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[Patch, 3.3] Fix broken stack-based output stdio_filebuf


Hi,

I noticed the problem while working on libstdc++/10063 (3.4)
and I'm proposing the one-liner below for 3.3 (*).

In a nutshell, this code in stdio_filebuf:

     if (__size > 0 && __size < 4)
       {
         // Specify unbuffered.
         _M_buf = _M_unbuf;
         _M_buf_size = __size;
         _M_buf_size_opt = 0;
       }
     else
       {
         _M_buf_size_opt = __size;
         _M_allocate_internal_buffer();
       }

leads to _M_buf_size_opt == 0 not only when actually
_M_buf_size == 0 but also when _M_buf_size < 4 (> 0) and
a small stack-based buffer is in place.

Therefore, at the beginning of _M_really_overflow, this check:

bool __testunbuffered = _M_file.is_open() && !_M_buf_size_opt;

is *true* for small stack-based buffers and they are not considered
for flush out below:

   if (!__testunbuffered)
     _M_convert_to_external(_M_out_beg,  _M_out_end - _M_out_beg,
                            __elen, __plen);

The simple fix I'm proposing only amends a couple of comments and
changes the check to ... && !_M_buf_size which really means no buffer
to flush out.

Note that the issue is quite hidden since output stdio_filebufs are
usually constructed either zero buffered (when sync_with_stdio true)
or BUFSIZ sized.

Tested x86-linux, Ok?

Thanks,
Paolo.

/////////

(*) As regards mainline, as soon as 27_io reopens I will propose an
unified fix for this issue and libstdc++/10063.
2003-03-29  Paolo Carlini  <pcarlini at unitus dot it>

	* include/ext/stdio_filebuf.h
	(stdio_filebuf::stdio_filebuf(int, openmode, bool, size_t),
	stdio_filebuf::stdio_filebuf(__c_file*, openmode, size_t):
	_M_buf_size_opt == 0 means only "not to use an allocated buffer"
	since a stack-based buffer is used for small values of the size_t
	parameter.
	* include/bits/fstream.tcc (basic_filebuf::_M_really_overflow).
	If _M_buf_size != 0 flush out the buffer (any kind, stack-based too).
	* testsuite/ext/stdio_filebuf_2.cc: New testfile.
diff -urN libstdc++-v3-orig/include/bits/fstream.tcc libstdc++-v3/include/bits/fstream.tcc
--- libstdc++-v3-orig/include/bits/fstream.tcc	2003-03-21 09:53:35.000000000 +0100
+++ libstdc++-v3/include/bits/fstream.tcc	2003-03-29 13:24:53.000000000 +0100
@@ -345,7 +345,7 @@
     {
       int_type __ret = traits_type::eof();
       bool __testput = _M_out_cur && _M_out_beg < _M_out_end;
-      bool __testunbuffered = _M_file.is_open() && !_M_buf_size_opt;
+      bool __testunbuffered = _M_file.is_open() && !_M_buf_size;
 
       if (__testput || __testunbuffered)
 	{
diff -urN libstdc++-v3-orig/include/ext/stdio_filebuf.h libstdc++-v3/include/ext/stdio_filebuf.h
--- libstdc++-v3-orig/include/ext/stdio_filebuf.h	2003-02-13 13:20:03.000000000 +0100
+++ libstdc++-v3/include/ext/stdio_filebuf.h	2003-03-29 13:22:49.000000000 +0100
@@ -126,7 +126,7 @@
 	  _M_mode = __mode;
 	  if (__size > 0 && __size < 4)
 	    {
-	      // Specify unbuffered.
+	      // Specify not to use an allocated buffer.
 	      _M_buf = _M_unbuf;
 	      _M_buf_size = __size;
 	      _M_buf_size_opt = 0;
@@ -151,7 +151,7 @@
 	  _M_mode = __mode;
 	  if (__size > 0 && __size < 4)
 	    {
-	      // Specify unbuffered.
+	      // Specify not to use an allocated buffer.
 	      _M_buf = _M_unbuf;
 	      _M_buf_size = __size;
 	      _M_buf_size_opt = 0;
diff -urN libstdc++-v3-orig/testsuite/ext/stdio_filebuf_2.cc libstdc++-v3/testsuite/ext/stdio_filebuf_2.cc
--- libstdc++-v3-orig/testsuite/ext/stdio_filebuf_2.cc	1970-01-01 01:00:00.000000000 +0100
+++ libstdc++-v3/testsuite/ext/stdio_filebuf_2.cc	2003-03-29 13:52:22.000000000 +0100
@@ -0,0 +1,61 @@
+// 2003-03-29  Paolo Carlini  <pcarlini at unitus dot it>
+
+// Copyright (C) 2003 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 2, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING.  If not, write to the Free
+// Software Foundation, 59 Temple Place - Suite 330, Boston, MA 02111-1307,
+// USA.
+
+// stdio_filebuf.h
+
+#include <ext/stdio_filebuf.h>
+#include <cstdio>
+#include <fstream>
+#include <testsuite_hooks.h>
+
+// Small stack-based buffers (i.e., using _M_unbuf) were not flushed
+// out by _M_really_overflow upon overflow.
+void test01()
+{
+  
+  using namespace std;
+  bool test = true;
+
+  const char* name = "tmp_file1";
+  FILE* file = fopen(name, "w");
+  {
+    using namespace __gnu_cxx;
+    
+    // One char big stack-based buffer.
+    stdio_filebuf<char> sbuf(file, ios_base::out, 1); 
+    sbuf.sputc('T');
+    sbuf.sputc('S');
+  }
+  fclose(file);
+
+  filebuf fbuf;
+  fbuf.open(name, ios_base::in);
+  char buf[10];
+  streamsize n = fbuf.sgetn(buf, sizeof(buf));	
+  fbuf.close();
+  
+  VERIFY( n == 2 );
+  VERIFY( !memcmp(buf, "TS", 2) );
+}
+
+int main()
+{
+  test01();
+}

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