largefile: improve and document

[Bruno Haible]

The 'largefile' module already supports native Windows since 2012.
This patch improves it: on mingw, with _FILE_OFFSET_BITS set to 64,
there is no need to redefine off_t since mingw already defines it to
a 64-bit type.


2017-05-13  Bruno Haible  <address@hidden>

        largefile: Improve and document.
        * m4/largefile.m4 (gl_LARGEFILE): Set WINDOWS_64_BIT_ST_SIZE to 0 if
        the mingw headers already define 'stat' appropriately.
        * modules/largefile (Description): Clarify.
        * doc/largefile.texi: New file.
        * doc/gnulib.texi: Include it.
        * doc/posix-headers/sys_types.texi: Update.

diff --git a/doc/gnulib.texi b/doc/gnulib.texi
index 094a98c..9a25553 100644
--- a/doc/gnulib.texi
+++ b/doc/gnulib.texi
@@ -6323,6 +6323,7 @@ to POSIX that it can be treated like any other Unix-like 
platform.
 
 @menu
 * Libtool and Windows::
+* Large File Support::
 * Precise file timestamps on Windows::
 * Windows sockets::
 * Native Windows Support without MSVC Support::
@@ -6330,6 +6331,8 @@ to POSIX that it can be treated like any other Unix-like 
platform.
 
 @include windows-libtool.texi
 
address@hidden largefile.texi
+
 @include windows-stat-timespec.texi
 
 @include windows-sockets.texi
diff --git a/doc/largefile.texi b/doc/largefile.texi
new file mode 100644
index 0000000..61e110d
--- /dev/null
+++ b/doc/largefile.texi
@@ -0,0 +1,8 @@
address@hidden Large File Support
address@hidden Large File Support
+
+The module provides support for files larger than 2 GB.
+To this effect, it ensures that @code{off_t} is a 64-bit integer type
+at least on the following platforms:
+glibc, Mac OS X, FreeBSD, NetBSD, OpenBSD, AIX, HP-UX, IRIX, OSF/1, Solaris,
+Cygwin, mingw, MSVC.
diff --git a/doc/posix-headers/sys_types.texi b/doc/posix-headers/sys_types.texi
index cf44bea..0f018b7 100644
--- a/doc/posix-headers/sys_types.texi
+++ b/doc/posix-headers/sys_types.texi
@@ -37,4 +37,4 @@ glibc x32
 
 This module, together with the module @code{largefile}, also defines the type
 @code{off_t} to a 64-bit integer type on some platforms:
-mingw (except mingw64), MSVC 9.
+mingw, MSVC 9.
diff --git a/m4/largefile.m4 b/m4/largefile.m4
index 790f7c0..edc1a9b 100644
--- a/m4/largefile.m4
+++ b/m4/largefile.m4
@@ -126,9 +126,24 @@ AC_DEFUN([gl_LARGEFILE],
       else
         WINDOWS_64_BIT_OFF_T=0
       fi
-      dnl But all native Windows platforms (including mingw64) have a 32-bit
-      dnl st_size member in 'struct stat'.
-      WINDOWS_64_BIT_ST_SIZE=1
+      dnl Some mingw versions define, if _FILE_OFFSET_BITS=64, 'struct stat'
+      dnl to 'struct _stat32i64' or 'struct _stat64' (depending on
+      dnl _USE_32BIT_TIME_T), which has a 32-bit st_size member.
+      AC_CACHE_CHECK([for 64-bit st_size], [gl_cv_member_st_size_64],
+        [AC_COMPILE_IFELSE(
+           [AC_LANG_PROGRAM(
+              [[#include <sys/types.h>
+                struct stat buf;
+                int verify_st_size_size[sizeof (buf.st_size) >= 8 ? 1 : -1];
+              ]],
+              [[]])],
+           [gl_cv_member_st_size_64=yes], [gl_cv_member_st_size_64=no])
+        ])
+      if test $gl_cv_member_st_size_64 = no; then
+        WINDOWS_64_BIT_ST_SIZE=1
+      else
+        WINDOWS_64_BIT_ST_SIZE=0
+      fi
       ;;
     *)
       dnl Nothing to do on gnulib's side.
diff --git a/modules/largefile b/modules/largefile
index 4700bef..fcae113 100644
--- a/modules/largefile
+++ b/modules/largefile
@@ -1,5 +1,5 @@
 Description:
-Use large files.
+Support for files larger than 2 GB.
 
 Files:
 m4/largefile.m4