Signals UNIX

8/12/2019 Signals UNIX

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Difference System Calls n Signals


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Three Actions Possible Ignore the signal

Catch and handle the signal

Perform the default action There are two signals that cannot be ignored: SIGKILL

and SIGSTOP.


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Basics Eer! signal has a s!mbolic name that starts with the

"refi# SIG.

$or e#am"le% SIGI&T is the signal sent when the user"resses Ctrl'C% SIG()*T is the signal sent when the"rocess calls the abort+ , function% and SIGKILL is thesignal sent when a "rocess is forcefull! terminated.


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Basics Signals are all defined in a header file included from

-signal.h.

Eer! signal is also associated with an integeridentifier.

The name'to'integer ma""ing for the signals isim"lementation'de"endent% and aries among /ni#

s!stems.


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Common Signals


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Basics /ses signal+, s!stem call

#include

typedef void (*sighandler_t)(int); sighandler_t signal (int signo, sighandler_t handler);

( successful call to signal+ , remoes the current actionta0en on recei"t of the signal signo% and instead handles

the signal with the signal handler s"ecified b! handler. ( "rocess can catch neither SIGKILL nor SIGSTOP% so

setting u" a handler for either of these two signals ma0esno sense.


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Basics The handler function must return oid% which

ma0es sense because +unli0e with normal

functions, there is no standard "lace in the"rogram for this function to return.

The function ta0es one argument% an integer%which is the signal identifier +for e#am"le%

SIG/S*1, of the signal being handled. This allows a single function to handle multi"le

signals.


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Basics2hen it raises a signal to a "rocess that has registered

a signal handler% the 0ernel sus"ends e#ecution of the"rogram3s regular instruction stream% and calls thesignal

handler.

The handler is "assed the alue of the signal% which is

the signo originall! "roided to signal+ ,.


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Basics 4ou ma! also use signal+ , to instruct the 0ernel to ignore a

gien signal for the current "rocess% or to reset the signal tothe default behaior.

This is done using s"ecial alues for the handler "arameter:

SIG56$L ' Set the behaior of the signal gien b! signoto its default.

SIG5IG& ' Ignore the signal gien b! signo.


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Waiting for a Signal "ause+ , s!stem call "uts a "rocess to slee" until it

receies a signal that either is handled or

terminates the "rocess: #include

int pause (void);

"ause+ , returns onl! if a caught signal is receied%in which case the signal is handled% and "ause+ ,returns '7% and sets errno to EI&T*.


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Pause "erforms onl! two actions. $irst% it "uts the"rocess in the interru"tible slee" state. &e#t% it callsschedule+ , to ino0e the Linu# "rocess scheduler tofind another "rocess to run.

(s the "rocess is not actuall! waiting for an!thing% the0ernel will not wa0e it u" unless it receies a signal.


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Execution and Inheritance2hen a "rocess is first e#ecuted% all signals are set

to their default actions% unless the

"arent "rocess +the one e#ecuting the new "rocess,is ignoring them8

In this case% the newl! created "rocess will alsoignore those signals.

Put another wa!% an! signal caught b! the "arent isreset to the default action in the new "rocess% andall other signals remain the same.


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Sending a Signal The 0ill+ , s!stem call% the basis of the common kill utility, sends a

signal from one "rocess to another:

#include

#include int kill (pid_t pid, int signo);

In its normal use +i.e.% if "id is greater than 9,% 0ill+ , sends the signalsigno to the "rocess identified b! "id.

If "id is 9% signo is sent to eer! "rocess in the ino0ing "rocess3

"rocess grou". If "id is '7% signo is sent to eer! "rocess for which the ino0ing "rocess

has "ermission to send a signal% e#ce"t itself and init.


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Sending a Signal to Yourself The raise+ , function is a sim"le wa! for a "rocess to

send a signal to itself:

#include int raise (int signo);

This call: raise +signo,8 is euialent to the followingcall: 0ill +get"id + ,% signo,8


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eentrancy2hen the 0ernel raises a signal% a "rocess can be

e#ecuting code an!where.

$or e#am"le% it might be in the middle of an im"ortanto"eration that% if interru"ted% would leae the "rocessis an inconsistent state


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Signal handlers cannot tell what code the "rocess ise#ecuting when a signal hits8 the handler can run inthe middle of an!thing.

It is thus er! im"ortant that an! signal handler !our"rocess installs be er! careful about the actions it"erforms and the data it touches


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It is a good idea for a signal handler neer to touchglobal data


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eentrant !unction( reentrant function is a function that is safe to call

from within itself +or concurrentl!% from another

thread in the same "rocess,. In order to ualif! as reentrant% a function must

not mani"ulate static data% must mani"ulate onl!stac0'allocated data or data "roided to it b! the

caller% and must not ino0e an! nonreentrantfunction

Signals UNIX

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