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Linux Device Drivers, Second Edition by Alessandro Rubini, Jonathan Corbet

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Chapter 4. Debugging Techniques

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.

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

Debugging by Printing

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.


We used the printk function in earlier chapters with the simplifying assumption that it works like printf. 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. For example, KERN_INFO, which we saw prepended to some of the earlier print statements, is one of the possible loglevels of the message. 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 printk commands, a debug message and a critical message:

printk(KERN_DEBUG "Here I am: %s:%i\n", __FILE__, __LINE_&_);
printk(KERN_CRIT "I'm trashed; giving up on %p\n", ptr);

There are eight possible loglevel strings, defined in the header <linux/kernel.h>:


Used for emergency messages, usually those that precede a crash.


A situation requiring immediate action.


Critical conditions, often related to serious hardware or software failures.


Used to report error conditions; device drivers will often use KERN_ERR to report hardware difficulties.


Warnings about problematic situations that do not, in themselves, create serious problems with the system.


Situations that are normal, but still worthy of note. A number of security-related conditions are reported at this level.


Informational messages. Many drivers print information about the hardware they find at startup time at this level.


Used for debugging messages.

Each string (in the macro expansion) represents an integer in angle brackets. Integers range from 0 to 7, with smaller values representing higher priorities.

A printk statement with no specified priority defaults to DEFAULT_MESSAGE_LOGLEVEL, specified in kernel/printk.c as an integer. 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. If the priority is less than the integer variable console_loglevel, the message is displayed. If both klogd and syslogd are running on the system, kernel messages are appended to /var/log/messages (or otherwise treated depending on your syslogd configuration), independent of console_loglevel. If klogd is not running, the message won’t reach user space unless you read /proc/kmsg.

The variable console_loglevel is initialized to DEFAULT_CONSOLE_LOGLEVEL and can be modified through the sys_syslog system call. 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 klogd and then restart it with the -c option. Alternatively, you can write a program to change the console loglevel. You’ll find a version of such a program in misc-progs/setlevel.c in the source files provided on the O’Reilly FTP site. The new level is specified as an integer value between 1 and 8, inclusive. If it is set to 1, only messages of level 0 (KERN_EMERG) will reach the console; if it is set to 8, all messages, including debugging ones, will be displayed.

You’ll probably want to lower the loglevel if you work on the console and you experience a kernel fault (see Section 4.4 later in this chapter), because the fault-handling code raises the console_loglevel to its maximum value, causing every subsequent message to appear on the console. 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.

From version 2.1.31 on it is possible to read and modify the console loglevel using the text file /proc/sys/kernel/printk. The file hosts four integer values. You may be interested in the first two: the current console loglevel and the default level for messages. With recent kernels, for instance, you can cause all kernel messages to appear at the console by simply entering

 #echo 8 > /proc/sys/kernel/printk

If you run 2.0, however, you still need the setlevel tool.

It should now be apparent why the hello.c sample had the <1> markers; they are there to make sure that the messages appear on the console.

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. To select a different virtual terminal to receive messages, you can issue ioctl(TIOCLINUX) on any console device. 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:

int main(int argc, char **argv)
    char bytes[2] = {11,0}; /* 11 is the TIOCLINUX cmd number */

    if (argc==2) bytes[1] = atoi(argv[1]); /* the chosen console */
    else {
        fprintf(stderr, "%s: need a single arg\n",argv[0]); exit(1);
    if (ioctl(STDIN_FILENO, TIOCLINUX, bytes)<0) {    /* use stdin */
        fprintf(stderr,"%s: ioctl(stdin, TIOCLINUX): %s\n",
                argv[0], strerror(errno));

setconsole uses the special ioctl command TIOCLINUX, which implements Linux-specific functions. To use TIOCLINUX, you pass it an argument that is a pointer to a byte array. 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. The complete description of TIOCLINUX can be found in drivers/char/tty_io.c, in the kernel sources.

How Messages Get Logged

The printk function writes messages into a circular buffer that is LOG_BUF_LEN (defined in kernel/printk.c) bytes long. It then wakes any process that is waiting for messages, that is, any process that is sleeping in the syslog system call or that is reading /proc/kmsg. These two interfaces to the logging engine are almost equivalent, but note that reading from /proc/kmsg consumes the data from the log buffer, whereas the syslog system call can optionally return log data while leaving it for other processes as well. In general, reading the /proc file is easier, which is why it is the default behavior for klogd.

