Problem with line buffering and getc function on 1.7.33.

Kaz Kylheku 920-082-4242@kylheku.com
Sun Mar 13 00:37:00 GMT 2016


On 12.03.2016 14:29, Corinna Vinschen wrote:
> On Mar 12 20:39, Corinna Vinschen wrote:
>> On Mar 11 16:05, Kaz Kylheku wrote:
>> > We can reproduce the problem with just file streams using
>> > a much simpler program:
>> >
>> > #include <stdio.h>
>> >
>> > int main(void)
>> > {
>> >   FILE *out = fopen("file", "w+");
>> >   setvbuf(out, (char *) NULL, _IOLBF, 0);
>> >   getc(out);
>> >   clearerr(out);
>> >   fseek(out, 0, SEEK_SET);
>> >   putc('a', out);
>> >   putc('b', out);
>> >   putc('c', out);
>> >   putc('d', out);
>> >   putc('e', out);
>> >   putc('\n', out);
>> >   fclose(out);
>> >   return 0;
>> > }
>> >
>> > The contents of file end up being "\n": one empty
>> > line, instead of "abcde\n":
>> >
>> >   $ cat file
>> >
>> >   $
>> 
>> Thanks for the testcase.  I can reproduce the issue and I see where 
>> the
>> problem occurs, but I'm still puzzled.  Comparing the code in our 
>> newlib
>> C library with its BSD counterparts, I could swear the same behaviour
>> happens on OpenBSD as well.  If not (which needs testing), I wonder 
>> why
>> and where newlib's actually different.  Right now I don't see the
>> difference.
> 
> I do now.  Basically it's setvbuf screwing up the internal flags in the
> FILE structure.  I took the liberty to update newlib's setvbuf to the
> OpenBSD version locally and I'm going to apply my patches to newlib
> soon.  I'll provide a new 2.5.0 test release of Cygwin with this patch
> tomorrow or early next week.


Indeed, I thought it would have something to do with the
stream->_flags, because when I single-stepped over the getc(stream)
line (necessary for the problem to repro, IIRC) the only apparent state
change in the stream was to the _flags member:

(gdb) n
8         getc(out);
(gdb) p *out
$2 = {_p = 0x80039c28 "", _r = 0, _w = 0, _flags = -30575, _file = 3, 
_bf = {
                                           ^^^^^^^^^^^^^^^^

     _base = 0x80039c28 "", _size = 1024}, _lbfsize = -1024, _data = 0x0,
   _cookie = 0x80039a30, _read = 0x61164cb0 <__sread>,
   _write = 0x61164f30 <__swrite64>, _seek = 0x61164e50 <__sseek>,
   _close = 0x61164ea0 <__sclose>, _ub = {_base = 0x0, _size = 0}, _up = 
0x0,
   _ur = 0, _ubuf = "\000\000", _nbuf = "", _lb = {_base = 0x0, _size = 
0},
   _blksize = 0, _flags2 = 0, _offset = 0, _seek64 = 0x61164ed0 
<__sseek64>,
   _lock = 0x80039bf8, _mbstate = {__count = 0, __value = {__wch = 0,
       __wchb = "\000\000\000"}}}
(gdb) n
9         clearerr(out);
(gdb) p *out
$3 = {_p = 0x80039c28 "", _r = 0, _w = 0, _flags = -22347, _file = 3, 
_bf = {
                                          ^^^^^^^^^^^^^^^^^^

     _base = 0x80039c28 "", _size = 1024}, _lbfsize = -1024, _data = 0x0,
   _cookie = 0x80039a30, _read = 0x61164cb0 <__sread>,
   _write = 0x61164f30 <__swrite64>, _seek = 0x61164e50 <__sseek>,
   _close = 0x61164ea0 <__sclose>, _ub = {_base = 0x0, _size = 0}, _up = 
0x0,
   _ur = 0, _ubuf = "\000\000", _nbuf = "", _lb = {_base = 0x0, _size = 
0},
   _blksize = 0, _flags2 = 0, _offset = 0, _seek64 = 0x61164ed0 
<__sseek64>,
   _lock = 0x80039bf8, _mbstate = {__count = 0, __value = {__wch = 0,
       __wchb = "\000\000\000"}}}



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