Signals and Signal Processing
CIS 370 – Lab 5
UMassD
Good Programming
4 It is always good practice to write several
small programs that do specific things and
combine them to do a task than write a large
monolithic program.
4 Ideally you would like several cooperating
processes.
4 UNIX provides a rich environment for IPCs
- some of these are signals, pipes and
FIFOs.
Signals
4 We will focus on signals in this chapter and
get to pipes and FIFOs later.
4 Suppose you’re running a UNIX command
$cc largeprog.c
4 If the command does not terminate in a
reasonable period of time, one typically
terminates this by pressing ctrl-c.
4 The command is terminated and the shell
prompt returns.
What actually happens?
4 The part of the kernel responsible for the
keyboard input sees the interrupt character.
4 The kernel then sends a signal called
SIGINT to all the processes that recognize
the terminal as their controlling terminal.
4 This includes the invocation of cc.
4 When cc receives the signal, it performs the
default action associated with SIGINT and
terminates.
What actually happens contined
4 The shell process also receives the signal.
4 It sensibly ignores the signal !
4 Programs can elect to “catch” SIGINT.
4 When they catch the signal they execute a
special interrupt routine (aka interrupt
service routine (ISR)).
Signals and the kernel
4 Signals are also used by the kernel to deal
with certain kinds of severe error.
4 Suppose a corrupt program containing
illegal machine instructions is executed
(accidentally or intentionally).
4 A process begins execution of the program.
Signal and the kernel
4 The kernel will detect the attempt to execute
the illegal instructions and send the process
the signal SIGILL (ILL stands for illegal) to
terminate it. It may look something like this
$faultyprog
Illegal instruction. Core dumped.
Process to process signals
4 Signals can also be sent between processes.
4 Consider the following:
$ gcc verybigprog.c &
[1] 1034
$ // after a long time
$ kill -9 1034
1034 terminated
$
The command kill sends a SIGTERM signal.
SIGTERM will terminate the process.
Process response to signals
A process can do three things with signals:
4 Choose the way it responds when it receives
a particular signal (signal handling).
4 Block out signals (that is, leave them for
later) for a specific piece of critical code.
4 Send a signal to another process.
Signal names
4 Signals are given mnemonic names.
4 They are defined in <signal.h>, with the
pre-processor directive #define.
4 They stand for small positive integers.
4 Most are intended for the kernel, although a
few are provided to be sent from process to
process.
Signal names
4 SIGABRT - Process abort signal (abnormal
termination - core dumped)
4 SIGALRM - Alarm clock (sent by the
kernel after timer has expired).
4 SIGBUS - Bus error (sent if a hardware
fault is detected - abnormal termination)
4 SIGCHLD - Child process terminated
(whenever a child process stops or
terminates, the kernels sends this to the
parent).
4 SIGCONT - Continue executing if stopped
(This is a job control signal which will
continue if the process is stopped, else
ignored, inverse of SIGSTOP)
4 SIGFPE - Floating-point exception (AT).
4 SIGHUP - The hangup signal. (The kernel
sends this to all processes attached to a
control terminal when the terminal is
disconnected - applies to sessions as well).
4 SIGILL - Illegal instruction. (AT)
4 SIGKILL - Kill (special signal sent from
one process to another to terminate the
receiver - cannot be ignored).
4 SIGPIPE - Write on pipe or socket when
recipient has terminated. (discussed later).
4 SIGPOLL - Pollable event. (signal is
generated by the kernel when an open file
descriptor is ready for input or output).
4 SIGPROF - Profiling time expired. (signal
is generated when timer expires and can be
used by interpreters to profile execution).
4 SIGQUIT - Quit. (similar to SIGINT)
4 SIGSEGV - Invalid memory reference. (AT)
(segmentation violation, generated when a
process attempts to access invalid memory)
4 SIGSTOP - Stop execution. (job control
signal, cannot be ignored)
4 SIGSYS - Invalid system call. (AT) (sent by
kernel when a process attempts to execute a
machine instruction which is not a sys call)
4 SIGTERM - Software termination signal
(used to terminate a process)
4 SIGTRAP - Trace trap. (AT) (special signal
used by debuggers such as sdb and adb,
along with ptrace system call).
4 SIGTSTP - Terminal stop signal. (signal
generated by the user typing ctrl-z, can be
ignored)
4 SIGTTIN - Background process attempting
read. (when a bg process attempts to read
from the controlling terminal, this signal is
sent and the process is typically stopped)
4 SIGTTOU - Background process
attempting write. (generated when the bg
process attempts to write to the controlling
terminal)
4 SIGURG - High bandwidth data is
available at a socket. (signal tells a process
that urgent or out of band data has been
received on a network connection)
4 SIGUSR1 and SIGUSR2 - user defined
signals
4 SIGVTALRM - Virtual timer expired. (sent
when user-mode time expires).
