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Title

Reserved filenames in Windows

Description

Did you know that there are some filenames you're not allowed to use in Windows? I knew that there are characters that you cannot use in a path, like <c>:</c> and <c>?</c> and <c>*</c>, but I didn't know that otherwise innocuous-looking names like <c>aux.txt</c> and <c>con.txt</c> are verboten. I learned about this from the article <a href="https://threadreaderapp.com/thread/1058676834940776450.html" source="Thread Reader" author="foone">It is 2018 and this error message is a mistake from 1974. This limitation, which is still found in the very latest Windows 10, dates back to []</a>, which has a pretty click-bait--y title and is quite long, to boot. The author includes a lot of detail and conjecture about how it is that Windows 10 still has some forbidden/reserved filenames (and then takes half of it back or corrects it in several long footnotes). Instead of citing the article, I included the upshot that I found interesting, which is better explained in the official Microsoft documentation <a href="https://docs.microsoft.com/en-us/windows/desktop/fileio/naming-a-file" source="Windows Dev Center">Naming Files, Paths, and Namespaces</a>, referenced from the article above <bq>Do not use the following reserved names for the name of a file: CON, PRN, AUX, NUL, COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9, LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, and LPT9. Also avoid these names followed immediately by an extension; for example, NUL.txt is not recommended.</bq> While it's possible that this is only a recommendation, <c>Explorer.exe</c> does not allow these filenames at all, as shown in the screenshot below (take on an up-to-date Windows 10 Enterprise installation). <img src="{att_link}con_txt_is_illegal.png" href="{att_link}con_txt_is_illegal.png" align="none" scale="50%"> I tried <c>nul</c> and <c>con</c> with several different extensions (<c>bmp</c>, <c>exe</c>, <c>txt</c>, <c>doc</c>, etc.) and none of them are allowed. I have not checked whether this restriction applies to files created programmatically (i.e. is the restriction enforced by <c>Explorer</c> or by <c>CreateFileEx</c>?) I'm just wondering how I've never noticed or heard of this before. Talk about a restriction that Microsoft was wise never to fix because people don't run into it! I guess the filenames are so short I've never tried to use them.