Signals in LINUX Harish' M 7th December 2006

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Signals in LINUXHarish. M7th December 2006
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Agenda

• Brief introduction
• Signals available in linux
• Different ways of sending signals
• Different ways of catching and handling signals.
• Kernel code
• Quick summary
• References

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Introduction

• What Are Signals?
• Signals are notifications sent to a process
• o C, Z, Alarm, . . .
• Each signal may have a signal handler
• o When a signal is sent to a process,
• the OS stops the process
  immediately
• o Handler executes and finishes
• o Resume the process
• Signals are not interrupts
• o Interrupts are sent to OS by HW
• o Signals are sent to processes by OS
• Each UNIX signal has an integer number
• and a symbolic name
• o Defined in ltsignal.hgt

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Interprocess Communication Mechanisms

• Signals
• // discuss later
• Message Queues
• Allows processes to write messages which are read
  by other messages
• ipc_perm data structure.
• Semaphores
• Is a location in the memory whose value can be
  tested and set.
• Depending on the value which is set, the process
  either enters the memory location or sleeps.
• Shared Memory
• Communicating through memory which appears in all
  participating processes virtual memory.
• Synchronize access to memory.

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Process
Receives a signal
Function
Ignore
Catch execute
Defaullt action
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Signals
Standard
Real-time
1-32
33-63
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Catchable and Non-Catchable Signals

• Non-catch able signals
• o KILL
• Terminate the process immediately
• Catch able termination signal is SIGKILL
• o STOP
• Suspend the process immediately
• Catch able suspension signal is SIGSTOP
• Can resume the process with signal CONT
• Catch able signals
• o All other predefined signals
• o All user-defined signals

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Real-time signals

• 32 real time signals
• Notation
• 32 (SIGRTMIN)
• 63 (SIGRTMAX)
• SIGRTMINn
• Description
• No definite meaning
• Application defined purposes
• First three real-time signals
• Default action --- terminate

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Real-time signalscont
Sigqueue()

• Real time signals are delivered
  in a guaranteed order
• RT signals Vs Std signals pending for a process
• _POSIX_SIGQUEUE_MAX (32)

RT Signal
pid receiving process Sig - signal value int
or pointer
Can be queued
Instance 1
Instance 2
sent
Receiving process
Std Signal
handles
Cannot be queued
Instance 1
Instance 2
SA-SIGACTION
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Sync Vs Async

• Synchronous

Sent to
Causes
Sync signal
Unrecoverable error
Termination

• Asynchronous

Un related
Sent to
Causes
Async signal
Executing process
Termination
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Where is information about a signal stored
Kernel
Proc Table
User area
Sig handling info
Proc Table
Sig handler
Rel information
ignored
handled
blocked
posted
Each type of signal
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Sending signals

• A user can send a signal mainly in three ways.
• Using Keyboard
• Using the shell
• Through a program

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Sending Signals from Keyboard Steps o
Pressing keys generates interrupts to OS o OS
interprets a key sequence and sends a signal to
the running process Examples o
Ctrl-C causes the OS to send an INT signal to the
running process. By default, this causes the
process to immediately terminate. o Ctrl-Z
causes the OS to send a TSTP signal to the
running process. By default, this causes the
process to suspend execution.
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Sending Signals From The Shell
kill -ltsignalgt ltPIDgt o If no signal name or
number is specified, the default is to send an
SIGTERM signal to the process, o Signal SIGKILL
or 9 is special it cannot be caught o Example
send the INT signal to process with PID
1234 kill -INT 1234 The same affect as
pressing Ctrl-C if process 1234 is
running. fg o The command is foreground o
On UNIX shells, this command will send a CONT
signal o Resume execution of the process (that
was suspended with Ctrl-Z or a command
bg) o See man pages for fg and
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Effect of signals on process groups
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Sending Signals with kill Program
kill program sends arbitrary signal to a
process or process group Just a front-end for
the kill system call Examples ?? kill 9
24818 Send SIGKILL to process 24818 ??
kill 9 24817 Send SIGKILL to every
process in process group 24817.
gt ./kill_example gt Child1 pid24818
pgrp24817 Child2 pid24819 pgrp24817 gt ps PID
TTY TIME CMD 24788 pts/2 000000 tcsh 24818
pts/2 000002 forks 24819 pts/2 000002
forks 24820 pts/2 000000 ps gt kill -9
-24817 gt ps PID TTY TIME CMD 24788 pts/2
000000 tcsh 24823 pts/2 000000 ps gt
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Installing A Signal Handler
Predefined signal handlers o SIG_DFL default
handler o SIG_IGN Ignore the signal To
install a handler, use include
ltsignal.hgt typedef void (sighandler_t)(int) si
ghandler_t signal(int sig, sighandler_t
handler) o Handler handler will be invoked,
when signal sig occurs o Return the old handler
on success SIG_ERR on error o On most non-Linux
UNIX systems, after the handler executes, the
OS resets the handler to SIG_DFL
Example include ltsignal.hgt ... if
(signal(SIGINT, SIG_IGN) SIG_ERR) exit(1)
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Signal system call
Signal
Arg 1
Arg 2
Signum
Pointer
User defined
Pre defined
SIG_IGN
SIG_DFL
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Implementing signal system call

