Just borrowed a book from National Library (PNM) entitled UNIX Network Programming by Stevens , Fenner and Rudoff. Previously I had post a blog about the book , that covers about TCP Client/Server. Now , I’m going to touch on the chapter about I/O Multiplexing and Socket Options.
Nonblocking I/O Model
When we set a socket to be nonblocking, we are telling the kernel “when an I/O operation that I request cannot be completed without putting the process to sleep, do not put the process to
sleep, but return an error instead.” We will describe nonblocking I/O in Chapter 16.
The first three times that we call recvfrom, there is no data to return, so the kernel immediately returns an error of EWOULDBLOCK instead. The fourth time we call recvfrom, a datagram is ready, it is copied into our application buffer, and recvfrom returns successfully. We then process the data. When an application sits in a loop calling recvfrom on a nonblocking descriptor like this, it is called polling. The application is continually polling the kernel to see if some operation is ready. This is often a waste of CPU time, but this model is occasionally encountered, normally on systems dedicated to one function.
I/O Multiplexing Model
With I/O multiplexing, we call select or poll and block in one of these two system calls, instead of blocking in the actual I/O system call.
We block in a call to select, waiting for the datagram socket to be readable. When select returns that the socket is readable, we then call recvfrom to copy the datagram into our application buffer.
Comparing Figure 6.3 to Figure 6.1, there does not appear to be any advantage, and in fact, there is a slight disadvantage because using select requires two system calls instead of one. But the advantage in using select, which we will see later in this chapter, is that we can wait for more than one descriptor to be ready. Another closely related I/O model is to use multithreading with blocking I/O. That model very closely resembles the model described above, except that instead of using select to block on multiple file descriptors, the program uses multiple threads (one per file descriptor), and each thread is then free to call blocking system calls like recvfrom.
Signal-Driven I/O Model
We can also use signals, telling the kernel to notify us with the SIGIO signal when the descriptor is ready. We call this signal-driven I/O .
We first enable the socket for signal-driven I/O (as we will describe in Section 25.2) and install a signal handler using the sigaction system call. The return from this system call is immediate and our process continues; it is not blocked. When the datagram is ready to be read, the SIGIO signal is generated for our process. We can either read the datagram from the
signal handler by calling recvfrom and then notify the main loop that the data is ready to be processed (this is what we will do in Section 25.3), or we can notify the main loop and let it read the datagram.
Regardless of how we handle the signal, the advantage to this model is that we are not blocked while waiting for the datagram to arrive. The main loop can continue executing and just wait to be notified by the signal handler that either the data is ready to process or the datagram is ready to be read.
Asynchronous I/O Model
Asynchronous I/O is defined by the POSIX specification, and various differences in the realtime functions that appeared in the various standards which came together to form the current POSIX specification have been reconciled. In general, these functions work by telling the kernel to start the operation and to notify us when the entire operation (including the copy of the data from the kernel to our buffer) is complete. The main difference between this model and the
signal-driven I/O model in the previous section is that with signal-driven I/O, the kernel tells us when an I/O operation can be initiated, but with asynchronous I/O, the kernel tells us when an I/O operation is complete.
We call aio_read (the POSIX asynchronous I/O functions begin with aio_ or lio_) and pass the kernel the descriptor, buffer pointer, buffer size (the same three arguments for read), file offset (similar to lseek), and how to notify us when the entire operation is complete. This system call returns immediately and our process is not blocked while waiting for the I/O to complete. We assume in this example that we ask the kernel to generate some signal when the operation is complete. This signal is not generated until the data has been copied into our application buffer, which is different from the signal-driven I/O model. As of this writing, few systems support POSIX asynchronous I/O. We are not certain, for
example, if systems will support it for sockets. Our use of it here is as an example to compare against the signal-driven I/O model.
IPv4 Socket Options
These socket options are processed by IPv4 and have a level of IPPROTO_IP. We defer discussion of the multicasting socket options until Section 21.6.
IP_HDRINCL Socket Option
If this option is set for a raw IP socket (Chapter 28), we must build our own IP header for all the datagrams we send on the raw socket. Normally, the kernel builds the IP header for datagrams sent on a raw socket, but there are some applications (notably traceroute) that build their own IP header to override values that IP would place into certain header fields. When this option is set, we build a complete IP header, with the following exceptions:
IP always calculates and stores the IP header checksum.
If we set the IP identification field to 0, the kernel will set the field.
If the source IP address is INADDR_ANY, IP sets it to the primary IP address of the
Setting IP options is implementation-dependent. Some implementations take any IP options that were set using the IP_OPTIONS socket option and append these to the header that we build, while others require our header to also contain any desired IP options. Some fields must be in host byte order, and some in network byte order. This is
implementation-dependent, which makes writing raw packets with IP_HDRINCL not as portable as we’d like.
We show an example of this option in Section 29.7.
Well , I just touch about IPV4 socket options . For the rest of the chapter and info , you all have to borrow the book from National Library or buy it in a bookstore.
p/S:- For you all info , PIKOM PC Fair is just around the corner- starting on 19th December till 21th December , 2014. I’m looking forward to attend the fair that will be held at the KL Convention Center. Some of this article is an excerpt from the book UNIX Network Programming – written by Stevens , Fenner and Rudoff published by Addison-Wesley.