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4.2. 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.

4.2.1. 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>; we list them in order of decreasing severity:


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 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. In the 2.6.10 kernel, DEFAULT_MESSAGE_LOGLEVEL is KERN_WARNING, but that has been known to change in the past.

Based on the loglevel, the kernel may print the message to the current console, be it a text-mode terminal, a serial port, or a parallel printer. If the priority is less than the integer variable console_loglevel, the message is delivered to the console one line at a time (nothing is sent unless a trailing newline is provided). 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 (which is often most easily done with the dmesg command). When using klogd, you should remember that it doesn't save consecutive identical lines; it only saves the first such line and, at a later time, the number of repetitions it received.

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 O'Reilly's 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) reach the console; if it is set to 8, all messages, including debugging ones, are displayed.

It is also possible to read and modify the console loglevel using the text file /proc/sys/kernel/printk. The file hosts four integer values: the current loglevel, the default level for messages that lack an explicit loglevel, the minimum allowed loglevel, and the boot-time default loglevel. Writing a single value to this file changes the current loglevel to that value; thus, for example, you can cause all kernel messages to appear at the console by simply entering:

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

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

4.2.2. Redirecting Console Messages

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.

The following is the program in its entirety. You should invoke it with a single argument specifying the number of the console that is to receive messages.

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.

4.2.3. How Messages Get Logged

The printk function writes messages into a circular buffer that is _ _LOG_BUF_LEN bytes long: a value from 4 KB to 1 MB chosen while configuring the kernel. The function 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 and is the default behavior for klogd. The dmesg command can be used to look at the content of the buffer without flushing it; actually, the command returns to stdout the whole content of the buffer, whether or not it has already been read.

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. Therefore, the logging process 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 customize /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,[1] or issue the command cat /proc/kmsg from an unused xterm.

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

4.2.4. 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 implement a new feature in the driver (or somebody finds a bug), and you 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 is defined or undefined, depending on whether SCULL_DEBUG is defined, and 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.

4.2.5. Rate Limiting

If you are not careful, you can find yourself generating thousands of messages with printk, overwhelming the console and, possibly, overflowing the system log file. When using a slow console device (e.g., a serial port), an excessive message rate can also slow down the system or just make it unresponsive. It can be very hard to get a handle on what is wrong with a system when the console is spewing out data nonstop. Therefore, you should be very careful about what you print, especially in production versions of drivers and especially once initialization is complete. In general, production code should never print anything during normal operation; printed output should be an indication of an exceptional situation requiring attention.

On the other hand, you may want to emit a log message if a device you are driving stops working. But you should be careful not to overdo things. An unintelligent process that continues forever in the face of failures can generate thousands of retries per second; if your driver prints a "my device is broken" message every time, it could create vast amounts of output and possibly hog the CPU if the console device is slow—no interrupts can be used to driver the console, even if it is a serial port or a line printer.

In many cases, the best behavior is to set a flag saying, "I have already complained about this," and not print any further messages once the flag gets set. In others, though, there are reasons to emit an occasional "the device is still broken" notice. The kernel has provided a function that can be helpful in such cases:

int printk_ratelimit(void);

This function should be called before you consider printing a message that could be repeated often. If the function returns a nonzero value, go ahead and print your message, otherwise skip it. Thus, typical calls look like this:

if (printk_ratelimit(  ))
    printk(KERN_NOTICE "The printer is still on fire\n");

printk_ratelimit works by tracking how many messages are sent to the console. When the level of output exceeds a threshold, printk_ratelimit starts returning 0 and causing messages to be dropped.

The behavior of printk_ratelimit can be customized by modifying /proc/sys/kernel/printk_ratelimit (the number of seconds to wait before re-enabling messages) and are /proc/sys/kernel/printk_ratelimit_burst (the number of messages accepted before rate-limiting).

4.2.6. Printing Device Numbers

Occasionally, when printing a message from a driver, you will want to print the device number associated withp the hardware of interest. It is not particularly hard to print the major and minor numbers, but, in the interest of consistency, the kernel provides a couple of utility macros (defined in <linux/kdev_t.h>) for this purpose:

int print_dev_t(char *buffer, dev_t dev);
char *format_dev_t(char *buffer, dev_t dev);

Both macros encode the device number into the given buffer; the only difference is that print_dev_t returns the number of characters printed, while format_dev_t returns buffer; therefore, it can be used as a parameter to a printk call directly, although one must remember that printk doesn't flush until a trailing newline is provided. The buffer should be large enough to hold a device number; given that 64-bit device numbers are a distinct possibility in future kernel releases, the buffer should probably be at least 20 bytes long.

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