.\" Copyright (C) 1995 Andries Brouwer (aeb@cwi.nl) .\" .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" .\" Written 11 June 1995 by Andries Brouwer .TH SYSLOG 2 "11 June 1995" "Linux 1.2.9" "Linux Programmer's Manual" .SH NAME syslog \- read and/or clear kernel message ring buffer; set console_loglevel .SH SYNOPSIS .nf .B #include .sp .B #include .sp .B _syscall3(int, syslog, int, type, char *, bufp, int, len); .sp .BI "int syslog(int " type ", char *" bufp ", int " len ); .fi .SH DESCRIPTION This is probably not the function you are interested in. Look at .BR syslog (3) for the C library interface. This page only documents the bare kernel system call interface. The \fItype\fP argument determines the action taken by .BR syslog . Quoting from .IR kernel/printk.c : .nf /* * 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 printk's to console * 7 -- Enable printk's to console * 8 -- Set level of messages printed to console */ .fi Only function 3 is allowed to non-root processes. .B The kernel log buffer .br The kernel has a cyclic buffer of length LOG_BUF_LEN (4096) in which messages given as argument to the kernel function \fIprintk\fP() are stored (regardless of their loglevel). The call .B syslog .RI (2, buf , len ) waits until this kernel log buffer is nonempty, and then reads at most \fIlen\fP bytes into the buffer \fIbuf\fP. 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 .IR /proc/kmsg . The call .B syslog .RI (3, buf , len ) will read the last \fIlen\fP 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 .B syslog .RI (4, buf , len ) does precisely the same, but also executes the `clear ring buffer' command. The call .B syslog .RI (5, dummy , idummy ) only executes the `clear ring buffer' command. .B The loglevel .br The kernel routine \fIprintk\fP() will only print a message on the console, if it has a loglevel less than the value of the variable .I 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 .B syslog .RI (8, dummy , value ). The calls .B syslog .RI ( type , dummy , idummy ) with \fItype\fP 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 where \fId\fP is a digit in the range 1-7, in which case the level is \fId\fP. The conventional meaning of the loglevel is defined in .I as follows: .nf #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 */ .fi .SH "RETURN VALUE" In case of error, -1 is returned, and \fIerrno\fP is set. Otherwise, for \fItype\fP equal to 2, 3 or 4, \fBsyslog\fP() returns the number of bytes read, and otherwise 0. .SH "ERRORS" .TP .B EPERM An attempt was made to change console_loglevel or clear the kernel message ring buffer by a process without root permissions. .TP .B EINVAL Bad parameters. .TP .B ERESTARTSYS System call was interrupted by a signal - nothing was read. .SH "CONFORMING TO" This system call is Linux specific and should not be used in programs intended to be portable. .SH "SEE ALSO" .BR syslog (3)