Originální popis anglicky:
syslog, klogctl - read and/or clear kernel message ring buffer; set
console_loglevel
Návod, kniha: Linux Programmer's Manual
/* The glibc interface */
#include <sys/klog.h>
int klogctl(int type, char *bufp, int len);
/* The handcrafted system call */
#include <unistd.h>
#include <linux/unistd.h>
_syscall3(int, syslog, int, type, char *, bufp, int, len);
int syslog(int type, char *bufp, int len);
If you need the libc function
syslog(), (that talks to
syslogd(8)), then look at
syslog(3). The system call of this
name is about controlling the kernel
printk() buffer, and the glibc
version is called
klogctl().
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 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
* 9 -- Return number of unread characters in the log buffer
*/
Only function 3 is allowed to non-root processes. (Function 9 was added in
2.4.10.)
The kernel log buffer
The kernel has a cyclic buffer of length LOG_BUF_LEN (4096, since 1.3.54: 8192,
since 2.1.113: 16384; in recent kernels the size can be set at compile time)
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.
- EINVAL
- Bad parameters.
- EPERM
- An attempt was made to change console_loglevel or clear the
kernel message ring buffer by a process without root permissions.
- ERESTARTSYS
- System call was interrupted by a signal - nothing was read.
(This can be seen only during a trace.)
This system call is Linux specific and should not be used in programs intended
to be portable.
From the very start people noted that it is unfortunate that kernel call and
library routine of the same name are entirely different animals. In libc4 and
libc5 the number of this call was defined by
SYS_klog. In glibc 2.0 the
syscall is baptised
klogctl.
syslog(3)
