Switch to POSIX API on Cygwin.

This follows other Boost libraries (in particular, Boost.System) in
treating Cygwin as a POSIX platform rather than Windows. This is
a breaking change, but apparently downstream Boost.Filesystem packages
on Cygwin are patched to the same effect.

As part of this change, AT_NO_AUTOMOUNT is made an optional requirement
to enable POSIX *at APIs. This flag is not POSIX-standard and is not
supported on Cygwin, while *at APIs are.

Author: Lastique

build/Jamfile.v2

@@ -39,7 +39,7 @@ rule select-windows-crypto-api ( properties * )
 {
     local result ;
 
-    if <target-os>windows in $(properties) || <target-os>cygwin in $(properties)
+    if <target-os>windows in $(properties)
     {
         if ! [ has-config-flag BOOST_FILESYSTEM_DISABLE_BCRYPT : $(properties) ] &&
             [ configure.builds ../config//has_bcrypt : $(properties) : "has BCrypt API" ]
@@ -175,10 +175,6 @@ project
       <target-os>windows:<define>_WIN32_WINNT=0x0A00
       <target-os>windows:<define>WIN32_LEAN_AND_MEAN
       <target-os>windows:<define>NOMINMAX
-      <target-os>cygwin:<define>BOOST_USE_WINDOWS_H
-      <target-os>cygwin:<define>_WIN32_WINNT=0x0A00
-      <target-os>cygwin:<define>WIN32_LEAN_AND_MEAN
-      <target-os>cygwin:<define>NOMINMAX
 
       # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105329
       # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105651
@@ -207,7 +203,7 @@ rule select-platform-specific-sources ( properties * )
 {
     local result ;
 
-    if <target-os>windows in $(properties) || <target-os>cygwin in $(properties)
+    if <target-os>windows in $(properties)
     {
         result += <source>windows_file_codecvt.cpp ;
     }

config/has_posix_at_apis.cpp

@@ -22,7 +22,7 @@ int main()
     res |= renameat(AT_FDCWD, "x", fd1, "y");
 
     struct stat st;
-    res |= fstatat(fd1, "y", &st, AT_NO_AUTOMOUNT | AT_SYMLINK_NOFOLLOW);
+    res |= fstatat(fd1, "y", &st, AT_SYMLINK_NOFOLLOW);
 
     res |= linkat(fd1, "y", fd1, "z", 0);
     res |= symlinkat("foo/z", fd1, "sz");

doc/index.htm

@@ -338,8 +338,8 @@ <h3>Note for <a name="Cygwin">Cygwin</a> users</h3>
 <p> <a href="http://www.cygwin.com/">Cygwin</a> version 1.7 or later is
 required because only versions of GCC with wide character strings are supported.</p>
 
-<p> The library's implementation code treats Cygwin as a Windows platform, and
-thus uses the Windows API and uses Windows path syntax as the native path
+<p> The library's implementation code treats Cygwin as a POSIX platform, and
+thus uses the POSIX API and POSIX path syntax as the native path
 syntax.</p>
 
 <h2><a name="Change-history">Version history</a></h2>

doc/release_history.html

@@ -42,8 +42,17 @@
 
 <h2>1.91.0</h2>
 <ul>
-  <li>Due to a change in Boost.System, Boost.Filesystem now defines and uses its own platform macros <code>BOOST_FILESYSTEM_POSIX_API</code> and <code>BOOST_FILESYSTEM_WINDOWS_API</code>. These macros may not necessarily match the old <code>BOOST_POSIX_API</code> and <code>BOOST_WINDOWS_API</code> macros defined by Boost.System.</li>
+  <li>Boost.Filesystem now defines and uses its own platform macros <code>BOOST_FILESYSTEM_POSIX_API</code> and <code>BOOST_FILESYSTEM_WINDOWS_API</code>. The old <code>BOOST_POSIX_API</code> and <code>BOOST_WINDOWS_API</code> macros are still defined by Boost.System.</li>
   <li>On POSIX platforms not supporting <code>openat</code> and related APIs, fixed an error reported by <code>recursive_directory_iterator</code> increment when the iterator encounters a dangling symlink and following symlinks is disabled.</li>
+  <li><b>Breaking change for Cygwin users.</b> The library now treats Cygwin as a POSIX platform. This has several user-facing consequences, in particular:
+    <ul>
+      <li>The <code>path::value_type</code> type has changed from <code>wchar_t</code> to <code>char</code>. As on other POSIX platforms, wide character paths will be converted to the narrow character encoding using the locale set by <code>path::imbue</code>. Note that since Cygwin is running on top of Windows, it will also perform character code conversion internally. It is important that locale in the Cygwin environment is configured correctly.</li>
+      <li>Path syntax now follows POSIX conventions, Windows-specific paths (e.g. UNC paths, drive names and path prefixes) are not supported.</li>
+      <li>Certain Windows-specific parts of the API will become inaccessible. Since reparse points are a Windows-only feature, <code>file_type::reparse_file</code> will not be reported for files. Reparse point handling is dependent on Cygwin runtime behavior.</li>
+      <li>Cygwin has its own implementation of symlinks that is incompatible with native Windows symlinks. Boost.Filesystem will now create and operate on Cygwin symlinks.</li>
+      <li>Error codes reported by Boost.Filesystem will now use POSIX <code>errno</code> values.</li>
+    </ul>
+  </li>
 </ul>
 
