C :: Function That Gets As Parameter File And Folder Path

I'm writing a small function that gets as parameter a file's path and a folder's path, and copies the given file to that folder.

Code:
int copy_file(const char* source, const char* folder) {
char copy[PATH_MAX];
strcpy(copy, folder);
strcat(copy, "/");
strcat(copy, source);

[Code] ....

Basically, this function purpose is to make a backup of source in folder every X minutes (depending on user's input).

The problem is the second call to open():

This call attempts to open the file for writing, and creates it if it is not already exist.

It also truncates it before writing to it - and that's my concern:

Let's say this is the second time this function runs, so copy is already exist. open() will then truncate it, and then one of the system calls in the while loop fails.

In this situation, I might be left with no backup file.

The problem also arises for when source is a read-only file:

If source is a read-only file, and copy is not already exist (meaning - it's the first backup attempt), then everything's fine, but, if source is a read-only file and copy is already exist, then I have to first remove copy altogather, and make a fresh copy of source.

Making a backup with new name for copy every time copy_file() runs, will solve this problem, and how can this be accomplished?

I should say that I'd really prefer that copy and source will have the same names when copy_file() returns...