Linux Signal - required program (2023)


  • 1. Cognitive signals
  • 2. Generate a signal
  • 3. Process signal
  • 4. The signal is expressed in the nucleus
  • 5. Signal set processing function
  • 6. Signal Trap

Signals are uniquenotification mechanismEssential When a signal is sent to a process, the operating system interrupts the normal process of process control. If the process defines a signal handler, execute it, otherwise execute the default handler.

use command
kill -l View system signal list
Linux Signal - required program (1)
Each signal has a number and a name defined by a macro. These macro definitions can be found in Signal.h. For example:

#define signal 2

A live signal above 34 is a signal below 34.

1. Signaling via buttons

The default processing action for SIGINT (Ctrl+C hotkey) is to exit processing
The default processing action of SIGQUIT (Ctrl+key combination) is to terminate processing and dump the core.

For example, use the Ctrl+C key combination to generate a SIGINT signal. Press CTRL+C to stop the sleep command early before the timer expires;
The CTRL+Z key combination generates the SIGSTP signal, which can terminate any process running in the shell.
The termination process is not the same as the termination process. When a stop process stops, the program remains in memory and can continue running from where it stopped.

2. Call the system function to send a signal to the process
Write an infinite loop program and run it
Linux Signal - required program (2)
Linux Signal - required program (3)
Send the SIGSEGV signal with the kill command:
Linux Signal - required program (4)

Linux Signal - required program (5)
The previously discovered error was accessing illegal memory, and this program itself is correct. Sending Sigsegv will also result in a segfault.

The kill command is implemented by calling the KILL functionThe Essence Kill function can send a specific signal to a specific process.
add featureYou can send a specific signal to the current process (send a signal to yourself)

#includeint kill(pid_t pid, int char); int raise(int char);// Both functions return 0 correctly and -1 on error.

3. Signals about the state of the software
Example: Sigalrm signal and alarm functions

#includeunsigned int alarm (usigned int sekunder);

After a few seconds, a Sigalrm signal is sent to the calling process. The default action of the sigalrm signal is to terminate the process of calling the alarm function.
return value:
If the timer has not been set before or the timer setting times out, return 0;
Otherwise, the remaining seconds of the timer are returned and the timer is reset.

4. Hardware abnormality generates signal
hardware failure is hardwareTest the hardware in a wayAnd notify the kernel and then send the corresponding signal to the current process. For example, if the current process executes an instruction that divides by 0, the CPU operation unit will fail. The kernel sees this exception as a SIGFPE signal sent to the process.

After the process receives the signal, there are three processing methods:

  1. ignore
  2. The default handler for this signal
  3. The core switches to execution of user-defined processing functions based on the user. This method is calledsignal capture

The actual execution of the signal processing action is calledsignal lever
The state of the signal at the time of delivery is calleddivorce
process can chooseblock signal
A blocked signal remains in the decision state until a process that blocks this signal is raised.

Linux Signal - required program (6)
SignalBoth have two flags, which representBlocking and unreasonable (PENDING)There is still onefeature flagDirect healing function.
When the signal is generated, the core on the PCB is configured to set the fail character of the signal., only when the signal is passed to the flag.
In the figure above, the SIGINT signal (signal #2) has been generated, but it is being blocked, so it cannot be sent at this time. Although its processing action is ignored, this signal cannot be ignored until the lock is released, because the process still has the opportunity to change the processing action before the lock is released.
The SIGQUIT signal (signal 3) has not been generated. It blocks as soon as the SIGQUIT signal is generated. Its processing action is a Sighandler user-defined function.

Under Linux it is a regular signalOnly once Only once before delivery to warehouseLive signals can be queued multiple times before being transmitted.

signal set_t

From the picture above, each signal has only one unsatisfactory BIT character, either -0 or 1, and a lock flag.
Unreasonable and blocking characters can be stored with the same sigset_t data type. SIGSET_T is calledlogo set

The type can indicate the "active" or "inactive" state of each signal, iLock plate group"Valid" and "Invalid" mean whether the signal is shielded or not. This set of blocking signals is also known as the current process.signal shielding

Iincompatible signal setThe meaning of "valid" and "invalid" is whether the signal is in an outstanding state

You must include the header file:

#includeint sigemptyset(sigset_t *set); //SIGEMPTYSET function initializes the signal set pointed to by SET, so that the BITs corresponding to all signals are clearint sigfillset(sigset_t *set); //The signal set pointed to by the SIGFILLSET function initializes SET to set the corresponding bits of all signals set to 1

notice meBefore using variables of type Sigset_TBe sure to call SIGEMPTYSET or SIGFILLSET as initialization so that the token set is in a specific state.

