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[thirdparty/man-pages.git] / man2 / syslog.2

.\" Copyright (C) 1995 Andries Brouwer (aeb@cwi.nl)
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.\" Written 11 June 1995 by Andries Brouwer <aeb@cwi.nl>
.TH SYSLOG 2  2001-11-25 "Linux 1.2.9" "Linux Programmer's Manual"
.SH NAME
syslog, klogctl \- read and/or clear kernel message ring buffer;
set console_loglevel
.SH SYNOPSIS
.nf
/* The glibc interface */
.br
.B "#include <sys/klog.h>"
.sp
.BI "int klogctl(int " type ", char *" bufp ", int " len );
.sp
/* The handcrafted system call */
.br
.B #include <unistd.h>
.br
.B #include <linux/unistd.h>
.br
.B #include <errno.h>
.sp
.B _syscall3(int, syslog, int, type, char *, bufp, int, len)
          /* Using \fBsyscall\fP(2) may be preferable; see \fBintro\fP(2) */
.sp
.BI "int syslog(int " type ", char *" bufp ", int " len );
.fi
.SH DESCRIPTION
If you need the libc function
.BR syslog (),
(that talks to
.BR syslogd (8)),
then look at
.BR syslog (3).
The system call of this name is about controlling the kernel
.IR printk ()
buffer, and the glibc version is called
.BR klogctl ().

The \fItype\fP argument determines the action taken by this function.

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
 *      9 \-\- Return number of unread characters in the log buffer
 */
.fi

Only function 3 is allowed to non-root processes.
(Function 9 was added in 2.4.10.)

.B The kernel log buffer
.br
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 \fIprintk\fP() are stored
(regardless of their loglevel).

The call
.BR 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
.BR 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
.BR syslog ()
.RI (4, buf , len )
does precisely the same, but also executes the `clear ring buffer' command.

The call
.BR 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
.IR console_loglevel .
This variable initially has the value DEFAULT_CONSOLE_LOGLEVEL (7),
but is set to 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).
This variable is set (to a value in the range 1-8) by the call
.BR syslog ()
.RI (8, dummy , value ).
The calls
.BR 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 <d>
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 <linux/kernel.h>
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 EINVAL
Bad parameters.
.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 ERESTARTSYS
System call was interrupted by a signal; nothing was read.
(This can be seen only during a trace.)
.SH "CONFORMING TO"
This system call is Linux specific and should not be used in programs
intended to be portable.
.SH NOTES
From the very start people noted that it is unfortunate that
kernel call and library routine of the same name are entirely
different animals.
In libc4 and libc5 the number of this call was defined by
.BR SYS_klog .
In glibc 2.0 the syscall is baptised
.BR klogctl ().
.SH "SEE ALSO"
.BR syslog (3)