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Date: Fri, 1 Dec 2023 10:51:57 +0100
From: Corinna Vinschen <corinna-cygwin@cygwin.com>
To: cygwin@cygwin.com
Subject: Re: Cygwin tools to read/write NTFS alternate data streams?
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On Nov 30 04:55, Martin Wege via Cygwin wrote:
> Hello,
> 
> Does Cygwin have tools (modified /usr/bin/dd ?) to read/write NTFS
> alternate data streams?

No.  As you know, the colon is translated to a normal filename
character, and there's no POSIX-like API to expose ADS raw to user
space.

There is, however, an old function we still expose to user space
for backward compat:

  #include <sys/cygwin.h>

  int cygwin_attach_handle_to_fd (char *name,
				  int fd,
				  HANDLE handle,
				  mode_t bin,
				  DWORD myaccess);

This allows to sneak in a HANDLE into a Cygwin file descriptor
representation, kind of like this:

  HANDLE h;
  int fd;

  h = CreateFile ("foo:bar", GENERIC_READ, FILE_SHARE_VALID_FLAGS,
		  NULL, OPEN_EXISTING, 0, NULL);
  if (h != INVALID_HANDLE_VALUE)
    {
      fd = cygwin_attach_handle_to_fd ("foo", -1, h, 0, GENERIC_READ);
      if (fd < 0)
	bail_out;
    }

For the bin parameter, only 0, O_BINARY or O_TEXT are acceptable,
for myaccess, only GENERIC_READ and/or GENERIC_WRITE are acceptable.


Corinna