CS 241 Spring 2007: Signal Handling and Threads Lecture Notes, Study notes of Computer Science

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CS 241 Spring 2007System Programming

Signal Handling and Threads

Lecture 16 Klara Nahrstedt

CS241 Administrative †^ Read Chapter 8 in R&R and Chapter 13.5 inR&R †^ SMP4 due on Monday, 2/26 †^ Homework 1 will be posted this evening, 2/23 †^ Deadline for Homework 1 is Friday, March2, at 5pm^ „^

Homework 1 is a preparation for Midterm „ Homework 1 is an individual effort – NOWORKING IN GROUPS/PAIRS!! †^ MIDTERM – MONDAY, March 5, 11am

Catching and Ignoring Signals -SIGACTION †^ sigaction

function allows the caller to examine or specify action associated witha specific signal † Program installs

signal handler

by calling

sigaction

with the name of a user-written function. † The function

sigaction

is used to specify

what is to happen to a signal when it isdelivered.

SIGACTION

Process

Default Signal Handler

SignalMask^ Process Resumed

SignalMask

SignalMask

New Signal Handler

SignalMask

System Booking Keeping

SIGACTION changes Signal handler (and returns old handler)

Example: Set up Signal Handler for SIGINT struct sigaction newact; newact.sa_handler = mysighand

; /set new handler/

newact.sa_flags = 0;

/* no special options */

if (( sigemptyset(&newact,sa_mask)

== -1) ||

( sigaction(SIGINT, &newact, NULL)

== -1))

perror(“Failed to install SIGINT signal handler”);

Set up Signal Handler that CatchesSIGINT Generated by Ctrl-C void^

catchctrlc(int signo)

char handmsg[] = "I found Ctrl-C\n"; int msglen = sizeof(handmsg); write(STDERR_FILENO, handmsg, msglen); } … struct sigaction act; act.sa_handler = catchctrlc; act.sa_flags = 0; if (( sigemptyset(&act.sa_mask)

( sigaction(SIGINT, &act, NULL)

perror("Failed to set SIGINT to handle Ctrl-C"); Note: write is async-signal safe – meaning it can be called inside a signalhandler.^ Not so for

printf^

or strlen.

sigsuspend sigsuspend

function sets signal mask and suspends process until signal is caught by theprocess sigsuspend

returns when signal handler of the caught signal returns #include <signal.h> int sigsupend(const sigset_t *sigmask);

Example: sigsuspend What’s wrong? sigfillset(&sigmost); sigdelset(&sigmost, signum); sigsuspend(&sigmost); signum

is the only signal unblocked that can cause

sigsuspend

to return

If the signal

signum

is delivered before the start of

the code segment, the process still suspendsitself and deadlocks if another

signum

is not

generated

Use sigwait This function is much cleaner and avoids races and errors !!!! 1. First block all signals 2. Put the signals you want to wait for in

sigset_t

3. Call

sigwait

4.^ sigwait

blocks the process until at least one of these signals is pending. 5. It removes one of the pending signals and gives you thecorresponding signal number in the second parameter.. 6. Do what you want: no signal handler needed. 7. It returns 0 on success and -1 on error with

errno

set.

#include <signal.h> int sigwait(const sigset_t restrict sigmask, intrestrict signo);

Signal Handling & Threads (Ch13.5 p473-475)^ type

Delivery action

asynchronous

Delivered to some thread that has itunblocked synchronous

Delivered to thread that caused it

directed

Delivered to the identified thread

Masking Signals for Threads †^ Signal handlers are process-wide †^ Each thread has its own signal mask forprocess wide signals – only use sigprocmaskfor initialization before creating threads †^ Thread can^ „^

Examine its signal mask „ Set its signal mask †^ sigprocmask

function should not be used

when the process has multiple threads

Masking Signals for Threads #include <pthread.h> #include <signal.h> int pthread_sigmask(int how,

**const sigset_t restrict set, sigset_t restrict pset);

If ‘how

’ is^ SIG_SETMASK

, then thread’s signal mask is

replaced by ‘

set’

If ‘how

’ is^ SIG_BLOCK

, then additional signals in ‘

set’ are

blocked by the thread If ‘how

’ is^ SIG_UNBLOCK

, then any of blocked signals in ‘

set’

are removed from the thread’s current signal mask

Dedicating Threads for Signal Handling †^ Signal handlers are process-wide and installed with^ sigaction

as in single-threaded processes †^ Distinction between process-wide signal handlers andthread-specific signal masks is important!!! †^ Remember:^ „^

When a signal is caught, the signal that caused the event isautomatically blocked on entry to the signal handler „ With multi-threaded application, nothing prevents anothersignal of the same type from being delivered to another threadthat has the signal unblocked †^ A recommended strategy for dealing with signals inmulti-threaded processes is to dedicate particularthreads to signal handling

Dedicating Threads for Signal Handling †^ Dedicate particular threads to handlesignals †^ Main thread blocks all signals beforecreating any child threads †^ Signal mask is inherited by children †^ Thread dedicated to signal handlingexecutes

sigwait

on specific signals to

be handled or uses

pthread_sigmask

to

unblock signal