int sigaddset(sigset_t *set, int character); int sigdelset(sigset_t *set, int character); // After initializing the Sigset_t variable, you can call Sigaddset and SigDelset to add or remove a valid signal in this centralized signal

The first four functions arereturn success, return error -1

int sigismember(const sigset_t *set, int sign); // sigismember is a Boolean function used to determine whether the valid signal in the signal set contains a specific signal // If it contains 1, it returns 1, if it does not contain 0, it returns 0 And return -1int sigprocmask(int how, const sigset_t *set, sigset_t *oset); // Call the SIGPROCMASK function to read or change the signal mask of the process

If OSET is a non-null pointer, the current signal mask of the read process is passed through the OSET parameter.
If a non-air pointer is set, change the process signal screen and how to change how to set the parameters.
If OSET and Set are not null pointers, back up the original signal guard word in OSET first, and then modify the signal guard word according to the Set and HOW parameters.

HOW parameters and functions:

Linux Signal - required program (7)

int sigpending(sigset_t *set) // Read the set of unsatisfied signals from the current process and distribute them among the SET parameters. The call returns 0 on success, and -1 on error

Linux Signal - required program (8)

As shown in the figure above, if the signal processing gesture is a user-defined function, when the signal is sent, the function will be calledcapture signal
The time to capture the signal is when the kernel state changes to the user state
If there is no custom processing function, there will be no processing of 3 to 4 above. After performing default rendering operations in kernel space, do 5

Signal acquisition function:

  1. divorced

    typedef void (*sighandler_t)(int);
    señal sighandler_t (int signum, kontrolador sighandler_t);

signal() takes two arguments: the signal number and afeature flag(sighandler_t, pointing to a user-defined processing function), the return value is also of type SIGHANDLER_T, the previous signal processing function

Regulators can also have two special values:
signal to blocking signal
SIG_DFL restore default behavior

A custom signal handler is a function with an INT parameter returning VOID.

  1. follow-up

    int sigaction(int signo, konst struct sigaction *act, struct sigaction *oact);

The SIGACTION function can read and modify the processing action associated with the specified signal
The call returns 0 on success, and -1 on error
Detailed reference here

When calling a signal handler,The kernel automatically adds the current signal to the process signal mask.When the signal processing function returns, the original signal mask is automatically restored, ieIt is guaranteed that after processing this signal, if this signal is generated again, it will be blocked until the end of this processing.


How do I know if my signal is blocked Linux? ›

Under Linux, you can find the PID of your process, then look at /proc/$PID/status . It contains lines describing which signals are blocked (SigBlk), ignored (SigIgn), or caught (SigCgt).

How to stop signal Linux? ›

The most common way to kill a process in Linux is by using 'CONTROL-C', when we press 'CONTROL-C' SIGINT signal is sent to the program and by default, the program terminates. The signal SIGINT is also called a Interrupt signal or key.

What is the reason for SIGKILL? ›

The most common reason for a SIGKILL(9) error is when the machine suddenly runs out of memory. the environment you've selected. Maybe you need a machine type with more memory or a GPU? any 3rd party libraries you might have for known issues around the features you're using.

What is signal () in Linux? ›

signal() sets the disposition of the signal signum to handler, which is either SIG_IGN, SIG_DFL, or the address of a programmer- defined function (a "signal handler"). If the signal signum is delivered to the process, then one of the following happens: * If the disposition is set to SIG_IGN, then the signal is ignored.

How do I unblock a signal? ›

Steps to unblock a number or group:
  1. Go to Signal Settings. > Privacy > Blocked.
  2. Select the contact or group to unblock.
  3. Confirm by choosing Unblock.

How do I know if my firewall is blocking a port Linux? ›

netstat -an | grep PORTNUMBER | grep -i listen If the output is empty, the port is not in use. nc -w5 -z -v <ip_address> <port_number> , you should get something like Connection to 127.0. 0.1 9000 port [tcp/*] succeeded! , otherwise port is closed.

How do I update my signal in Linux? ›

  1. Updates are handled through APT.
  2. Debian and Ubuntu desktop environments check for new APT packages automatically, and your system will prompt you when updates are available.
  3. You can also launch the “Software Updater” app to check for new updates.

How to trap a signal in Linux? ›

The signals most commonly used with the trap command are:
  1. SIGHUP (1) - Clean tidy-up.
  2. SIGINT (2) - Interrupt.
  3. SIGQUIT (3) - Quit.
  4. SIGABRT (6) - Cancel.
  5. SIGALRM (14) - Alarm clock.
  6. SIGTERM (15) - Terminate.
Dec 16, 2021

What command is used to stop a service in Linux? ›

Stop the Service

Run "service MotionBoard60 stop". To stop the service as the user for running MotionBoard, run "<InstallDir>/system/bin/ stop".

