问题
I am debugging a driver for linux (specifically ubuntu server 9.04), and there are several printf statements in the code.
Where can I view the output of these statements?
EDIT1: What i'm trying to do is write to kernel using the proc file-system. The print code is
static int proc_fractel_config_write(struct file *file, const char *argbuf, unsigned long count, void *data)
{
printk(KERN_DEBUG "writing fractel config\n");
...
In kern.log, I see the following message when i try to overwrite the file /proc/net/madwifi/ath1/fractel_config (with varying time of course).
[ 8671.924873] proc write
[ 8671.924919]
Any explainations?
回答1:
It depends on the distribution, but many use klogd(8) to get the messages from the kernel and will either log them to a file (sometimes /var/log/dmesg or /var/log/kernel) or to the system log via syslog(3). In the latter case, where the log entries end up will depend on the configuration of syslogd(8).
One note about the dmesg command: Kernel messages are stored in a circular buffer, so large amounts of output will be overwritten.
回答2:
Many times KERN_DEBUG level messages are filtered and you need to explicitly increase the logging level. You can see what the system defaults are by examining /proc/sys/kernel/printk. For example, on my system:
# cat /proc/sys/kernel/printk
4 4 1 7
the first number shows the console log level is KERN_WARNING (see proc(5) man pages for more information). This means KERN_NOTICE, KERN_INFO, and KERN_DEBUG messages will be filtered from the console. To increase the logging level or verbosity, use dmesg
$ sudo dmesg -n 7
$ cat /proc/sys/kernel/printk
7 4 1 7
Here, setting the level to 7 (KERN_DEBUG) will allow all levels of messages to appear on the console. To automate this, add loglevel=N to the kernel boot parameters where N is the log level you want going to the console or ignore_loglevel to print all kernel messages to the console.
回答3:
You'll get the output with the command dmesg
回答4:
dmesg outputs all the messages from the kernel. Finding your desired messages would be difficult. Better use dmesg and grep combination and use a driver specific label in all your printk messages. That will ease in eliminating all the unwanted messages.
printk("test: hello world")
dmesg | grep test
回答5:
I had this problem on Ubuntu 11.10 and 10.04 LTS, on the former I edited /etc/rsyslog.d/50-default.conf, then restarted rsyslog using "sudo service rsyslog restart" to restart rsyslogd. Then it worked.
Note that Ubuntu uses *r*syslogd, not syslogd.
回答6:
You might try a higher level than KERN_DEBUG, for example KERN_INFO. Depending on your configuration the lowest priority messages might not be displayed.
回答7:
In centos (Atleast in centos 6.6) the output will be in /var/log/messages
来源：https://stackoverflow.com/questions/4518420/where-does-output-of-print-in-kernel-go