Since when does Windows support forward slash as path separator?

Question

Historically, Windows used CR/LF and backslash, first for new lines, second for path separator.

But it turns out that it supports forward slashes as well:

Note that Windows supports either the forward slash (which is returned by the AltDirectorySeparatorChar field) or the backslash (which is returned by the DirectorySeparatorChar field) as path separator characters, while Unix-based systems support only the forward slash.

So, when did Windows start supporting forward slash as a path separator?

Answer 1

TL;DR: Windows did not explicitly support both, but DOS did since 2.0

The answer is rather a clear "yes but" (*1):

• DOS 2.0 and later supported both ways, thus all DOS functions will work properly either way.
• Newer Windows Versions (NT and later) copied that behaviour
• Windows applications did not always follow and may or may not accept either when parsing for filenames.

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The Long Read

Since when does Windows support forward slash as path separator?

This depends quite on the viewpoint taken. After all, early Windows is a GUI on top of DOS, originally using DOS and adding a layer to it, not replacing it. DOS in turn supported both slashes as path separators since 2.0. (*1)

The MS-DOS Encyclopedia notes on page 284 regarding path names:

Names in pathnames passed to Interrupt 21H functions can be separated by either a back slash (\) or a forward slash (/). (The forward slash is the separator character used in path names in UNIX/XENIX systems.) For example, the pathnames C:/MSP/SOURCE/ROSE.PAS and C:\MSP\SOURCE\ROSE.PAS are equivalent when passed to an Interrupt 21H function.

Thus all DOS functions that take a path name - and all Windows function that forward a path name to DOS - will support both ways (*2) since Windows 1.0 (*3)

This can easily be tried online using PCjs with a Windows 1.01 image (*4)

(Thanks to DL444 for the idea :))

Opening Notepad with a full filename using forward slashes.

This results in Notepad opening that file and showing the path name in its title bar:

Resulting opened file in Notepad

Except, the title line is a bit strange. Normally Notepad shows, like all early windows applications, only the file name, without path or drive (*5), as seen here when navigating with the open dialog:

Opening the test-file using the open dialog box.

But when using a path a command line argument the drive is gone but the path still visible. Doing the same with forward slashes shows a result like using the opening dialog:

Now with back slashes

Using back slashes gives the same output as using the dialog box:

Now it only shows the file name, no path

Ergo, the (DOS?) file function used work with either slash (*6), but applications like Notepad did not at all care about a forward slash. Despite presumably being written by the very same team that did Windows and for sure according to all guidelines at the time.

Many Years Later (Win10)

Using Win 10 CMD to start Notepad using forward slashes works better than expected:

Obviously Notepad has learned a bit within the last 38 years :))

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Conclusion: It Does, But it's Incoherent.

The DOS/file system parts of Windows always did and still do support both separators, other parts may or may not (*7) - although they did improve over time.

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*1 - BTW: The item has already been asked and answered.

*2 - That's why slash is a forbidden character in file name entries - and a known way to obfuscate software.

*3 - Windows 1.0 requires DOS2.0 or later - but then again, there are no path names prior to 2.0, so that's moot anyway :))

*4 - All following screenshots and observations taken with this setup of PCjs, DOS 2.0 and Windows 1.01.

*5 - And all uppercase.

*6 - Starting with 3.0 DOS itself uses an internal function (INT 2Fh function 1204h) to convert slashes. Interestingly using a WCHAR - I would assume to support MBCS.

*7 - Which in turn is not exactly unusual for Windows

Answer 2

A quick experiment in 86Box made me believe that this is supported starting from the very beginning with Windows 1.0. At least notepad and its standard file open dialog would accept paths delimited by either \ or /.

Screenshot of notepad opening a file with forward-slashed path in Windows 1.0. See the title bar for confirmation.

Answer 3

Since MS-DOS 2.0:

From https://en.wikipedia.org/wiki/Backslash#Filenames

MS-DOS 2 copied the hierarchical file system from Unix and thus used the forward slash, but (possibly on the insistence of IBM) added the backslash to allow paths to be typed into the command shell while retaining compatibility with MS-DOS 1 and CP/M where the slash was the command-line option indicator. For instance, in a Windows command shell, you can add the "wide" option to the "dir" command by typing "dir/w", yet you can run a program called "w" in a subdirectory "dir" with "dir\w". Except for COMMAND.COM, all other parts of the operating system accept both characters in a path, but the Microsoft convention remains to use a backslash, and APIs that return paths use backslashes.

Further, the question asks about Windows APIs. The DOS kernel, on which early versions of Windows are based has accepted forward slash as a path separator since before Windows was invented, so Windows APIs have always accepted forward slash. However, the Windows 3 API function openfile has a bug when the first character of a filename is a forward slash.

Windows NT and onward have maintained this for backward compatibility, even though internally they will normalise paths to regular backward slash notation.