Can a program catch SIGKILL? ›

The program actually never receives the SIGKILL signal, as SIGKILL is completely handled by the operating system/kernel. When SIGKILL for a specific process is sent, the kernel's scheduler immediately stops giving that process any more CPU time for running user-space code.

Can you catch a SIGKILL? ›

The SIGKILL signal is sent to a process to cause it to terminate immediately (kill). In contrast to SIGTERM and SIGINT, this signal cannot be caught or ignored, and the receiving process cannot perform any clean-up upon receiving this signal.

In what cases does SIGKILL fail? ›

SIGKILL instructs the process to terminate immediately. It cannot be ignored or blocked. The process is killed, and if it is running threads, those are killed as well. If the SIGKILL signal fails to terminate a process and its threads, this indicates an operating system malfunction.

What is the default signal in Linux? ›

A Default signal handler is associated with every signal that the kernel runs when handling that signal. The action that a script or program performs when it receives a signal is called the default actions. A default signal handler handles these types of different default actions. Terminate the process.

What is an example of signal in Linux? ›

The name of a LINUX signal begins with "SIG". Although signals are numbered, we normally refer to them by their names. For example: SIGINT is a signal generated when a user presses Control-C.

How many signals does Linux have? ›

The Linux kernel supports a range of 33 different real-time signals, numbered 32 to 64.

How can I restore my signal? ›

What if I do not have my old Android phone, restored my Android phone to factory settings, or lost my phone?
  1. On your new device, install and open Signal, select Continue and complete registration.
  2. If you selected the Transfer or restore account option, go back or swipe close then open Signal again and select Continue.

What could be blocking my signal? ›

Common building materials such as concrete, steel, brick and wood are cell signal killers, but you may not expect to find fiberglass insulation on the list of things that could also impede signal.

How to unblock a port in Linux? ›

Use the firewall-cmd command to open a port.

To make the change permanent, add the --permanent flag to the command: firewall-cmd --zone=public --permanent --add-port=22/tcp . To open a UDP port, replace tcp with udp . To open the port by service name, use firewall-cmd --zone=public --permanent .

How to check port block in Linux? ›

Check open ports in Linux
  1. Open a Linux terminal application.
  2. Use ss command to display all open TCP and UDP ports in Linux.
  3. Another option is to use the netstat command to list all ports in Linux.
  4. Apart from ss / netstat one can use the lsof command to list open files and ports on Linux based system.
Jul 22, 2019

How do I check firewall access in Linux? ›

  1. Besides using ufw on the terminal, we can install gufw. ...
  2. Aside from ufw, we can also use firewalld to manage our firewall rules. ...
  3. firewalld can run alongside ufw. ...
  4. We can also use iptable to check the status of our firewall.
May 7, 2023

Why won t my Signal work? ›

Basic Troubleshooting

Update to the latest version of Signal Android and enable all application permissions as listed above. Update the Google Play services app on your phone and enable all application permissions. Allow Signal to autostart and work in the background. Enable phone specific settings.

What is the alternative to Signal in Linux? ›

The 10 Best Instant Messaging Apps for Linux
  • Signal. Signal is an open-source, cross-platform application with top-notch end-to-end encryption facilities. ...
  • Telegram. ...
  • Franz. ...
  • Skype. ...
  • Jami. ...
  • Discord. ...
  • Gajim. ...
  • Wire.
Feb 26, 2022

Can you use Signal without a SIM card? ›

Absolutely. All Signal calling and messaging is done over the internet. As long as you are connected to the internet without any restrictions, then everything will work just as it did when you were home with your old SIM card.

What signals Cannot be trapped in Linux? ›

There are two signals which cannot be intercepted and handled: SIGKILL and SIGSTOP.

Which signal Cannot be handled in Linux? ›

The SIGTERM signal is a generic signal used to cause program termination. Unlike SIGKILL , this signal can be blocked, handled, and ignored. It is the normal way to politely ask a program to terminate. The shell command kill generates SIGTERM by default.

What is Ctrl D signal in Linux? ›


If you're in a bash prompt and it want it to exit (like if you're remotely connected to a bash server for example), if you hit CTRL + D it'll tell the bash session to end. You also could close the window or just type exit and it'll exit too.

How do I check service status in Linux? ›

To check a service's status, use the systemctl status service-name command. I like systemd's status because of the detail given. For example, in the above listing, you see the full path to the unit file, the status, the start command, and the latest status changes.