 <h2>1.90.0</h2>

doc/v3.html

@@ -134,7 +134,7 @@ <h3>Compiler support</h3>
   supported.<br/>
 &nbsp;</li>
   <li>Cygwin versions prior to 1.7 are no longer supported because they lack
-  wide string support. Cygwin now compiles only for the Windows API and path
+  wide string support. Cygwin now compiles only for the POSIX API and path
   syntax.<br/>
 &nbsp;</li>
   <li>MinGW versions not supporting wide strings are no longer supported.<br/>

include/boost/filesystem/config.hpp

@@ -59,8 +59,8 @@
 #error BOOST_FILESYSTEM_WINDOWS_API and BOOST_FILESYSTEM_POSIX_API must not be defined by users
 #endif
 
-// Cygwin is treated as Windows to minimize path character code conversions
-#if defined(_WIN32) || defined(__WIN32__) || defined(WIN32) || defined(__CYGWIN__)
+//  Note: Cygwin is treated as a POSIX platform
+#if defined(BOOST_WINDOWS)
 #define BOOST_FILESYSTEM_WINDOWS_API
 #else
 #define BOOST_FILESYSTEM_POSIX_API

src/operations.cpp

@@ -186,6 +186,11 @@ using boost::system::system_category;
 #define BOOST_FILESYSTEM_NO_O_CLOEXEC
 #endif
 
+// At least Cygwin (cygwin1.dll 3.6.5, as of 2025-12-25) doesn't support AT_NO_AUTOMOUNT
+#ifndef AT_NO_AUTOMOUNT
+#define AT_NO_AUTOMOUNT 0
+#endif
+
 #if defined(_POSIX_SYNCHRONIZED_IO) && _POSIX_SYNCHRONIZED_IO > 0
 #define BOOST_FILESYSTEM_HAS_FDATASYNC
 #endif

src/platform_config.hpp

@@ -68,8 +68,11 @@
 #define _INCLUDE_STDCSOURCE_199901
 #endif
 
-#if defined(_WIN32) || defined(_WIN64) || defined(__WIN32__) || defined(__TOS_WIN__) || defined(__WINDOWS__) || \
-    defined(__CYGWIN__)
+#if defined(__CYGWIN__)
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+#elif defined(_WIN32) || defined(_WIN64) || defined(__WIN32__) || defined(__TOS_WIN__) || defined(__WINDOWS__)
 // Define target Windows version macros before including any other headers
 #include <boost/winapi/config.hpp>
 #endif

test/Jamfile.v2

@@ -49,6 +49,7 @@ rule check-mklink ( properties * )
 project
     : requirements
       <include>.
+      <include>../src
       <target-os>windows:<define>_SCL_SECURE_NO_WARNINGS
       <target-os>windows:<define>_SCL_SECURE_NO_DEPRECATE
       <target-os>windows:<define>_CRT_SECURE_NO_WARNINGS

test/deprecated_test.cpp

@@ -78,7 +78,7 @@ int cpp_main(int /*argc*/, char* /*argv*/[])
     // The choice of platform is make at runtime rather than compile-time
     // so that compile errors for all platforms will be detected even though
     // only the current platform is runtime tested.
-    platform = (platform == "Win32" || platform == "Win64" || platform == "Cygwin") ? "Windows" : "POSIX";
+    platform = (platform == "Win32" || platform == "Win64") ? "Windows" : "POSIX";
     std::cout << "Platform is " << platform << '\n';
 
     l.push_back('s');