Operating Systems

Signals

Signals

provide a very primitive interprocess communication (ipc) facility

many signals result when a hardware interrupt is handled by UNIX and converted into a "software interrupt" (signal)

originally designed in UNIX so that one process could terminate another process

Kill

system call that would be better called "send_signal"

sends a signal from one process to another

kill 9: a sure kill

Signal Table

a table of (say) 32 entries kept for each process

the exact number of entries depends on the version of UNIX

each entry specifies the action to be taken by the process if it receives that type (number) of signal

some control keys are translated by UNIX into signals:

< cntrl -c> => SIGINT = #2
SIGHUP = #1
SIGQUIT = #3
SIGILL = #4
SIGFPE = #8
a process can have up to one signal of each type pending
the entry for number 9 cannot be changed; it is always "terminate", which is why signal #9 is the sure kill
SIG_DFL = (void (*)(int))0 ==> 0 (zero) cast to look like a function pointer
SIG_IGN = (void (*)(int))1 ==> 1 (one) cast to look like a function pointer
< cntrl - z> = SIGTSTP = suspend process until further notice. Default: S = suspend => SIG_DFL = S
SIGSTOP - guaranteed to suspend the process; like #9, this cannot be changed in the table; default: S
each entry in the table is a function pointer (in a C program, this looks like the name of a function)
SIGDFL corresponds to 0; i.e. a null pointer
SIGCONT - is the opposite of suspend

suspend: removed from the ready list, i.e., it is not available for the cpu scheduler to schedule until it is `continue'd.

Kill Utility Program

UNIX utility program that uses the kill system call to send a signal (by default SIGINT) to a process

The signal can be specified by name or by number

see man 1 kill

Examples

% kill 147 - Sends the default signal (SIGINT) to process 147

% kill -KILL 147 - Sends the KILL signal, instead of the default signal, to process 147

% kill -9 147 - Sends signal number 9, instead of the default signal, to process 147; this particular command has the same effect as kill -KILL 147

% kill -9 -1 => Kills all your processes

System Calls that Affect the Signal Table
kill: sends signals
signal: change the response to a signal, i.e., change the entry in the signal table
fork(): creates a new copy of the signal table for the new process

Note: the calls to signal are typically all placed at the beginning of a program

Example 1:

Program says "Ha! Ha!" whenever someone types control-C (which is translated to SIGINT).

Source code for signal1.cpp

Output for signal1.cpp

Example 2:

Program creates a child process that starts looping, and then terminates it.

Source code for signal2.cpp

Output for signal2.cpp

Example 3:

Program shows that the child can be set up to ignore a signal from its parent.

Source code for signal3.cpp