Events and Signals

20 April 2020

Obed N Munoz

Cloud Software Engineer

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.

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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 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.

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The inotify API: key 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.

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The inotify API: events

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The inotify API: reading events (1/2)

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 */
};

Take a look on inotify/demo_inotify.c from The Linux Programming Interface.

In a first terminal, run:

mkdir dir1 dir2
./demo_inotify dir1 dir2
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The inotify API: reading events (2/2)

In a second terminal, run:

cat > dir1/aaa
Hello World

# Type Control-D

mv dir1/aaa dir2/bbb

mkdir dir2/ddd

rmdir dir1
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Let's Code: File/Directory Monitor

8

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:

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Process' actions on signal

Depending on the signal, a process may act different:

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

10

Signal types and default actions

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Signal Handlers (changing 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.

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Signal Handler example:

#include <signal.h>
#include "tlpi_hdr.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);
   }
}

Take a look on signals/ouch.c and signals/intquit.c from The Linux Programming Interface.

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Sending Signals

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

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);

The killpg() function sends a signal to all of the members of a process group.

Take a look on signals/t_kill.c from The Linux Programming Interface.

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Let's Code: Progress Notifier with Signals

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Resources and Credits

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

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Thank you

Obed N Munoz

Cloud Software Engineer

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