• include ltsignal.hgt
• void my_handler (int sig) / function prototype
  /
• int main ( void )
• / Part I Catch SIGINT /
• signal (SIGINT, my_handler)
• printf ("Catching SIGINT\n")
• sleep(3)
• printf (" No SIGINT within 3 seconds\n")
• / Part II Ignore SIGINT /
• signal (SIGINT, SIG_IGN)
• printf ("Ignoring SIGINT\n")
• sleep(3)
• printf ("No SIGINT within 3 seconds\n")

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Cont

• / Part III Default action for SIGINT /
• signal (SIGINT, SIG_DFL)
• printf ("Default action for SIGINT\n")
• sleep(3)
• printf ("No SIGINT within 3 seconds\n")
• return 0
• / User-defined signal handler function /
• void my_handler (int sig)
• printf ("I got SIGINT, number d\n", sig)
• exit(0)

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sigaction

• include ltsignal.hgt
• Acts as an interface for reliable signals.
• Allows the calling process to specify the action
  associated with a specific signal
• Structure
• struct sigaction
• sighandler_t sa_handler
• unsigned long sa_flags
• sigrestore_t sa_restorer
• sigset_t sa_mask
• sa_handler pointer to a signal catching
  function
• sa_flags special flags to effect the behaviour
  of the signal.

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Sigactioncont

• Sa_flags
• SA_NOCLDSTOP
• Do not generate SIGCHLD when children stop or
  stopped children continue.
• SA_ONSTACK
• The signal shall be delivered to the calling
  process on that stack.
• SA_RESETHAND
• The signal shall be reset to SIG_DFL or SIG_INFO
• SA_RESTART
• SA_NODEFER

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Implementing sigaction()
include ltsignal.hgt include ltstdlib.hgt include
ltstdio.hgt include ltunistd.hgt / Simple signal
handler / void handler(int sig) if (sig
SIGINT) printf("got SIGINT\n")
Else printf("got SIGTSTP\n") int
main(void) struct sigaction sa struct
sigaction oldint
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Sigaction().(Cont)

• / Initialize the sa structure /
• sa.sa_handler handler
• sigemptyset(sa.sa_mask)
• sa.sa_flags 0
• / Set up the signal handlers/
• sigaction(SIGINT, sa, oldint)
• sigaction(SIGTSTP, sa, NULL)
• / Sleep for five seconds, or until a signal
  wakes us up/
• sleep(5)
• / Restore the behavior of SIGINT just to show
  it can be done. /
• sigaction(SIGINT, oldint, NULL)
• return 0

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Summary

• Over view of signals
• Different methods used
• Different ways to handle signals
• Sending and receiving signals
• Different programming techniques

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References

• http//linuxboxadmin.com/articles/bashdaemon.php
• http//www.linuxjournal.com/article/6483
• http//www.linuxjournal.com/article/3985
• http//cs-pub.bu.edu/fac/richwest/cs591_w1/notes/w
  k3_pt2.PDF
• http//www.linuxforums.org/forum/linux-programming
  -scripting/46017-threads-signals.html
• http//www.die.net/doc/linux/man/man1/strace.1.htm
  l
• http//www-128.ibm.com/developerworks/linux/librar
  y/l-sigdebug.html?cadgr-lnxw06SignalDebug
• http//www.linux-tutorial.info/modules.php?nameTu
  torialpageid289

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Questions ?