How do I restart a network service in Linux? ›

This can be done by using the command ¡°systemctl restart network¡±. This command will restart the network service and it should be up and running again. It is important to note that this command will only restart the network service and not any other services that may be running on the system.

How do I restart all services in Linux? ›

System Logging
  1. Log into your server using the terminal program of your choice via SSH. ...
  2. If your server uses CentOS, CloudLinux, or RHEL 7, type: systemctl restart syslog.service and press Enter If your server uses CentOS, CloudLinux, or RHEL 5 or 6, type: service syslog restart.

What is the difference between signal SIGTERM and signal SIGKILL? ›


SIGTERM gracefully kills the process whereas SIGKILL kills the process immediately. SIGTERM signal can be handled, ignored and blocked but SIGKILL cannot be handled or blocked. SIGTERM doesn't kill the child processes. SIGKILL kills the child processes as well.

What is the difference between SIGKILL and exit? ›

Typically, an app would want to terminate gracefully if at all possible. SIGKILL is an instant death for your process - your exit handlers won't be called, for example. But in your case, you also have to call the getpid as well as the kill itself. exit instantly initiates the graceful exit process.

What is a signal mask? ›

The collection of signals that are currently blocked is called the signal mask. Each process has its own signal mask. When you create a new process (see Creating a Process), it inherits its parent's mask. You can block or unblock signals with total flexibility by modifying the signal mask.

Can SIGQUIT be caught? ›

It's SIGKILL that cannot be caught, not SIGQUIT . For signals that can be caught, a process can set a trap or signal handler that gets called when the process receives that signal.

What is the difference between SIGKILL and Sigabrt? ›

SIGABRT is equivalent of "kill -6" and is used to terminate/abort running processes. SIGKILL signal cannot be caught or ignored and the receiving process cannot perform any clean-up upon receiving this signal. SIGABRT signal can be caught, but it cannot be blocked.

What is the difference between SIGKILL and SIGINT? ›

The default action for SIGINT, SIGTERM, SIGQUIT, and SIGKILL is to terminate the process. However, SIGTERM, SIGQUIT, and SIGKILL are defined as signals to terminate the process, but SIGINT is defined as an interruption requested by the user.

What is the difference between SIGKILL and SIGHUP? ›

The SIGHUP signal is used by some utilities as a way to notify the process to do something, such as re-read its configuration file. The SIGHUP signal is also sent to a process if the remote connection is lost or hangs up. The SIGKILL signal is used to abort a process, and the SIGSTOP signal is used to pause a process.

Can SIGTERM be ignored? ›

Using SIGTERM is a preferred method to prematurely terminate a process. SIGTERM lets the application clean up, but it can be ignored by the application.

How to use SIGKILL in Linux? ›

The kill -9 command sends a SIGKILL signal indicating to a service to shut down immediately. An unresponsive program will ignore a kill command, but it will shut down whenever a kill -9 command is issued. Use this command with caution. It bypasses the standard shutdown routine so any unsaved data will be lost.

What is Linux signal code 6? ›

Signal 6 ( SIGABRT ) = SIGABRT is commonly used by libc and other libraries to abort the program in case of critical errors. For example, glibc sends an SIGABRT in case of a detected double-free or other heap corruptions.

What is Linux signal number 9? ›

(signal 9) is a directive to kill the process immediately. This signal cannot be caught or ignored. Zombie processes continue to exist in the process table until the parent process dies or the system is shut down and restarted. In the example shown above, the parent process (PPID) is the ksh command.

What is Linux signal code 11? ›

Signal 11, also known as SIGSEGV (signal segmentation violation) is an error signal in Unix systems (Linux). The error appears when a program or code tries to access an invalid memory location. The operating system sends the signal 11 error to the program as a response.

What is the default action of Sigtstp? ›

This is the signal sent when the user hits Control+Z on the terminal. (SIGTSTP is short for “terminal stop”) The only difference between SIGTSTP and SIGSTOP is that pausing is only the default action for SIGTSTP but is the required action for SIGSTOP.

Which command is used to signal processes in Linux? ›

The ps command shows the processes. By default, it shows only processes that maintain a connection with a terminal, a console, a serial line, or a pseudo terminal.

What is an unreliable signal? ›

Unreliable signals are those for which the signal handler does not remain installed once called. These ``one-shot'' signals must re-install the signal handler within the signal handler itself, if the program wishes the signal to remain installed.

How do I see all networks in Linux? ›

netstat command – It is used to display network connections, routing tables, interface statistics, masquerade connections, and multicast memberships. ifconfig command – It is used to display or configure a network interface. nmcli command – A command to show or configure a network interface on Linux.

