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GNU/Linux |
RedHat 5.2(Apollo) |
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syslog(2) |
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syslog − 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);
This 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 kernel has a cyclic buffer of length LOG_BUF_LEN (4096)
in which messages given as argument to the kernel function
printk() are stored (regardless of their
loglevel).
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.
The
loglevel
The kernel routine printk() will only print a message
on the console, if it has a loglevel less than the value of
the variable console_loglevel (initially
DEFAULT_CONSOLE_LOGLEVEL (7), but set to 10 if the kernel
commandline contains the word ’debug’, and to 15
in case of a kernel fault - the 10 and 15 are just silly,
and equivalent to 8). This variable is set (to a value in
the range 1-8) by the call syslog
(8,dummy,value). The calls syslog
(type,dummy,idummy) with type
equal to 6 or 7, set it to 1 (kernel panics only) or 7 (all
except debugging messages), respectively.
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. Otherwise, for type equal to 2, 3 or 4, syslog() returns the number of bytes read, and otherwise 0.
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EPERM |
An attempt was made to change console_loglevel or clear the kernel message ring buffer by a process without root permissions. | ||
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EINVAL |
Bad parameters. |
ERESTARTSYS
System call was interrupted by a signal - nothing was read.
This system call is Linux specific and should not be used in programs intended to be portable.
syslog(3)
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syslog(2) | |