linux app read kernel log

man klogctl:

int klogctl(int type, char *bufp, int len);

SYSLOG(2)                                        Linux Programmer's Manual                                       SYSLOG(2)

NAME
       syslog, klogctl - read and/or clear kernel message ring buffer; set console_loglevel

SYNOPSIS
       int syslog(int type, char *bufp, int len);
                       /* No wrapper provided in glibc */

       /* The glibc interface */
       #include <sys/klog.h>

       int klogctl(int type, char *bufp, int len);

DESCRIPTION
       Note:  Probably,  you are looking for the C library function syslog(), which talks to syslogd(8); see syslog(3) for
       details.

       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().

   The kernel log buffer
       The  kernel  has  a cyclic buffer of length LOG_BUF_LEN in which messages given as arguments to the kernel function
       printk() are stored (regardless of their log level).  In early kernels, LOG_BUF_LEN had the value 4096; from kernel
       1.3.54,  it  was 8192; from kernel 2.1.113, it was 16384; since kernel 2.4.23/2.6, the value is a kernel configura-
       tion option (CONFIG_LOG_BUF_SHIFT, default value dependent on the architecture).  Since Linux 2.6.6, the  size  can
       be queried with command type 10 (see below).

   Commands
       The type argument determines the action taken by this function.  The list below specifies the values for type.  The
       symbolic names are defined in the kernel source, but are not exported to user space; you will either  need  to  use
       the numbers, or define the names yourself.

       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  de-
              termines 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_con-
              sole_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  (inclu-
              sive).  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  ca-
       pability (now deprecated for this purpose) or the (new) CAP_SYSLOG capability.

/* kernel\printk.c */

...
SYSCALL_DEFINE3(syslog, int, type, char __user *, buf, int, len)
{
	return do_syslog(type, buf, len, SYSLOG_FROM_CALL);
}
...