syslog, klogctl - read and/or clear kernel message ring buffer; set console_loglevel
int syslog(int type, char *bufp, int len); /* No wrapper provided in glibc */ /* The glibc interface */
#include <sys/klog.h> int klogctl(int type, char *bufp, int len);
The type argument determines the action taken by this function.
Quoting from kernel/printk.c:
/* * Commands to sys_syslog: * * 0 -- Close the log. Currently a NOP. * 1 -- Open the log. Currently a NOP. * 2 -- Read from the log. * 3 -- Read all messages remaining in the ring buffer. * 4 -- Read and clear all messages remaining in the ring buffer * 5 -- Clear ring buffer. * 6 -- Disable printk to console * 7 -- Enable printk to console * 8 -- Set level of messages printed to console * 9 -- Return number of unread characters in the log buffer * 10 -- Return size of the log buffer */
Only command types 3 and 10 are allowed to unprivileged processes. Type 9 was added in 2.4.10; type 10 in 2.6.6.
The call syslog(2,buf,len) waits until this kernel log buffer is non-empty, and then reads at most len bytes into the buffer buf. It returns the number of bytes read. Bytes read from the log disappear from the log buffer: the information can only be read once. This is the function executed by the kernel when a user program reads /proc/kmsg.
The call syslog(3,buf,len) will read the last len bytes from the log buffer (non-destructively), but will not read more than was written into the buffer since the last "clear ring buffer" command (which does not clear the buffer at all). It returns the number of bytes read.
The call syslog(4,buf,len) does precisely the same, but also executes the "clear ring buffer" command.
The call syslog(5,dummy,dummy) executes just the "clear ring buffer" command. (In each call where buf or len is shown as "dummy", the value of the argument is ignored by the call.)
The call syslog(6,dummy,dummy) sets the console log level to minimum, so that no messages are printed to the console.
The call syslog(7,dummy,dummy) sets the console log level to default, so that messages are printed to the console.
The call syslog(8,dummy,level) sets the console log level to level, which must be an integer between 1 and 8 (inclusive). See the loglevel section for details.
The call syslog(9,dummy,dummy) returns the number of bytes currently available to be read on the kernel log buffer.
The call syslog(10,dummy,dummy) returns the total size of the kernel log buffer.
Every text line in a message has its own loglevel. This level is DEFAULT_MESSAGE_LOGLEVEL - 1 (6) unless the line starts with <d> where d is a digit in the range 1-7, in which case the level is d. The conventional meaning of the loglevel is defined in <linux/kernel.h> as follows:
#define KERN_EMERG "<0>" /* system is unusable */ #define KERN_ALERT "<1>" /* action must be taken immediately */ #define KERN_CRIT "<2>" /* critical conditions */ #define KERN_ERR "<3>" /* error conditions */ #define KERN_WARNING "<4>" /* warning conditions */ #define KERN_NOTICE "<5>" /* normal but significant condition */ #define KERN_INFO "<6>" /* informational */ #define KERN_DEBUG "<7>" /* debug-level messages */
In case of error, -1 is returned, and errno is set to indicate the error.
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Created 1996-2023 by Maxim Chirkov
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