Signals, Part 3: Raising signals

How do I send a signal to a process from the shell?

You already know one way to send a SIG_INT just type CTRL-C From the shell you can use kill (if you know the process id) and killall (if you know the process name)

# First let's use ps and grep to find the process we want to send a signal to
ps au | grep myprogram
angrave  4409   0.0  0.0  2434892    512 s004  R+    2:42PM   0:00.00 myprogram 1 2 3

#Send SIGINT signal to process 4409 (equivalent of `CTRL-C`)
kill -SIGINT 4409

#Send SIGKILL (terminate the process)
kill -SIGKILL 4409
kill -9 4409

killall is similar except that it matches by program name. The next two example, sends a SIGINT and then SIGKILL to terminate the processes that are running myprogram

# Send SIGINT (SIGINT can be ignored)
killall -SIGINT myprogram

# SIGKILL (-9) cannot be ignored! 
killall -9 myprogram

How do I send a signal to a process from the running shell?

Use raise or kill

int raise(int sig); // Send a signal myself!
int kill(pid_t pid, int sig); // Send a signal to another process

For non-root processes, signals can only be sent to processes of the same user i.e. you cant just SIGKILL my processes! See kill(2) i.e. man -s2 for more details.

How do I send a signal to a specific thread?

Use pthread_kill

int pthread_kill(pthread_t thread, int sig)

In the example below, the newly created thread executing func will be interrupted by SIGINT

pthread_create( &tid, NULL, func, args);
pthread_kill( tid, SIGINT);
pthread_kill( pthread_self() , SIGKILL); // send SIGKILL to myself

Will pthread_kill( threadid, SIGKILL) kill the process or thread?

It will kill the entire process. Though individual threads can set a signal mask, the signal disposition (the table of handlers/action performed for each signal) is per-process not per-thread. This means sigaction can be called from any thread because you will be setting a signal handler for all threads in the process.

How do I catch (handle) a signal ?

You can choose a handle pending signals asynchronously or synchronously.

Install a signal handler to asynchronously handle signals use sigaction (or, for simple examples, signal ).

To synchronously catch a pending signal use sigwait (which blocks until a signal is delivered) or signalfd (which also blocks and provides a file descriptor that can be read() to retrieve pending signals).

See Signals, Part 4 for an example of using sigwait