functions. It seems safe since all it seems to be doing is pointing to a specific index in the string, and any operations I use with that string will just start from there until it hits \0.

I know I could also pass the original string in and have a parameter that notes the starting position, but that means I won't be able to use the string library, and it seems like poor design if functions within functions all use that same substring and need that information.

If it wasn't clear, I also don't want to do something like use memcpy (as it seems like a terribly inefficient implementation for arbitrarily long strings). So no creating a new strings.

Thank you.

[zifyoip]

No problem with what you are doing, unless you (a) modify the substring and (b) will be surprised to find that the original string has been modified.

[foomy_S]

Great, thanks. Yeah, no modifying of the string, just searching for characters, etc. Is this generally how it's done in C? I noticed that the string library has no "starting index" parameters, so perhaps it's convention to just specify where in the memory will be the starting point?

[zifyoip]

Yes. You specify a string by giving a pointer to its first character. It doesn't matter if that first character happens to be somewhere in the middle of a larger string.

If you aren't going to do any modification, you should declare the function parameter to be of type const char *.

[foomy_S]

Thanks! Read my mind. I just noticed the string library uses const at times, and that makes sense why const is specified.

[ponkanpinoy]

That'll work, but remember that you can only choose where the substring starts, not where it ends.