I compiled a small test program in C, but when I run it, I get no output!

You probably compiled the program into a binary named test, didn’t you? Linux has a program called test, which tests if a certain condition is true, it never produces any output on the screen. Instead of just typing test, try: ./test

What is IP masquerading and when is it of use?

IP masquerading is a process where one computer acts as an IP gateway for a network. All computers on the network send their IP packets through the gateway, which replaces the source IP address with its own address and then forwards it to the internet. Perhaps the source IP port number is also replaced with another port number, although that is less interesting. All hosts on the internet see the packet as originating from the gateway.

Any host on the Internet which wishes to send a packet back, ie in reply, must necessarily address that packet to the gateway. Remember that the gateway is the only host seen on the internet. The gateway rewrites the destination address, replacing its own address with the IP address of the machine which is being masqueraded, and forwards that packet on to the local network for delivery.

This procedure sounds simple, and it is. It provides an effective means by which you can provide second class internet connections for a complete LAN using only one (internet) IP address.

Command Pipelines

Pipes are easy. The Unix shells provide mechanisms which you can use them to allow you to generate remarkably sophisticated `programs’ out of simple components. We call that a pipeline. A pipeline is composed of a data generator, a series of filters, and a data consumer. Often that final stage is as simple as displaying the final output on stdout, and sometimes the first stage is as simple as reading from stdin. I think all shells use the “|” character to separate each stage of a pipeline. So:

data-generator | filter | … | filter | data-consumer

Each stage of the pipeline runs in parallel, within the limits which the system permits. Hey, look closely, because that last phrase is important. Are you on a uni-processor system because if you are, then obviously only one process runs at a time, although that point is simply nitpicking. But pipes are buffers capable of holding only finite data. A process can write into a pipe until that pipe is full. When the pipe is full the process writing into it blocks until some of the data already in the pipe has been read. Similarly, a process can read from a pipe until that pipe is empty. When it’s empty the reading process is blocked until some more data has been written into the pipe.