Part 1 fundamental techniques in handling

Teflon Virtually all materials, including water and dirt particles in the air, can be triboelectrically charged. Insulative Materials A material that prevents or limits the flow of electrons across its surface or through its volume is called an insulator. Because an insulative material does not readily allow the flow of electrons, both positive and negative charges can reside on insulative surface at the same time, although at different locations.

Part 1 fundamental techniques in handling

Debuggers and Related Tools One of the most compelling problems for anyone writing kernel code is how to approach debugging. Kernel code cannot be easily executed under a debugger, nor can it be easily traced, because it is a set of functionalities not related to a specific process.

Kernel code errors can also be exceedingly hard to reproduce and can bring down the entire system with them, thus destroying much of the evidence that could be used to track them down.

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This chapter introduces techniques you can use to monitor kernel code and trace errors under such trying circumstances.

The most common debugging technique is monitoring, which in applications programming is done by calling printf at suitable points. When you are debugging kernel code, you can accomplish the same goal with printk. Now it's time to introduce some of the differences.

One of the differences is that printk lets you classify messages according to their severity by associating different loglevels, or priorities, with the messages.

You usually indicate the loglevel with a macro. The loglevel macro expands to a string, which is concatenated to the message text at compile time; that's why there is no comma between the priority and the format string in the following examples. Here are two examples of printkcommands, a debug message and a critical message: A number of security-related conditions are reported at this level.

Many drivers print information about the hardware they find at startup time at this level. Each string in the macro expansion represents an integer in angle brackets.

Integers range from 0 to 7, with smaller values representing higher priorities. The default loglevel value has changed several times during Linux development, so we suggest that you always specify an explicit loglevel.

Based on the loglevel, the kernel may print the message to the current console, be it a text-mode terminal, a serial line printer, or a parallel printer. One way to change it is by specifying the -c switch when invoking klogd, as specified in the klogd manpage.

Note that to change the current value, you must first kill klogdand then restart it with the -c option. Alternatively, you can write a program to change the console loglevel. The new level is specified as an integer value between 1 and 8, inclusive.

You'll want to raise the loglevel if you need to see your debugging messages; this is useful if you are developing kernel code remotely and the text console is not being used for an interactive session.

The file hosts four integer values. You may be interested in the first two: It should now be apparent why the hello. Linux allows for some flexibility in console logging policies by letting you send messages to a specific virtual console if your console lives on the text screen.

By default, the "console" is the current virtual terminal. The following program, setconsole, can be used to choose which console receives kernel messages; it must be run by the superuser and is available in the misc-progs directory. This is how the program works: The first byte of the array is a number that specifies the requested subcommand, and the following bytes are subcommand specific.

In setconsole, subcommand 11 is used, and the next byte stored in bytes[1] identifies the virtual console. Obviously, you can't read messages this way if klogd or another process is already reading the same data because you'll contend for it. If the circular buffer fills up, printk wraps around and starts adding new data to the beginning of the buffer, overwriting the oldest data.

The logging process thus loses the oldest data. This problem is negligible compared with the advantages of using such a circular buffer. For example, a circular buffer allows the system to run even without a logging process, while minimizing memory waste by overwriting old data should nobody read it.

Another feature of the Linux approach to messaging is that printk can be invoked from anywhere, even from an interrupt handler, with no limit on how much data can be printed. The only disadvantage is the possibility of losing some data.

If klogd isn't running, data remains in the circular buffer until someone reads it or the buffer overflows. Yet another possibility is to take the brute-force approach: Turning the Messages On and Off During the early stages of driver development, printk can help considerably in debugging and testing new code.

When you officially release the driver, on the other hand, you should remove, or at least disable, such print statements. Unfortunately, you're likely to find that as soon as you think you no longer need the messages and remove them, you'll implement a new feature in the driver or somebody will find a bug and you'll want to turn at least one of the messages back on.

There are several ways to solve both issues, to globally enable or disable your debug messages and to turn individual messages on or off.

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Why a Scientific Format? The scientific format may seem confusing for the beginning science writer due to its rigid structure which is so different from writing in the humanities. One reason for using this format is that it is a means of efficiently communicating scientific findings to the broad community of scientists in a .

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Part 1 fundamental techniques in handling

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