|SYSLOG(2)||Linux Programmer's Manual||SYSLOG(2)|
#include <sys/klog.h> /* Definition of SYSLOG_* constants */ #include <sys/syscall.h> /* Definition of SYS_* constants */ #include <unistd.h>
int syscall(SYS_syslog, int type, char *bufp, int len);
/* The glibc interface */ #include <sys/klog.h>
int klogctl(int type, char *bufp, int len);
This page describes the kernel syslog() system call, which is used to control the kernel printk() buffer; the glibc wrapper function for the system call is called klogctl().
- SYSLOG_ACTION_CLOSE (0)
- Close the log. Currently a NOP.
- SYSLOG_ACTION_OPEN (1)
- Open the log. Currently a NOP.
- SYSLOG_ACTION_READ (2)
- Read from the log. The call waits until the kernel log buffer is nonempty, and then reads at most len bytes into the buffer pointed to by bufp. The call returns the number of bytes read. Bytes read from the log disappear from the log buffer: the information can be read only once. This is the function executed by the kernel when a user program reads /proc/kmsg.
- SYSLOG_ACTION_READ_ALL (3)
- Read all messages remaining in the ring buffer, placing them in the buffer pointed to by bufp. The call reads 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 (see command 5 below)). The call returns the number of bytes read.
- SYSLOG_ACTION_READ_CLEAR (4)
- Read and clear all messages remaining in the ring buffer. The call does precisely the same as for a type of 3, but also executes the "clear ring buffer" command.
- SYSLOG_ACTION_CLEAR (5)
- The call executes just the "clear ring buffer" command. The bufp and len arguments are ignored.
- This command does not really clear the ring buffer. Rather, it sets a kernel bookkeeping variable that determines the results returned by commands 3 (SYSLOG_ACTION_READ_ALL) and 4 (SYSLOG_ACTION_READ_CLEAR). This command has no effect on commands 2 (SYSLOG_ACTION_READ) and 9 (SYSLOG_ACTION_SIZE_UNREAD).
- SYSLOG_ACTION_CONSOLE_OFF (6)
- The command saves the current value of console_loglevel and then sets console_loglevel to minimum_console_loglevel, so that no messages are printed to the console. Before Linux 2.6.32, the command simply sets console_loglevel to minimum_console_loglevel. See the discussion of /proc/sys/kernel/printk, below.
- The bufp and len arguments are ignored.
- SYSLOG_ACTION_CONSOLE_ON (7)
- If a previous SYSLOG_ACTION_CONSOLE_OFF command has been performed, this command restores console_loglevel to the value that was saved by that command. Before Linux 2.6.32, this command simply sets console_loglevel to default_console_loglevel. See the discussion of /proc/sys/kernel/printk, below.
- The bufp and len arguments are ignored.
- SYSLOG_ACTION_CONSOLE_LEVEL (8)
- The call sets console_loglevel to the value given in len, which must be an integer between 1 and 8 (inclusive). The kernel silently enforces a minimum value of minimum_console_loglevel for len. See the log level section for details. The bufp argument is ignored.
- SYSLOG_ACTION_SIZE_UNREAD (9) (since Linux 2.4.10)
- The call returns the number of bytes currently available to be read from the kernel log buffer via command 2 (SYSLOG_ACTION_READ). The bufp and len arguments are ignored.
- SYSLOG_ACTION_SIZE_BUFFER (10) (since Linux 2.6.6)
- This command returns the total size of the kernel log buffer. The bufp and len arguments are ignored.
All commands except 3 and 10 require privilege. In Linux kernels before 2.6.37, command types 3 and 10 are allowed to unprivileged processes; since Linux 2.6.37, these commands are allowed to unprivileged processes only if /proc/sys/kernel/dmesg_restrict has the value 0. Before Linux 2.6.37, "privileged" means that the caller has the CAP_SYS_ADMIN capability. Since Linux 2.6.37, "privileged" means that the caller has either the CAP_SYS_ADMIN capability (now deprecated for this purpose) or the (new) CAP_SYSLOG capability.
- Only messages with a log level lower than this value will be printed to the console. The default value for this field is DEFAULT_CONSOLE_LOGLEVEL (7), but it is set to 4 if the kernel command line contains the word "quiet", 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). The value of console_loglevel can be set (to a value in the range 1–8) by a syslog() call with a type of 8.
- This value will be used as the log level for printk() messages that do not have an explicit level. Up to and including Linux 2.6.38, the hard-coded default value for this field was 4 (KERN_WARNING); since Linux 2.6.39, the default value is a defined by the kernel configuration option CONFIG_DEFAULT_MESSAGE_LOGLEVEL, which defaults to 4.
- The value in this field is the minimum value to which console_loglevel can be set.
- This is the default value for console_loglevel.
|Kernel constant||Level value||Meaning|
|KERN_EMERG||0||System is unusable|
|KERN_ALERT||1||Action must be taken immediately|
|KERN_NOTICE||5||Normal but significant condition|
The kernel printk() routine will print a message on the console only if it has a log level less than the value of console_loglevel.
In case of error, -1 is returned, and errno is set to indicate the error.
- 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).
- This syslog() system call is not available, because the kernel was compiled with the CONFIG_PRINTK kernel-configuration option disabled.
- 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 or CAP_SYSLOG capability).
- System call was interrupted by a signal; nothing was read. (This can be seen only during a trace.)