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 sys-
log(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 [Toc] [Back]
The kernel has a cyclic buffer of length LOG_BUF_LEN (4096, since
1.3.54: 8192, since 2.1.113: 16384) 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 [Toc] [Back]
The kernel routine printk() will only print a message on the console,
if it has a loglevel less than the value of the variable con-
sole_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.
EPERM An attempt was made to change console_loglevel or clear the kernel
message ring buffer by a process without root permissions.
EINVAL Bad parameters.
ERESTARTSYS [Toc] [Back]
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)
Linux 1.2.9 1995-06-11 SYSLOG(2)
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