Is there any character that is illegal in file paths on every OS?

Question

Is there any character that is guaranteed not to appear in any file path on Windows or Unix/Linux/OS X?

I need this because I want to join together a few file paths into a single string, and then split them apart again later.

Answer 1

In the comments, Harry Johnston writes:

The generic solution to this class of problem is to encode the file paths before joining them. For example, if you're dealing with single-byte strings, you could convert them to hex strings; so "hello" becomes "68656c6c6f". (Obviously that isn't the most efficient solution!)

That is absolutely correct. Please don't try to do anything "tricky" with filenames and reserved characters, because it will eventually break in some weird corner case and your successor will have a heck of a time trying to repair the damage.

In fact, if you're trying to be portable, I strongly recommend that you never attempt to create any filenames including any characters other than [a-z0-9_]. (Consider that common filesystems on both Windows and OS X can operate in case-insensitive mode, where FooBar.txt and FOOBAR.TXT are the same identifier.)

A decently compact encoding scheme for practical use would be to make a "whitelisted set" such as [a-z0-9_], and encode any character ch outside your "whitelisted set" as printf("_%2x", ch). So hello.txt becomes hello_2etxt, and hello_world.txt becomes hello_5fworld_2etxt. Since every _ is escaped, you can use double-_ as a separator: the encoded string hello_2etxt__goodbye___2e_2e uniquely identifies the list of filenames ['hello.txt', 'goodbye', '..'].

Answer 2

You can use a newline character, or specifically CR (decimal code 13) or LF (decimal code 10) if you like. Whether this is suitable or not depends on what requirements you have with regard to displaying the concatenated string to the user - with this approach, it will print its parts on separate lines - which may be very good or very bad for the purpose (or you may not care...).

If you need the concatenated string to print on a single line, edit your question to specify this additional requirement; and we can go from there then.