What are reliable signals in Linux? ›

Linux supports both POSIX reliable signals (hereinafter "standard signals") and POSIX real-time signals. Each signal has a current disposition, which determines how the process behaves when it is delivered the signal. Default action is to terminate the process.

How do I send a signal to a process in Linux? ›

The kill Command

Thus, if you want to send signals SIGINT, SIGQUIT and SIGKILL, use INT, QUIT and KILL, respectively. If the signal name is missing, the default SIGTERM is used. Following the signal name is a list of process IDs to which the signal will be sent.

How do I know if my network port is blocked? ›

Check for Blocked Port using the Command Prompt
  1. Type cmd in the search bar.
  2. Right-click on the Command Prompt and select Run as Administrator.
  3. In the command prompt, type the following command and hit enter. netsh firewall show state.
  4. This will display all the blocked and active port configured in the firewall.
Apr 3, 2023

How do I know if port 22 is blocked in Linux? ›

How to check if port 22 is open in Linux
  1. Run the ss command and it will display output if port 22 opened: sudo ss -tulpn | grep :22.
  2. Another option is to use the netstat: sudo netstat -tulpn | grep :22.
  3. We can also use the lsof command to see if ssh port 22 status: sudo lsof -i:22.
Sep 21, 2020

How do I know if port 25 is blocked Linux? ›

You can use nmap -sT localhost to determine which ports are listening for TCP connections from the network. To check for UDP ports, you should use -sU option. To check for port 25, you can easily use nmap -p25 localhost . And if you do not have access to the system, you can use nmap -sS -p25 yourTargetIP .

How to check network connectivity on Linux? ›

[LINUX] Checking the Network Connection
  1. Open Terminal (CTRL+ALT+T)
  2. Type ip addr show.
  3. NOTE: If no IP Address is assigned, please restart the device.
Dec 27, 2022

How do I unblock a port on my network? ›

Answer: From the Control Panel, navigate to System and Security, and click on Windows Firewall. Go to the Advanced settings and right-click on Inbound Rules on the left pane. Select New Rule, add the port and click Next. Pick the Protocol and the Port Number, click Next again.

How to check ports open in Linux? ›

To check the listening ports and applications on Linux:
  1. Open a terminal application i.e. shell prompt.
  2. Run any one of the following command on Linux to see open ports: $ sudo lsof -i -P -n | grep LISTEN. $ sudo netstat -tulpn | grep LISTEN. ...
  3. For the latest version of Linux use the ss command. For example, ss -tulw.
5 days ago

What happens when a port is blocked? ›

Port blocking thus affects the traffic associated with a particular combination of port number and transport protocol on that ISP, regardless of source or destination IP address. The practice can potentially prevent the use of particular applications altogether by blocking the ports those applications use.

How do I enable 22 port in Linux? ›

A user with sudo access.
  1. Check UFW Status. First, let's check the UFW firewall status. ...
  2. Allowing SSH on Port 22. As mentioned before by default enabling UFW will deny all incoming connections and same time allow all outgoing connections. ...
  3. Enable UFW. Now you can go ahead and enable UFW firewall. ...
  4. Verify.
Jan 21, 2023

How do I check my firewall status in Linux? ›

  1. Besides using ufw on the terminal, we can install gufw. ...
  2. Aside from ufw, we can also use firewalld to manage our firewall rules. ...
  3. firewalld can run alongside ufw. ...
  4. We can also use iptable to check the status of our firewall.
May 7, 2023

How do I know if port 443 is blocked Linux? ›

Type the ss command or netstat command to see if a TCP port 443 is in use on Linux? The port 443 is in use and opened by nginx service. Where, -t : Display TCP sockets/port.

How to unblock port 22 in Linux? ›

Keep reading the rest of the series:
  1. Install UFW firewall on Ubuntu 16.04 LTS server.
  2. Open ssh port 22 using ufw on Ubuntu/Debian Linux.
  3. Configure ufw to forward port 80/443 to internal server hosted on LAN.
  4. Block an IP address with ufw on Ubuntu Linux server.
  5. Limit SSH (TCP port 22) connections with ufw on Ubuntu Linux.
Mar 24, 2023

How do I know if port 8080 is blocked? ›

Use the Windows netstat command to identify which applications are using port 8080:
  1. Hold down the Windows key and press the R key to open the Run dialog.
  2. Type “cmd” and click OK in the Run dialog.
  3. Verify the Command Prompt opens.
  4. Type “netstat -a -n -o | find "8080"". A list of processes using port 8080 are displayed.
May 15, 2023


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