From: Johannes Sixt <j6t@kdbg.org>
Date: Thu, 18 Dec 2025 18:13:22 GMT
Subject: Re: [PATCH 13/18] mingw: implement `readlink()`
Message-ID: <8826825b-79ad-4700-aeb5-71e7847ca5dc@kdbg.org>
In-Reply-To: <8fef8220f4d60b48358882fbc961c4604eab3abf.1765980535.git.gitgitgadget@gmail.com>

Am 17.12.25 um 15:08 schrieb Karsten Blees via GitGitGadget:
> From: Karsten Blees <blees@dcon.de>
> 
> Implement `readlink()` by reading NTFS reparse points via the
> `read_reparse_point()` function that was introduced earlier to determine
> the length of symlink targets. Works for symlinks and directory
> junctions. If symlinks are disabled, fail with `ENOSYS`.

This last sentence is obsolete, I think, because I cannot see how the
patch achieves a failure with ENOSYS.

> 
> Signed-off-by: Karsten Blees <blees@dcon.de>
> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> ---
>  compat/mingw-posix.h |  3 +--
>  compat/mingw.c       | 24 ++++++++++++++++++++++++
>  2 files changed, 25 insertions(+), 2 deletions(-)
> 
> diff --git a/compat/mingw-posix.h b/compat/mingw-posix.h
> index 0939feff27..896aa976b1 100644
> --- a/compat/mingw-posix.h
> +++ b/compat/mingw-posix.h
> @@ -121,8 +121,6 @@ struct utsname {
>   * trivial stubs
>   */
>  
> -static inline int readlink(const char *path UNUSED, char *buf UNUSED, size_t bufsiz UNUSED)
> -{ errno = ENOSYS; return -1; }
>  static inline int symlink(const char *oldpath UNUSED, const char *newpath UNUSED)
>  { errno = ENOSYS; return -1; }
>  static inline int fchmod(int fildes UNUSED, mode_t mode UNUSED)
> @@ -197,6 +195,7 @@ int setitimer(int type, struct itimerval *in, struct itimerval *out);
>  int sigaction(int sig, struct sigaction *in, struct sigaction *out);
>  int link(const char *oldpath, const char *newpath);
>  int uname(struct utsname *buf);
> +int readlink(const char *path, char *buf, size_t bufsiz);
>  
>  /*
>   * replacements of existing functions
> diff --git a/compat/mingw.c b/compat/mingw.c
> index 5d2a8c247c..b407a2ac07 100644
> --- a/compat/mingw.c
> +++ b/compat/mingw.c
> @@ -2698,6 +2698,30 @@ int link(const char *oldpath, const char *newpath)
>  	return 0;
>  }
>  
> +int readlink(const char *path, char *buf, size_t bufsiz)
> +{
> +	WCHAR wpath[MAX_PATH];
> +	char tmpbuf[MAX_PATH];
> +	int len;
> +	DWORD tag;
> +
> +	if (xutftowcs_path(wpath, path) < 0)
> +		return -1;
> +
> +	if (read_reparse_point(wpath, TRUE, tmpbuf, &len, &tag) < 0)
> +		return -1;
> +
> +	/*
> +	 * Adapt to strange readlink() API: Copy up to bufsiz *bytes*, potentially
> +	 * cutting off a UTF-8 sequence. Insufficient bufsize is *not* a failure
> +	 * condition. There is no conversion function that produces invalid UTF-8,
> +	 * so convert to a (hopefully large enough) temporary buffer, then memcpy
> +	 * the requested number of bytes (including '\0' for robustness).
> +	 */
> +	memcpy(buf, tmpbuf, min(bufsiz, len + 1));
> +	return min(bufsiz, len);
> +}
> +
>  pid_t waitpid(pid_t pid, int *status, int options)
>  {
>  	HANDLE h = OpenProcess(SYNCHRONIZE | PROCESS_QUERY_INFORMATION,

-- Hannes