If you happen to read the kernel messages by hand, after stopping klogd you’ll find that the /proc file looks like a FIFO, in that the reader blocks, waiting for more data. 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 the klogd process is running, it retrieves kernel messages and dispatches them to syslogd, which in turn checks /etc/syslog.conf to find out how to deal with them. syslogd differentiates between messages according to a facility and a priority; allowable values for both the facility and the priority are defined in <sys/syslog.h>. Kernel messages are logged by the LOG_KERN facility, at a priority corresponding to the one used in printk (for example, LOG_ERR is used for KERN_ERR messages). If klogd isn’t running, data remains in the circular buffer until someone reads it or the buffer overflows.

If you want to avoid clobbering your system log with the monitoring messages from your driver, you can either specify the -f (file) option to klogd to instruct it to save messages to a specific file, or modify /etc/syslog.conf to suit your needs. Yet another possibility is to take the brute-force approach: kill klogd and verbosely print messages on an unused virtual terminal,[21] or issue the command cat /proc/kmsg from an unused xterm.

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.

Here we show one way to code printk calls so you can turn them on and off individually or globally; the technique depends on defining a macro that resolves to a printk (or printf) call when you want it to.

  • Each print statement can be enabled or disabled by removing or adding a single letter to the macro’s name.

  • All the messages can be disabled at once by changing the value of the CFLAGS variable before compiling.

  • The same print statement can be used in kernel code and user-level code, so that the driver and test programs can be managed in the same way with regard to extra messages.

The following code fragment implements these features and comes directly from the header scull.h.

#undef PDEBUG             /* undef it, just in case */
#  ifdef __KERNEL__
     /* This one if debugging is on, and kernel space */
#    define PDEBUG(fmt, args...) printk( KERN_DEBUG "scull: " fmt, 
                                         ## args)
#  else
     /* This one for user space */
#    define PDEBUG(fmt, args...) fprintf(stderr, fmt, ## args)
#  endif
#  define PDEBUG(fmt, args...) /* not debugging: nothing */

#undef PDEBUGG
#define PDEBUGG(fmt, args...) /* nothing: it's a placeholder */

The symbol PDEBUG depends on whether or not SCULL_DEBUG is defined, and it displays information in whatever manner is appropriate to the environment where the code is running: it uses the kernel call printk when it’s in the kernel, and the libc call fprintf to the standard error when run in user space. The PDEBUGG symbol, on the other hand, does nothing; it can be used to easily “comment” print statements without removing them entirely.

To simplify the process further, add the following lines to your makefile:

# Comment/uncomment the following line to disable/enable debugging

# Add your debugging flag (or not) to CFLAGS
ifeq ($(DEBUG),y)
  DEBFLAGS = -O -g -DSCULL_DEBUG # "-O" is needed to expand inlines


The macros shown in this section depend on a gcc extension to the ANSI C preprocessor that supports macros with a variable number of arguments. This gcc dependency shouldn’t be a problem because the kernel proper depends heavily on gcc features anyway. In addition, the makefile depends on GNU’s version of make; once again, the kernel already depends on GNU make, so this dependency is not a problem.

If you’re familiar with the C preprocessor, you can expand on the given definitions to implement the concept of a “debug level,” defining different levels and assigning an integer (or bit mask) value to each level to determine how verbose it should be.

But every driver has its own features and monitoring needs. The art of good programming is in choosing the best trade-off between flexibility and efficiency, and we can’t tell what is the best for you. Remember that preprocessor conditionals (as well as constant expressions in the code) are executed at compile time, so you must recompile to turn messages on or off. A possible alternative is to use C conditionals, which are executed at runtime and therefore permit you to turn messaging on and off during program execution. This is a nice feature, but it requires additional processing every time the code is executed, which can affect performance even when the messages are disabled. Sometimes this performance hit is unacceptable.

The macros shown in this section have proven themselves useful in a number of situations, with the only disadvantage being the requirement to recompile a module after any changes to its messages.

[21] For example, use setlevel 8; setconsole 10 to set up terminal 10 to display messages.

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