Uwe Ligges wrote:
> Tony Plate wrote:
>
>> [EMAIL PROTECTED] wrote:
>>
>>> [EMAIL PROTECTED] wrote:
>>>
>>>
>>>> I've noticed that seek(con, 0, "end", rw="r") on a file connection
>>>> does not always work correctly after a write (R 2.1.0 on Windows).
>>>>
>>>> [Is a call to fflush() needed inside file_seek() in
>>>> main/connections.c?]
>>>
>>>
>>>
>>>
>>>
>>> If you have an idea where to fflush() precisely and your patch works,
>>> please tell it! I'll happily run some test cases where seeking matters.
>>>
>>
>> I couldn't see why the current code was returning a bad value under
>> some conditions. (That's why didn't offer anything more than a
>> suggestion). My suggestion to use an fflush() was a guess (hence the
>> question mark, but evidence for the guess being correct was that doing
>> a flush at the R command line made the whole thing work correctly.)
>> To be safe, I would try to put a flush() right at the beginning of
>> file_seek(), before the call to f_tell(). I tried this, and with the
>> modification the test case I gave produced correct output. Here's how
>> the beginning of my modified file_seek() function (in
>> main/connections.c) looks:
>
> >
>
>> static double file_seek(Rconnection con, double where, int origin, int
>> rw)
>> {
>> Rfileconn this = con->private;
>> FILE *fp = this->fp;
>> #if defined(HAVE_OFF_T) && defined(__USE_LARGEFILE)
>> off_t pos;
>> #else
>> #ifdef Win32
>> off64_t pos;
>> #else
>> long pos;
>> #endif
>> #endif
>> int whence = SEEK_SET;
>> fflush(fp);
>> pos = f_tell(fp);
>>
>> /* make sure both positions are set */
>>
>
>
> Works for your example, but I found another one where it introduces a
> worse bug when using origin="current". Hence it's not that easy.
>
> After reviewing this issue more closely, I think writeLines() into a
> binary connection might be the real problem and a misuse in this case.
> See the last paragrpah in the Details Section of ?writeLines. Hence,
> this might also be an issue related to the text mode connection problem
> on Windows.
>
> Using simple writeChar and readChar statements works as expected for me
> (at least, I was not able to produce anything unexpected). I'm no longer
> convinced that this is a bug in R.
I see the same (buggy) behavior when I replace the writeLines()
statements by writeChar() statments (but continue using readLines()). I
also see the same buggy behavior when I explicitly supply a 'sep'
argument to writeLines(). Transcripts of both of these are below.
[Also, in both cases, calling the R function flush() at indicated
position in the transcript results in correct output.]
Regarding the documentation for writeLines, it states:
Normally 'writeLines' is used with a text connection, and the
default separator is converted to the normal separator for that
platform (LF on Unix/Linux, CRLF on Windows, CR on Classic MacOS).
For more control, open a binary connection and specify the
precise value you want written to the file in 'sep'. For even
more control, use 'writeChar' on a binary connection.
The sentence beginning "For more control" seems to permit the use of
writeLines() for binary connections. What suggested to you that it was
a misuse?
> # seek(, rw="r") on a file does not always work a write
> # even when sep="\n" is supplied to writeLines
> f <- file("tmp3.txt", "w+b")
> # Write something earlier in the file
> seek(f, 10, rw="w")
[1] 0
> writeLines(c("ghi", "jkl"), f, sep="\n")
> seek(f, 20, rw="w")
[1] 18
> writeLines(c("abc"), f, sep="\n")
> seek(f, 0, "end", rw="w")
[1] 24
> # Try to read at the end of the file
> seek(f, 0, "end", rw="r")
[1] 0
> readLines(f, -1)
character(0)
> seek(f, 0, "end", rw="w")
[1] 18
> # write something at the end of the file
> writeLines(c("def"), f, sep="\n")
> # Try to read at the end of the file
> # flush(f) # flushing here makes the seek work correctly
> seek(f, 0, "end", rw="r")
[1] 24
> seek(f, NA, rw="r") # ***WRONG*** (should return 28)
[1] 24
> readLines(f, -1) # ***WRONG*** (should return character(0))
[1] "def"
> seek(f, 20, rw="r")
[1] 28
> readLines(f, -1)
[1] "abc" "def"
> seek(f, 0, "end", rw="r") # now it works correctly
[1] 28
> seek(f, NA, rw="r")
[1] 28
> readLines(f, -1)
character(0)
> close(f)
>
> # seek(, rw="r") on a file does not always work a write
> # even when writeChar is used instead of writeLines
> f <- file("tmp3.txt", "w+b")
> # Write something earlier in the file
> seek(f, 10, rw="w")
[1] 0
> writeChar(c("ghi\n", "jkl\n"), f, eos=NULL)
> seek(f, 20, rw="w")
[1] 18
> writeChar(c("abc\n"), f, eos=NULL)
> seek(f, 0, "end", rw="w")
[1] 24
> # Try to read at the end of the file
> seek(f, 0, "end", rw="r")
[1] 0
> readLines(f, -1)
character(0)
> seek(f, 0, "end", rw="w")
[1] 18
> # write something at the end of the file
> writeChar(c("def\n"), f, eos=NULL)
> # Try to read at the end of the file
> # flush(f) # flushing here makes the seek work correctly
> seek(f, 0, "end", rw="r")
[1] 24
> seek(f, NA, rw="r") # ***WRONG*** (should return 28)
[1] 24
> readLines(f, -1) # ***WRONG*** (should return character(0))
[1] "def"
> seek(f, 20, rw="r")
[1] 28
> readLines(f, -1)
[1] "abc" "def"
> seek(f, 0, "end", rw="r") # now it works correctly
[1] 28
> seek(f, NA, rw="r")
[1] 28
> readLines(f, -1)
character(0)
> close(f)
>
> version
_
platform i386-pc-mingw32
arch i386
os mingw32
system i386, mingw32
status
major 2
minor 1.0
year 2005
month 04
day 18
language R
>
-- Tony Plate
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