Re: [PLUG] C++, FIFOs, getline, and O_NDELAY

[Will Dyson <will_dyson@pobox.com>]

Michael F. Robbins wrote:

[...]
> are needed.  I would have thought that checking for the POLLIN event
> would have returned true since there were more data left to read. 
> However, my guess is that the ifstream class has an internal buffer
> which has the next command inside, so the poll() returns false.

Yeah, mixing iostreams with other io functions is quite the source of 
unhappyness. There is definatly a buffer (what you get a pointer to when 
you call rdbuf()).

FYI, I belive that the basic_filebuf class in libstdc++3 library (from g++ 
3.0) does not have the fd() member. It is certainly not part of the ANSI 
C++ spec. You could probably implement your own filebuf class to take care 
of this. Just something to be aware of, should you ever consider 
switching/upgrading compilers.

> Any ideas?  Right now I'm looking at setting O_NDELAY on the file
> descriptor.  This would certainly allow a simple read() function to work
> on it, but would the getline() work correctly?

You definatly don't want to set O_NDELAY on the fd behind the ifstream's 
back. I don't even see how you might do that, since O_NDELAY is a flag you 
pass to open(), and ifstream is opening the file for you.

couple approaches:

1) The basic_filebuf class has a public member in_avail(), which returns 
the number of charecters in the buffer. After poll tells you you have 
input waiting, loop on in_avail():

while(fifoIn.rdbuf()->in_avail()) {
	fifoIn.getline(...);
}

2) Set the ifstream to unbuffered ( fifoIn.rdbuf()->pubsetbuf(0,0) ). Not 
a huge performance hit, since you are never hitting the disk with a FIFO.

3) Do as Kyle Burton suggests and launch a thread to loop on getline() and 
dispatch commands. This is probably what I would do, but I learned to 
program on the BeOS, which probably warped my mind :)

--
Will Dyson
"Back off man, I'm a scientist!" -Dr. Peter Venkman


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