R devel - May 2018 - Buffering in R 3.5 connections causes incorrect data in readChar

I noticed an issue where readChar does not return the correct value after a
call to readline. It appears that readChar is not aware of the buffering,
so it reads from the end of the buffer, rather than the current position in
the file. This is a significant change of behavior from R-3.4.4.

Below is a test case that I used to home in on the problem.

---

p<-"test2.txt"
cat("abcdefg
hijklmn
opqrstu",file=p)

cat("read char after readline (h)\n")
con <- file(p,"r")
invisible(readLines(con,1))
print(readChar(con,1))
close(con)

cat("read char after readline and seek (h)\n")
con <- file(p,"r")
invisible(readLines(con,1))
invisible(seek(con,seek(con)))
print(readChar(con,1))
close(con)

cat("read lines after readline (hijklmn)\n")
con <- file(p,"r")
invisible(readLines(con,1) )
print(readLines(con,1))
close(con)


cat("read line after char (bcdefg):\n")
con <- file(p,"r")
invisible(readChar(con,1) )
print(readLines(con,1))
close(con)

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On 05/26/2018 05:15 AM, Aaron Goodman wrote:
> I noticed an issue where readChar does not return the correct value after a
> call to readline. It appears that readChar is not aware of the buffering,
> so it reads from the end of the buffer, rather than the current position in
> the file. This is a significant change of behavior from R-3.4.4.
>
> Below is a test case that I used to home in on the problem.
Thanks for the report and analysis, you are right, readChar ignores the 
buffer (and it also ignores the pushback). But please note that this 
behavior is in line with the documentation, see ?readChar: readChar must 
only be used with binary connections, but the example uses it on a text 
connection. Buffering and pushback are only used on (readable) text 
connections. I will check whether we could report a runtime error.

Best
Tomas
> ---
>
> p<-"test2.txt"
> cat("abcdefg
> hijklmn
> opqrstu",file=p)
>
> cat("read char after readline (h)\n")
> con <- file(p,"r")
> invisible(readLines(con,1))
> print(readChar(con,1))
> close(con)
>
> cat("read char after readline and seek (h)\n")
> con <- file(p,"r")
> invisible(readLines(con,1))
> invisible(seek(con,seek(con)))
> print(readChar(con,1))
> close(con)
>
> cat("read lines after readline (hijklmn)\n")
> con <- file(p,"r")
> invisible(readLines(con,1) )
> print(readLines(con,1))
> close(con)
>
>
> cat("read line after char (bcdefg):\n")
> con <- file(p,"r")
> invisible(readChar(con,1) )
> print(readLines(con,1))
> close(con)
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel

Tomas,

Thank you for the explanation. I see in the documentation: "These functions
are intended to be used with binary-mode connections."  So I see how using
it on a text connection is undefined, and not a bug. An error or warning
when attempting to use a it on a text connection would be helpful
considering how the behavior has changed in R-3.5.

On Tue, May 29, 2018 at 3:09 AM, Tomas Kalibera <tomas.kalibera at
gmail.com>
wrote:
> On 05/26/2018 05:15 AM, Aaron Goodman wrote:
>
>> I noticed an issue where readChar does not return the correct value
after
>> a
>> call to readline. It appears that readChar is not aware of the
buffering,
>> so it reads from the end of the buffer, rather than the current
position
>> in
>> the file. This is a significant change of behavior from R-3.4.4.
>>
>> Below is a test case that I used to home in on the problem.
>>
> Thanks for the report and analysis, you are right, readChar ignores the
> buffer (and it also ignores the pushback). But please note that this
> behavior is in line with the documentation, see ?readChar: readChar must
> only be used with binary connections, but the example uses it on a text
> connection. Buffering and pushback are only used on (readable) text
> connections. I will check whether we could report a runtime error.
>
> Best
> Tomas
>
> ---
>>
>> p<-"test2.txt"
>> cat("abcdefg
>> hijklmn
>> opqrstu",file=p)
>>
>> cat("read char after readline (h)\n")
>> con <- file(p,"r")
>> invisible(readLines(con,1))
>> print(readChar(con,1))
>> close(con)
>>
>> cat("read char after readline and seek (h)\n")
>> con <- file(p,"r")
>> invisible(readLines(con,1))
>> invisible(seek(con,seek(con)))
>> print(readChar(con,1))
>> close(con)
>>
>> cat("read lines after readline (hijklmn)\n")
>> con <- file(p,"r")
>> invisible(readLines(con,1) )
>> print(readLines(con,1))
>> close(con)
>>
>>
>> cat("read line after char (bcdefg):\n")
>> con <- file(p,"r")
>> invisible(readChar(con,1) )
>> print(readLines(con,1))
>> close(con)
>>
>>         [[alternative HTML version deleted]]
>>
>> ______________________________________________
>> R-devel at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>
>
>
>
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