------- Additional Comments From dannysmith at users dot sourceforge dot net
2005-04-07 08:35 -------
This optimization in basic_filebuf::xsgetn() causes problems on DOS based file
sytstem when ifstreams are opened in text mode (\r\n line endings) and the
user suppled buffer length exceeds _M_buf_size.
2004-09-13 Paolo Carlini <[EMAIL PROTECTED]>
PR libstdc++/11722
* include/std/std_fstream.h (xsgetn): Declare only.
* include/bits/fstream.tcc (xsgetn): Define, optimize for the
always_noconv() case: when __n > __buflen, copy the available
buffer and issue a direct read.
The problem is that the native read translates the input \r\n to
\n, returning the number of chars written, so this test:
fstream.tcc:line 550
if (__len == __n)
{
_M_set_buffer(0);
_M_reading = true;
}
fails.
Attached is a testcase submitted by a mingw user, The input file,
c:\winnt\schedlog.txt, has DOS line endings and is 32667 bytes in size.
The testcase stops after the first read with:
Is EOF? 1 gcount: 494
The first 512 bytes contain 18 line endings
After reverting the xsgetn patch, or disabling for test mode files
with:
*** fstream.tcc.orig Sun Jan 30 17:44:23 2005
--- fstream.tcc Wed Apr 6 21:48:11 2005
*************** namespace std
*** 521,530 ****
// future: when __n > __buflen we read directly instead of using the
// buffer repeatedly.
const bool __testin = _M_mode & ios_base::in;
const streamsize __buflen = _M_buf_size > 1 ? _M_buf_size - 1
: 1;
if (__n > __buflen && __check_facet(_M_codecvt).always_noconv()
! && __testin && !_M_writing)
{
// First, copy the chars already present in the buffer.
const streamsize __avail = this->egptr() - this->gptr();
--- 521,531 ----
// future: when __n > __buflen we read directly instead of using the
// buffer repeatedly.
const bool __testin = _M_mode & ios_base::in;
+ const bool __testbinary = _M_mode & ios_base::binary;
const streamsize __buflen = _M_buf_size > 1 ? _M_buf_size - 1
: 1;
if (__n > __buflen && __check_facet(_M_codecvt).always_noconv()
! && __testin && __testbinary && !_M_writing)
{
// First, copy the chars already present in the buffer.
const streamsize __avail = this->egptr() - this->gptr();
the testcase produces:
Is EOF? 0 gcount: 512
Is EOF? 0 gcount: 512
<snip 57 reads of 512 bytes >
Is EOF? 0 gcount: 512
Is EOF? 1 gcount: 294
Disabling this optimization for non-binary input streams on all platforms is a
bit extreme. Should this be conditional on an os_defines.h define?
Danny
Testcase modified from:
https://sourceforge.net/tracker/?
func=detail&atid=102435&aid=1171379&group_id=2435
#include <fstream>
using namespace std;
#define BS 512
int main (void)
{
char buf [BS+1];
// change to any DOS text file > BS bytes
ifstream f ("c:\\winnt\\schedlog.txt") ;
int r;
while (!f.eof ())
{
f.read (buf, BS);
r = f.gcount ();
buf [r] = 0;
fprintf (stderr, "%d %d: %s\n", f.eof (), r, buf);
}
return 0;
}
--
What |Removed |Added
----------------------------------------------------------------------------
Summary|basic_filebuf::xsgetn() |basic_filebuf::xsgetn()
|fails with text mode and |fails with text mode and DOS
| |line endings and large
| |buffers
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20806
