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 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).
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 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.