4 SIGXCPU - CPU time limit exceeded.
(signal generated when the process exceeds
its maximum CPU time limit).
4 SIGXFSZ - File size limit exceeded. (AT)
(generated when a process exceeds its
maximum file size limit)
Normal termination
4 For most signals normal termination occurs
when a signal is received.
4 The exit status returned to the parent in this
circumstance tells the parent what happened
4 There are macros defined in <sys/wait.h>
which allow the parent to determine the
cause of termination and in this particular
case the value of the signal which was
responsible.
Abnormal Termination
4 The signals SIGABRT, SIGBUS, SIGSEGV,
SIGQUIT, SIGILL, SIGTRAP, SIGSYS,
SIGXCPU, SIGXFSZ and SIGFPE cause
abnormal termination - core dumped.
4 The memory dump of the process is written
to a file called core.
4 The core file will include the values of the
program variables, h/w registers and control
information - in binary.
Signal Handling
4 Once a signal is received the process may:
– Take the default action, which is to terminate
the process. SIGSTOP will stop the process.
– Ignore the signal and carry on processing. In
case of batch programs signals may be
ignored.
– Take a specified user-defined action. Whenever
a program exits, whatever the cause, a
programmer might want to perform clean-up
operations such as removing work files.
Signal sets
4 Signal sets are one of the main parameters
passed to system calls that deal with signals
4 They simply specify a list of signals you
want to do something with.
4 Signal sets are defined of the type sigset_t
4 You can empty the set, or remove a few,
keep only the ones that interest you.
Signal sets
4 Usage
#include <signal.h>
// initialize
int sigempty(sigset_t *set);
int sigfillset(sigset_t *set);
// manipulate
int sigaddset(segset_t *set, int signo);
int sigdelset(segset_t *set, int signo);
Setting the signal action
4 Once you have defined a signal set, you can
choose a particular method of handling a
signal using sigaction.
4 Usage
#include <string.h>
int sigaction(int signo, const struct sigaction *act,
const struct sigaction *oact);
sigaction continued
4 sigaction structure contains a signal set.
4 The first parameter signo specifies the
signal for which we want to specify action
4 SIGSTOP and SIGKILL are exempt
4 The second parameter act gives the action
you want to set for signo.
4 The third parameter oact will be filled out
with the current settings.
Without interrupts
With an interrupt
Restoring a previous action
The sigsetjmp and siglongjmp
A program position is saved in in an object of
type sigjmp_buf (defined in <setjmp.h>
sigsetjmp and siglongjmp signals
4 Sometimes it is useful to go back to a
certain part of a program if a signal occurs
4 sigsetjmp saves the mask of the current
signal set and the current position in the
program
4 siglongjmp restores the mask if
encountered (like a long-distance, non-local
goto statement) and returns control to the
point in the program where it was saved.
Signal Blocking
4 If a program is performing a sensitive task,
it may well need to be protected from
interrupts.
4 Rather than ignore the signals, a process can
block the signals
4 The signals will be handled after the
completion of the task.
The sigprocmask system call
4 Usage
#include <signal.h>
int sigprocmak(int how, const sigset_t *set,
sigset_t *oset);
The how parameter tells the system call what
specific action to take. For example it coul be
set to SIG_SETMASK, which means block out
the signals in the second parameter set from
now on. The third parameter is simply filled
with the current mask of blocked signals.
Sending signals to other processes
4 The kill system call
4 Usage
#include <sys/types.h>
#include <signal.h>
int kill(pid_t pid, int sig);
Sending signals using raise and alarm
4 raise() simply sends a signal to the executing
process.
4 alarm() is a simple and useful call that sets up
a process alarm clock. Signals are used to tell
the process that the clock’s timer has expired.
4 alarm is not like sleep, which suspends the
process execution, alarm returns immediately.
4 After a fork, however the alarm clock is turned
off in a child process.
The pause system call
4 pause is a companion to alarm.
4 pause suspends the calling process, in a
way that will not waste CPU cycles, until
any signal, such as SIGALRM, is received.
4 If the process ignores the signal, the pause
also ignores the signal.
4 If the signal is caught, when the appropriate
interrupt routine is finished, pause returns a
-1.
Piping
4 Pipes and other IPCs