Using modified date of file for comparison. Is it safe?

I want to make a procedure that does one way syncrhonization of a folder between a server and a client. I was thinking to use ModifiedDate as a criterio, provided that only the date of the server files will be used. Procedure will not use Modified dates of files on the client at all. It will read dates from the server and compare them with dates read from the server last time the procedure run. Do you think t开发者_运维技巧his is safe? Is there any possibility that Modified Date will not be changed when a file is edited or it will be changed without touching the contents of the file (eg. from some strange antivirus programs)?

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Don't count on the modification date of a file.

Strange programs (antiviruses and such) are not the problem more than the fact that you simply can't count on the client and server clocks to be synchronized.

Why not do a straightforward diff or hash calculation? You can't get a better comparison than that.

Taking performance considerations into effect, you can use the following heuristic:

1. If date hasn't changed then file is obviously the same
2. If date has changed, file contents might have changed, and might have not (for example: it has been touched). In this case in order to get a definitive answer you must examine the file somehow.

Bottom line: modification date can always give you a true negative (file not changed), but might sometimes yield a false positive - and this case you must verify.

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You didn't mention what OS you're on, but on UNIX platforms the modification time can be set by client code to any value it wants (see the utimes() API or touch command). Therefore you shouldn't rely on modification times to tell you whether a file has changed or not. I imagine Windows is somewhat similar.