Re: Printf stops calling write() syscall after encountering a line feed

[Anthony DeRosa <anthony at ransomed dot us>]

Thanks, Jeff, for your reply.? I am not surprised that you and Corinna
do not see the problem, because it seems like a glaring issue that
would be addressed immediately if it ever arose.? I'm just kind of
puzzled as to what is going on and thought someone else may have seen
similar symptoms.? I have to put this project on the back-burner for
now, but will update when I get the time to instrument the system with
a debugger.

Thanks,

Anthony

On Mon, Nov 30, 2009 at 4:30 PM, Jeff Johnston <jjohnstn@redhat.com> wrote:
>
> On 25/11/09 03:52 AM, Corinna Vinschen wrote:
>>
>> On Nov 24 20:48, Anthony DeRosa wrote:
>>>
>>> Hi,
>>>
>>> I am having trouble with printf, and I am positive that its not my
>>> terminal's fault or a buffering problem. ?First I'll explain the
>>> problem, and second why I think it's not my terminal's fault.
>>>
>>> The problem is that printf stops calling the write() syscall after
>>> encountering a line feed character ('\n'). ?Below is some code and
>>> corresponding output to demonstrate.
>>>
>>> The code:
>>>
>>> ? ? printf("line 0\n");
>>> ? ? printf("line 1\nline 2\n");
>>> ? ? printf("line 3");
>>> ? ? printf("\nline 4\n");
>>> ? ? printf("line 5\n");
>>> ? ? fflush(stdout);
>>>
>>> The expected output:
>>>
>>> line 0
>>> line 1
>>> line 2
>>> line 3
>>> line 4
>>> line 5
>>>
>>> (my broken) Newlib's output:
>>>
>>> line 0
>>> line 1 ?// stops printing line 2 after line feed
>>> line 3 ?// stops printing line 4 after line feed
>>> line 5
>>>
>>>
>>> The reason I don't blame my terminal is because if I replace all calls
>>> to printf with calls to write(1, text, textlen), then everything works
>>> as expected with regards to line feeds. ?Things also work as expected
>>> if I use sprintf() followed by a call to write().
>>
>> I don't know whom to blame in your case, but the above testcase works
>> fine on Cygwin, which is using newlib as well. ?I think you will have
>> to put more debugging effort into this, like, say, running your testcase
>> in a debugger.
>>
>
> Corinna is correct. ?You should be calling the __sfvwrite_r routine (libc/stdio/fvwrite.c) for each printf call with the input string (i.e. there isn't any formatting done by the main _vfprintf_r routine). ?In __sfvwrite_r, you should fall into the line-buffered code which searches the string for a newline and after finding one, calls _fflush_r. ?You should make two calls to _fflush_r when the string has the two newlines in it and each of these calls should end up calling your write syscall. ?That should give you a place to start.
>
> FWIW: testing the above code using newlib and running on an mn10300-elf simulator also works fine.
>
> -- Jeff J.
>
>>
>> Corinna
>>
>