SYSLOG

NAME

SYNOPSIS

int syslog(int type, char *bufp, int len);
                /* No wrapper provided in glibc */

/* The glibc interface */


#include <sys/klog.h>

int klogctl(int type, char *bufp, int len);

DESCRIPTION

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 all messages remaining in the ring buffer.
 *      4 -- Read and clear all messages remaining in the ring buffer
 *      5 -- Clear ring buffer.
 *      6 -- Disable printk to console
 *      7 -- Enable printk to console
 *      8 -- Set level of messages printed to console
 *      9 -- Return number of unread characters in the log buffer
 *     10 -- Return size of the log buffer
 */

Only command types 3 and 10 are allowed to unprivileged processes. Type 9 was added in 2.4.10; type 10 in 2.6.6.  

The kernel log buffer

The call syslog(2,buf,len) waits until this kernel log buffer is non-empty, 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 (non-destructively), 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,dummy) executes just the "clear ring buffer" command. (In each call where buf or len is shown as "dummy", the value of the argument is ignored by the call.)

The call syslog(6,dummy,dummy) sets the console log level to minimum, so that no messages are printed to the console.

The call syslog(7,dummy,dummy) sets the console log level to default, so that messages are printed to the console.

The call syslog(8,dummy,level) sets the console log level to level, which must be an integer between 1 and 8 (inclusive). See the loglevel section for details.

The call syslog(9,dummy,dummy) returns the number of bytes currently available to be read on the kernel log buffer.

The call syslog(10,dummy,dummy) returns the total size of the kernel log buffer.  

The loglevel

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             */

RETURN VALUE

In case of error, -1 is returned, and errno is set to indicate the error.  

ERRORS

EINVAL

Bad arguments (e.g., bad type; or for type 2, 3, or 4, buf is NULL, or len is less than zero; or for type 8, the level is outside the range 1 to 8).

EPERM

An attempt was made to change console_loglevel or clear the kernel message ring buffer by a process without sufficient privilege (more precisely: without the CAP_SYS_ADMIN capability).

ERESTARTSYS

System call was interrupted by a signal; nothing was read. (This can be seen only during a trace.)

ENOSYS

This syslog() system call is not available, because the kernel was compiled with the CONFIG_PRINTK kernel-configuration option disabled.

CONFORMING TO

NOTES

SEE ALSO

COLOPHON