man2/syslog.2

   1 .\" Copyright (C) 1995 Andries Brouwer (aeb@cwi.nl)
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  23 .\" Written 11 June 1995 by Andries Brouwer <aeb@cwi.nl>
  24 .TH SYSLOG 2  2001-11-25 "Linux 1.2.9" "Linux Programmer's Manual"
  25 .SH NAME
  26 syslog, klogctl \- read and/or clear kernel message ring buffer; set console_loglevel
  27 .SH SYNOPSIS
  28 .nf
  29 /* The glibc interface */
  30 .br
  31 .B "#include <sys/klog.h>"
  32 .sp
  33 .BI "int klogctl(int " type ", char *" bufp ", int " len );
  34 .sp
  35 /* The handcrafted system call */
  36 .br
  37 .B #include <unistd.h>
  38 .br
  39 .B #include <linux/unistd.h>
  40 .sp
  41 .B _syscall3(int, syslog, int, type, char *, bufp, int, len)
  42 .sp
  43 .BI "int syslog(int " type ", char *" bufp ", int " len );
  44 .fi
  45 .SH DESCRIPTION
  46 If you need the libc function
  47 .BR syslog() ,
  48 (that talks to
  49 .BR syslogd (8)),
  50 then look at
  51 .BR syslog (3).
  52 The system call of this name is about controlling the kernel
  53 .I printk()
  54 buffer, and the glibc version is called
  55 .BR klogctl() .
  56
  57 The \fItype\fP argument determines the action taken by this function.
  58
  59 Quoting from
  60 .IR kernel/printk.c :
  61 .nf
  62 /*
  63  * Commands to sys_syslog:
  64  *
  65  *      0 -- Close the log.  Currently a NOP.
  66  *      1 -- Open the log. Currently a NOP.
  67  *      2 -- Read from the log.
  68  *      3 -- Read up to the last 4k of messages in the ring buffer.
  69  *      4 -- Read and clear last 4k of messages in the ring buffer
  70  *      5 -- Clear ring buffer.
  71  *      6 -- Disable printk's to console
  72  *      7 -- Enable printk's to console
  73  *      8 -- Set level of messages printed to console
  74  *      9 -- Return number of unread characters in the log buffer
  75  */
  76 .fi
  77
  78 Only function 3 is allowed to non-root processes.
  79 (Function 9 was added in 2.4.10.)
  80
  81 .B The kernel log buffer
  82 .br
  83 The kernel has a cyclic buffer of length LOG_BUF_LEN
  84 (4096, since 1.3.54: 8192, since 2.1.113: 16384; in recent kernels
  85 the size can be set at compile time) in which messages given as argument
  86 to the kernel function \fIprintk\fP() are stored
  87 (regardless of their loglevel).
  88
  89 The call
  90 .B syslog
  91 .RI (2, buf , len )
  92 waits until this kernel log buffer is nonempty, and then reads
  93 at most \fIlen\fP bytes into the buffer \fIbuf\fP. It returns
  94 the number of bytes read. Bytes read from the log disappear from
  95 the log buffer: the information can only be read once.
  96 This is the function executed by the kernel when a user program
  97 reads
  98 .IR /proc/kmsg .
  99
 100 The call
 101 .B syslog
 102 .RI (3, buf , len )
 103 will read the last \fIlen\fP bytes from the log buffer (nondestructively),
 104 but will not read more than was written into the buffer since the
 105 last `clear ring buffer' command (which does not clear the buffer at all).
 106 It returns the number of bytes read.
 107
 108 The call
 109 .B syslog
 110 .RI (4, buf , len )
 111 does precisely the same, but also executes the `clear ring buffer' command.
 112
 113 The call
 114 .B syslog
 115 .RI (5, dummy , idummy )
 116 only executes the `clear ring buffer' command.
 117
 118 .B The loglevel
 119 .br
 120 The kernel routine \fIprintk\fP() will only print a message on the
 121 console, if it has a loglevel less than the value of the variable
 122 .I console_loglevel
 123 (initially DEFAULT_CONSOLE_LOGLEVEL (7), but set to 10 if the
 124 kernel commandline contains the word `debug', and to 15 in case
 125 of a kernel fault - the 10 and 15 are just silly, and equivalent to 8).
 126 This variable is set (to a value in the range 1-8) by the call
 127 .B syslog
 128 .RI (8, dummy , value ).
 129 The calls
 130 .B syslog
 131 .RI ( type , dummy , idummy )
 132 with \fItype\fP equal to 6 or 7, set it to 1 (kernel panics only)
 133 or 7 (all except debugging messages), respectively.
 134
 135 Every text line in a message has its own loglevel. This level is
 136 DEFAULT_MESSAGE_LOGLEVEL - 1 (6) unless the line starts with <d>
 137 where \fId\fP is a digit in the range 1-7, in which case the level
 138 is \fId\fP. The conventional meaning of the loglevel is defined in
 139 .I <linux/kernel.h>
 140 as follows:
 141
 142 .nf
 143 #define KERN_EMERG    "<0>"  /* system is unusable               */
 144 #define KERN_ALERT    "<1>"  /* action must be taken immediately */
 145 #define KERN_CRIT     "<2>"  /* critical conditions              */
 146 #define KERN_ERR      "<3>"  /* error conditions                 */
 147 #define KERN_WARNING  "<4>"  /* warning conditions               */
 148 #define KERN_NOTICE   "<5>"  /* normal but significant condition */
 149 #define KERN_INFO     "<6>"  /* informational                    */
 150 #define KERN_DEBUG    "<7>"  /* debug-level messages             */
 151 .fi
 152
 153 .SH "RETURN VALUE"
 154 In case of error, \-1 is returned, and \fIerrno\fP is set. Otherwise,
 155 for \fItype\fP equal to 2, 3 or 4, \fBsyslog\fP() returns the number
 156 of bytes read, and otherwise 0.
 157 .SH ERRORS
 158 .TP
 159 .B EINVAL
 160 Bad parameters.
 161 .TP
 162 .B EPERM
 163 An attempt was made to change console_loglevel or clear the kernel
 164 message ring buffer by a process without root permissions.
 165 .TP
 166 .B ERESTARTSYS
 167 System call was interrupted by a signal - nothing was read.
 168 (This can be seen only during a trace.)
 169 .SH "CONFORMING TO"
 170 This system call is Linux specific and should not be used in programs
 171 intended to be portable.
 172 .SH NOTES
 173 From the very start people noted that it is unfortunate that
 174 kernel call and library routine of the same name are entirely
 175 different animals.
 176 In libc4 and libc5 the number of this call was defined by
 177 .BR SYS_klog .
 178 In glibc 2.0 the syscall is baptised
 179 .BR klogctl .
 180
 181 .SH "SEE ALSO"
 182 .BR syslog (3)