SYSLOGSection: Linux Programmer's Manual (2 )
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NAMEsyslog - read and/or clear kernel message ring buffer; set console_loglevel
#include <unistd.h> #include <linux/unistd.h> _syscall3(int, syslog, int, type, char *, bufp, int, len); int syslog(int type, char *bufp, int len);
DESCRIPTIONThis is probably not the function you are interested in. Look at syslog(3) for the C library interface. This page only documents the bare kernel system call interface.
The type argument determines the action taken by syslog.
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 up to the last 4k of messages in the ring buffer. * 4 -- Read and clear last 4k of messages in the ring buffer * 5 -- Clear ring buffer. * 6 -- Disable printk's to console * 7 -- Enable printk's to console * 8 -- Set level of messages printed to console */
Only function 3 is allowed to non-root processes.
The kernel log buffer
The call syslog (2,buf,len) waits until this kernel log buffer is nonempty, 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 (nondestructively), 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,idummy) only executes the `clear ring buffer' command.
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 */
RETURN VALUEIn case of error, -1 is returned, and errno is set. Otherwise, for type equal to 2, 3 or 4, syslog() returns the number of bytes read, and otherwise 0.
CONFORMING TOThis system call is Linux specific and should not be used in programs intended to be portable.
This document was created by man2html, using the manual pages.
Time: 10:15:43 GMT, June 19, 2013