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da questo dispositivo ha anche altre istruzioni :
Facilità d'uso
F
or
the
purp oses
of
illustration,
w
e
will
use
the
signal()
system
call,
whic
h
is
the
simplest
of
the
system
calls
dealing
with
so
c
k
ets,
to
describ e
signal
handling.
There
are
sev
eral
default
actions
p ossible
that
may
b e
p erformed
up on
receipt
of
a
signal.
Some
of
them
are:
ignore
the
signal,
terminate
the
pro cess,
susp end
(stop)
the
pro cess,
and
resume
(con
tinue)
the
pro cess.
Of
the
signals
generated
b
y
so
c
k
ets,
only
the
ignore
and
terminate
defaults
are
a
v
ailable.
Whic
h
of
these
actions
is
the
default
for
signal
reception
dep ends
on
the
signal.
When
signal()
is
called,
there
are
three
metho ds
for
handling
the
signal.
They
are
SIG
DFL,
SIG
IGN
or
a
user
sp ecied
signal
handler.
SIG
DFL
sets
the
signal
handler
to
the
default
signal
handler.
This
is
useful
if
the
signal
handler
w
as
c
hanged
and
the
pro cess
should
b e
returned
to
an
initial
state.
SIG
IGN
sets
the
signal
handler
to
ignore
this
signal.
The
third
c
hoice
can
b e
the
most
useful.
It
is
used
to
set
up
a
user
sp eciable
function
to
b e
the
signal
handler.
Whenev
er
that
signal
is
raised,
the
user
sp ecied
function
is
in
v
ok
ed.
This
is
generally
referred
to
as
catc
hing
a
signal.
A
t
ypical
example
of
this
t
yp e
of
handler
(for
a
signal
that
is
not
indicativ
e
of
an
error
state)
is
to
either
set
or
incremen
t
a
ag
indicating
that
a
signal
w
as
receiv
ed.
When
a
signal
is
sen
t
to
a
pro cess
while
p erforming
a
so
c
k
ets
Sock
ets
and
Incoming
function,
sev
eral
things
may
o ccur.
This
dep ends
on
whether
the
Signals
so
c
k
et
function
is
dened
as
a
slow
function.
A
slow
function
is
a
function
that
can
blo
c
k
indenitely
.
F
or
so
c
k
ets,
these
functions
are
read ,
write ,
recv ,
send ,
recvfrom ,
recvmsg ,
sendmsg ,
and
accept .
All
other
so
c
k
ets
functions
are
fast
.
F
ast
functions
are
not
in
terrupted
b
y
a
signal.
Instead,
the
signal
is
raised
when
these
so
c
k
et
functions
exit.
Slow
functions
are
in
terrupted
b
y
a
signal
if
they
are
blo
c
k
ed
w
aiting
for
IO
(if
they
are
pro cessing
IO,
they
are
not
in
terrupted).
They
are
in
terrupted
in
the
middle
of
pro cessing
b
y
the
raising
of
a
signal.
They
stop
what
pro cessing
they
are
doing
and
return
the
error
EINTR .
They
do
not
complete
the
IO
that
w
as
initiated.
The
user
program
m
ust
re-initiate
an
y
desired
IO
explicitly
.
There
are
t
w
o
signals
that
can
b e
generated
b
y
actions
on
a
so
c
k
et.
Signals
Generated
By
They
are
SIGPIPE
and
SIGIO.
A
SIGPIPE
is
generated
when
a
send
Sock
ets
op eration
is
attempted
on
a
br
oken
so
c
k
et.
One
w
a
y
of
br
e
aking
a
so
c
k
et
is
to
do
a
sh
utdo
wn(,2)
on
a
so
c
k
et.
The
default
action
is
to
terminate
the
pro cess.
The
target
of
the
signal
is
the
pro cess
attempting
the
send.
The
other
signal
is
SIGIO.
SIGIO
is
somewhat
more
complex
than
SIGPIPE.
First,
a
call
to
ioctl()
to
request
the
enabling
FIO
ASYNC
is
required
to
enable
generation
of
this
signal.
Second,
another
call
to
ioctl()
is
required
to
request
SIOCSPGRP
,
whic
h
2-34
Sock
et
System
Calls
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