Cygwin: define FILE as struct __sFILE64, not as __sFILE

Until Cygwin 3.3.6, we define __LARGE64_FILES unconditionally, so we
were using the type __sFILE64 even for 64 bit.  That was lazy and wrong.
so commit 2902b3a09e ("Cygwin: drop requirement to build newlib's
stdio64") tried to fix that.

Unfortunately this patch forgot to take the exposure of the typename
__sFILE64 in userspace into account.  This leads to trouble in C++ due
to name mangling.

Commit 0f376ae220 tried to fix this by just renaming __sFILE to
__sFILE64 by using a macro.  While __sFILE and __sFILE64 are the same
size, they are not exactly congruent.

To avoid backward compatibility problems, make sure to define FILE
as the  real __sFILE64, and make sure that __sFILE is not defined at
all on Cygwin.

Fixes: 0f376ae220 ("Cygwin: rename __sFILE to __sFILE64 for backward
compatibility")
Fixes: 2902b3a09e ("Cygwin: drop requirement to build newlib's stdio64")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

newlib/libc/include/sys/reent.h

@@ -144,6 +144,11 @@ struct __sbuf {
 
 #define _REENT_SMALL_CHECK_INIT(ptr) /* nothing */
 
+/* Cygwin must use __sFILE64 for backward compatibility, even though
+   it's not defining __LARGE64_FILES anymore.  To make sure that __sFILE
+   is never defined, disable it here explicitely. */
+#ifndef __CYGWIN__
+
 struct __sFILE {
   unsigned char *_p;	/* current position in (some) buffer */
   int	_r;		/* read space left for getc() */
@@ -195,13 +200,24 @@ struct __sFILE {
   int   _flags2;        /* for future use */
 };
 
+#endif /* !__CYGWIN__ */
+
 #ifdef __CUSTOM_FILE_IO__
 
 /* Get custom _FILE definition.  */
 #include <sys/custom_file.h>
 
 #else /* !__CUSTOM_FILE_IO__ */
-#ifdef __LARGE64_FILES
+/* Cygwin must use __sFILE64 for backward compatibility, even though
+   it's not defining __LARGE64_FILES anymore.  It also has to make
+   sure the name is the same to satisfy C++ name mangling.  Overloading
+   _fpos64_t just fixes a build problem.  The _seek64 function is
+   actually never used without __LARGE64_FILES being defined. */
+#if defined (__LARGE64_FILES) || defined (__CYGWIN__)
+#ifdef __CYGWIN__
+#define _fpos64_t _fpos_t
+#endif
+
 struct __sFILE64 {
   unsigned char *_p;	/* current position in (some) buffer */
   int	_r;		/* read space left for getc() */

winsup/cygwin/include/cygwin/config.h

@@ -49,12 +49,6 @@ extern inline struct _reent *__getreent (void)
 
 #define __FILENAME_MAX__ 4096	/* Keep in sync with PATH_MAX in limits.h. */
 
-/* Unfortunately we defined __LARGE64_FILES until Cygwin 3.3.6, so
-   FILE was based on `struct __sFILE64'.  The name is exposed into
-   userspace and consequentially used in C++ name mangling.  We must
-   redefine __sFILE as __sFILE64 to stay backward compatible. */
-#define __sFILE	__sFILE64
-
 /* The following block of macros is required to build newlib correctly for
    Cygwin.  Changing them in applications has no or not the desired effect.
    Just leave them alone. */