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If the file referenced by new exists prior to calling rename(), the behavior is implementation-defined. On POSIX systems, the file is removed. On Windows systems, the rename() fails, returns a nonzero value, and sets errno to EACCES.
This creates issues when trying to write portable code, or when trying to get the other behavior.
- What to do in an application targeted at only POSIX or only Windows if the "other" behavior is desired (i.e. non-clobbering on POSIX, clobbering on Windows). For non-clobbering on POSIX you use access() and only call rename() if the file does not exist. For clobbering on Windows you call remove() before rename(). In both cases there is a TOCTOU so the part about needing a secure directory applies.
- What to do in applications intended to be portable to any C99 implementation. This is a combination of the above two cases. I.e. if you want non-clobbering you do an existence check even though it wouldn't be needed on Windows and other non-clobbering implementations; if you want clobbering you do an explicit remove() even though it wouldn't be needed on POSIX and other clobbering implementations.
For portability, you must ensure that the file referenced by new does not exist when rename() is invoked.
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