Events

An event can be defined as a situation or occurence that is being
recognized by a program. An event will occur by one or multiple
actions that happened in the execution of the program.

• Event's handling will depend on each program flow and logic.
• An event can be generated by a signal.
• Events can cause new signals.
• Event-Driven architecture and development comes from this basic concept.

File Events

Some applications will require the ability to monitor files or directories in order to determine an event have occured in the specific monitored objects.

Example: A graphical file manager

A graphical file manager needs to be able to determine when files are added or removed from the directory that is currently being displayed, or a daemon may want to monitor its configuration file in order to know if the file has been changed.

inotify API: steps

1. Application calls inotify_init() to create an inotify instance.

2. The application informs the kernel about which files are of interest by using the inotify_add_watch()

3. In order to obtain event notifications, the application performs read() operations on the inotify file descriptor.

4. When the application has finished monitoring, it closes the inotify file descriptor.

inotify API: events

The inotify API: reading events

After events have occurred, each read() returns a buffer.

 struct inotify_event {
 	int      wd;       /* Watch descriptor */
 	uint32_t mask;     /* Mask describing event */
 	uint32_t cookie;   /* Unique cookie associating related events (for rename()) */
 	uint32_t len;      /* Size of name field */
 	char     name[];   /* Optional null-terminated name */
 };

Source Code: inotify/demo_inotify.c from TLPI.

In a terminal 1, run:

 mkdir dir1 dir2
 ./demo_inotify dir1 dir2

In terminal 2, run:

 echo "Hello World" > dir1/aaa
 mv dir1/aaa dir2/bbb
 mkdir dir2/ddd
 rmdir dir1

• What's happening on terminal 1?

Let's Code: File/Directory Monitor

• Follow instructions from:

https://github.com/CodersSquad/c-file-monitor

Signals

A signal is a notification to a process that an event has occurred. Signals are sometimes described as software interrupts.

The usual source of many signals sent to a process is the kernel.

Below the types of events that cause the kernel to generate a signal:

• A hardware exeption occured
  Examples: marformed machine-language instruction, dividing by 0, inaccessible memory referencing.

• The user typed one of the terminal special character
  Examples: Control-C, Control-Z

• A software event ocurred
  Examples: terminal windows was resized, timer went off, CPU limits or child process is terminated.

Process' actions on signal

Depending on the signal, a process may act different:

• The signal is ignored.

• The process is terminated (killed).

• A core dump file is generated.

• The process is stopped.

• Execution of the process is resumed after previously being stopped.

Alternatively, a program may change the default behavior (also known as disposition) by:

• Allowing default action occur.

• The signal is ignored.

• A signal handler is executed.

Signal types and default actions

Signal Handlers

(Signal dispositions)

A signal handler (also known as signal catcher) is a function, written by the programmer, that performs appropriate tasks in response to the delivery of a signal.

 #include <signal.h>
 static void sigHandler(int sig) {
    printf("Ouch!\n");   /* UNSAFE (see Section 21.1.2) */
}

 int main(int argc, char *argv[]) {
   int j;

  if (signal(SIGINT, sigHandler) == SIG_ERR) errExit("signal");
    for (j = 0; ; j++) {
        printf("%d\n", j);
        sleep(3);
    }
 }

Source Code: signals/ouch.c and signals/intquit.c from TLPI.

Sending Signals

• int kill(pid_t pid, int sig)
  One process can send a signal to another process using the kill() system call, which is the analog of the kill shell command.

• int raise(int sig)
  Sometimes, it is useful for a process to send a signal to itself.

• In a single-threaded program, a call to raise() is equivalent to the following call to kill()

  kill(getpid(), sig)

• int killpg(pid_t pgrp, int sig);
  The killpg() function sends a signal to all of the members of a process group.

Source Code: signals/t_kill.c from TLPI.

Resources and Credits

This material is generated thanks to some extracts from following resources:

• The C Programming Language - Brian W. Kernighan
• The Linux Programming Interface (Chapters 19, 20, 21, 22) - Michael Kerrisk
• Signal Handling from GNU Project Documentation

Thanks

• Obed N Muñoz Reynoso
  • Cloud Software Engineer
  • obed.n.munoz@gmail | tec | intel.com