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syslog.2: Rewrite parts of the page, and import /proc/sys/kernel/printk
* Move /proc/sys/kernel/printk from proc(5) to this page, and correct various details in the discussion of that file. * Rewrite and correct various other details on the page. * Clean out some crufty text. * Miscellaneous minor fixes. Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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Michael Kerrisk
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man2/syslog.2
113
man2/syslog.2
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@ -66,17 +66,18 @@ The kernel has a cyclic buffer of length
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.B LOG_BUF_LEN
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in which messages given as arguments to the kernel function
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.BR printk ()
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are stored (regardless of their loglevel).
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are stored (regardless of their log level).
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In early kernels,
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.B LOG_BUF_LEN
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had the value 4096;
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from kernel 1.3.54, it was 8192;
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from kernel 2.1.113, it was 16384;
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since kernel 2.4.23/2.6, the value is a kernel configuration option
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.RB ( CONFIG_LOG_BUF_SHIFT ).
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.RB ( CONFIG_LOG_BUF_SHIFT ,
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default value dependent on the architecture).
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.\" Under "General setup" ==> "Kernel log buffer size"
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.\" For 2.6, precisely the option seems to have appeared in 2.5.55.
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In recent kernels, the size can be queried with command type 10 (see below).
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Since Linux 2.6.6, the size can be queried with command type 10 (see below).
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.SS Commands
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The \fItype\fP argument determines the action taken by this function.
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The list below specifies the values for
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@ -107,7 +108,7 @@ This is the function executed by the kernel when a user program reads
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.TP
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.BR SYSLOG_ACTION_READ_ALL " (3)"
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Read all messages remaining in the ring buffer,
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placing then in the buffer pointed to by
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placing them in the buffer pointed to by
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.IR bufp .
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The call reads the last \fIlen\fP
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bytes from the log buffer (nondestructively),
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@ -154,6 +155,9 @@ the command simply sets
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.I console_loglevel
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to
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.IR minimum_console_loglevel .
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See the discussion of
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.IR /proc/sys/kernel/printk ,
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below.
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The
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.I bufp
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@ -174,6 +178,9 @@ this command simply sets
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.I console_loglevel
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to
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.IR default_console_loglevel .
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See the discussion of
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.IR /proc/sys/kernel/printk ,
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below.
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The
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.I bufp
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@ -182,11 +189,17 @@ and
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arguments are ignored.
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.TP
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.BR SYSLOG_ACTION_CONSOLE_LEVEL " (8)"
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The call sets the console log level to the value given in
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The call sets
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.I console_loglevel
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to the value given in
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.IR len ,
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which must be an integer between 1 and 8 (inclusive).
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The kernel silently enforces a minimum value of
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.IR minimum_console_loglevel
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for
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.IR len .
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See the
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.B loglevel
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.IR "log level"
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section for details.
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The
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.I bufp
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@ -227,36 +240,64 @@ Since Linux 2.6.37,
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capability (now deprecated for this purpose) or the (new)
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.BR CAP_SYSLOG
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capability.
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.SS The loglevel
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The kernel routine
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.BR printk ()
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will only print a message on the
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console, if it has a loglevel less than the value of the variable
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.IR console_loglevel .
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This variable initially has the value
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.\"
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.\"
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.SS /proc/sys/kernel/printk
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.I /proc/sys/kernel/printk
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is a writable file containing four integer values that influence kernel
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.I printk()
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behavior when printing or logging error messages.
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The four values are:
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.TP
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.I console_loglevel
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Only messages with a log level lower than this value will
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be printed to the console.
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The default value for this field is
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.B DEFAULT_CONSOLE_LOGLEVEL
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(7), but is set to 10 if the
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kernel command line contains the word "debug", and to 15 in case
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(7), but it is set to
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4 if the kernel command line contains the word "quiet", \" since Linux 2.4
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10 if the kernel command line contains the word "debug",
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and to 15 in case
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of a kernel fault (the 10 and 15 are just silly, and equivalent to 8).
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This variable is set (to a value in the range 1-8) by a
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The value of
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.IR console_loglevel
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can be set (to a value in the range 1-8) by a
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.BR syslog ()
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call with a
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.I type
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of 8.
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Calls to
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.BR syslog ()
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with
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.I type
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equal to 6 or 7 set the variable to 1 (kernel panics only)
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or 7 (all except debugging messages), respectively.
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Every text line in a message has its own loglevel.
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This level is
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.I "DEFAULT_MESSAGE_LOGLEVEL \- 1"
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(6) unless the line starts with <d>
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where \fId\fP is a digit in the range 1-7, in which case the level
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is \fId\fP.
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The conventional meaning of the loglevel is as follows:
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.TP
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.I default_message_loglevel
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This value will be used as the log level for
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.IR printk()
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messages that do not have an explicit level.
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Up to and including Linux 2.6.38,
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the hard-coded default value for this field was 4
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.RB ( KERN_WARNING );
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since Linux 2.6.39,
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.\" commit 5af5bcb8d37f99ba415a1adc6da71051b84f93a5
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the default value is a defined by the kernel configuration option
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.BR CONFIG_DEFAULT_MESSAGE_LOGLEVEL ,
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which defaults to 4.
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.TP
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.I minimum_console_loglevel
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The value in this field is the minimum value to which
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.I console_loglevel
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can be set.
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.TP
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.I default_console_loglevel
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This is the default value for
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.IR console_loglevel .
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.\"
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.\"
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.SS The log level
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Every
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.IR printk ()
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message has its own log level.
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If the log level is not explicitly specified as part of the message,
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it defaults to
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.IR default_message_loglevel .
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The conventional meaning of the log level is as follows:
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.TS
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lB lB lB
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@ -271,6 +312,12 @@ KERN_NOTICE 5 Normal but significant condition
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KERN_INFO 6 Informational
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KERN_DEBUG 7 Debug-level messages
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.TE
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The kernel
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.IR printk()
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routine will print a message on the
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console only if it has a log level less than the value of
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.IR console_loglevel .
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.SH RETURN VALUE
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For \fItype\fP equal to 2, 3, or 4, a successful call to
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.BR syslog ()
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@ -314,7 +361,9 @@ system call is not available, because the kernel was compiled with the
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kernel-configuration option disabled.
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.TP
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.B EPERM
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An attempt was made to change console_loglevel or clear the kernel
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An attempt was made to change
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.I console_loglevel
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or clear the kernel
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message ring buffer by a process without sufficient privilege
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(more precisely: without the
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.B CAP_SYS_ADMIN
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@ -329,7 +378,7 @@ System call was interrupted by a signal; nothing was read.
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This system call is Linux-specific and should not be used in programs
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intended to be portable.
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.SH NOTES
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From the very start people noted that it is unfortunate that
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From the very start, people noted that it is unfortunate that
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a system call and a library routine of the same name are entirely
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different animals.
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.\" In libc4 and libc5 the number of this call was defined by
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