syslog() - Unix, Linux System Call
syslog, klogctl - read and/or clear kernel message ring buffer;
int syslog(int type, char *bufp, int len);
/* No wrapper provided in glibc */
/* The glibc interface */
int klogctl(int type, char *bufp, int len);
If you need the libc function
syslog() (which talks to
then look at
The system call of this name is about controlling the kernel
printk() buffer, and the glibc version is called
The type argument determines the action taken by this function.
* 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 printks to console
* 7 -- Enable printks 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).
syslog() (2,buf,len) waits until this kernel log buffer is nonempty, and then reads
at most len bytes into the buffer buf.
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
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.
syslog() (4,buf,len) does precisely the same, but also executes the clear ring buffer command.
syslog() (5,dummy,idummy) only executes the clear ring buffer command.
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. This variable initially has the value DEFAULT_CONSOLE_LOGLEVEL (7),
but is set to 10 if the
kernel command line 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
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.
for type equal to 2, 3 or 4,
syslog() returns the number
of bytes read, and otherwise 0.
An attempt was made to change console_loglevel or clear the kernel
message ring buffer by a process without root permissions.
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
In libc4 and libc5 the number of this call was defined by
SYS_klog. In glibc 2.0 the syscall is baptised