Linux, and any POSIX-compliant OS utilize signals to talk between the kernel and user space.
In fact there are a number of ways that process can talk to each other including
- Message Queues
- Shared Memory
You can review more information on Inter-Process Communication Mechanisms (IPC) here: http://www.tldp.org/LDP/tlk/ipc/ipc.html
There are a number of different Unix signals that are all handled differently. Application code can implement a signal, so that when it is called, that signal is intercepted and that signal handler is invoked. If a custom signal handler is not implemented, the default handler is used. The only two signals that cannot be intercepted are SIGKILL and SIGSTOP.
The following webpage listed out the signals in an easy to read list: http://www.comptechdoc.org/os/linux/programming/linux_pgsignals.html
Source for below: https://en.wikipedia.org/wiki/Unix_signal
The list below documents the signals specified in the Single Unix Specification. All signals are defined as macro constants in <signal.h> header file. The name of the macro constant consists of a "SIG" prefix followed by a mnemonic name for the signal
|Signal||Portable number||Default Action||Description|
|SIGABRT||6||Terminate (core dump)||Process abort signal|
|SIGBUS||n/a||Terminate (core dump)||Access to an undefined portion of a memory object.|
|SIGCHLD||n/a||Ignore||Child process terminated, stopped, or continued.|
|SIGCONT||n/a||Continue||Continue executing, if stopped.|
|SIGFPE||n/a||Terminate (core dump)||Erroneous arithmetic operation.|
|SIGILL||n/a||Terminate (core dump)||Illegal instruction.|
|SIGINT||2||Terminate||Terminal interrupt signal.|
|SIGKILL||9||Terminate||Kill (cannot be caught or ignored).|
|SIGPIPE||n/a||Terminate||Write on a pipe with no one to read it.|
|SIGPROF||n/a||Terminate||Profiling timer expired.|
|SIGQUIT||3||Terminate (core dump)||Terminal quit signal.|
|SIGSEGV||n/a||Terminate (core dump)||Invalid memory reference.|
|SIGSTOP||n/a||Stop||Stop executing (cannot be caught or ignored).|
|SIGSYS||n/a||Terminate (core dump)||Bad system call.|
|SIGTRAP||n/a||Terminate (core dump)||Trace/breakpoint trap.|
|SIGTSTP||n/a||Stop||Terminal stop signal.|
|SIGTTIN||n/a||Stop||Background process attempting read.|
|SIGTTOU||n/a||Stop||Background process attempting write.|
|SIGUSR1||n/a||Terminate||User-defined signal 1.|
|SIGUSR2||n/a||Terminate||User-defined signal 2.|
|SIGURG||n/a||Ignore||High bandwidth data is available at a socket.|
|SIGVTALRM||n/a||Terminate||Virtual timer expired.|
|SIGXCPU||n/a||Terminate (core dump)||CPU time limit exceeded.|
|SIGXFSZ||n/a||Terminate (core dump)||File size limit exceeded|
Additional Signal Details
The SIGABRT signal is sent to a process to tell it to abort, i.e. to terminate. The signal is usually initiated by the process itself when it calls abort function of the C Standard Library, but it can be sent to the process from outside like any other signal.
SIGALRM, SIGVTALRM and SIGPROF
The SIGALRM, SIGVTALRM and SIGPROF signal is sent to a process when the time limit specified in a call to a preceding alarm setting function (such as setitimer) elapses. SIGALRM is sent when real or clock time elapses. SIGVTALRM is sent when CPU time used by the process elapses. SIGPROF is sent when CPU time used by the process and by the system on behalf of the process elapses.
The SIGBUS signal is sent to a process when it causes a bus error. The conditions that lead to the signal being sent are, for example, incorrect memory access alignment or non-existent physical address.
The SIGCHLD signal is sent to a process when a child process terminates, is interrupted, or resumes after being interrupted. One common usage of the signal is to instruct the operating system to clean up the resources used by a child process after its termination without an explicit call to the waitsystem call.
The SIGCONT signal instructs the operating system to continue (restart) a process previously paused by the SIGSTOP or SIGTSTP signal. One important use of this signal is in job control in the Unix shell.
The SIGFPE signal is sent to a process when it executes an erroneous arithmetic operation, such as division by zero (the name "FPE", standing for floating-point exception, is a misnomer as the signal covers integer-arithmetic errors as well).
The SIGHUP signal is sent to a process when its controlling terminal is closed. It was originally designed to notify the process of a serial line drop (a hangup). In modern systems, this signal usually means that the controlling pseudo or virtual terminal has been closed. Many daemons will reload their configuration files and reopen their logfiles instead of exiting when receiving this signal. nohup is a command to make a command ignore the signal.
The SIGILL signal is sent to a process when it attempts to execute an illegal, malformed, unknown, or privileged instruction.
The SIGINT signal is sent to a process by its controlling terminal when a user wishes to interrupt the process. This is typically initiated by pressing Ctrl+C, but on some systems, the "delete" character or "break" key can be used.
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.
The SIGPIPE signal is sent to a process when it attempts to write to a pipe without a process connected to the other end.
The SIGPOLL signal is sent when an event occurred on an explicitly watched file descriptor. Using it effectively leads to making asynchronous I/O requests since the kernel will poll the descriptor in place of the caller. It provides an alternative to active polling.
SIGRTMIN to SIGRTMAX
The SIGRTMIN to SIGRTMAX signals are intended to be used for user-defined purposes. They are real-time signals.
The SIGQUIT signal is sent to a process by its controlling terminal when the user requests that the process quit and perform a core dump.
The SIGSEGV signal is sent to a process when it makes an invalid virtual memory reference, or segmentation fault, i.e. when it performs a segmentation violation.
The SIGSTOP signal instructs the operating system to stop a process for later resumption.
The SIGSYS signal is sent to a process when it passes a bad argument to a system call. In practice, this kind of signal is rarely encountered since applications rely on libraries (e.g. libc) to make the call for them.
The SIGTERM signal is sent to a process to request its termination. Unlike the SIGKILL signal, it can be caught and interpreted or ignored by the process. This allows the process to perform nice termination releasing resources and saving state if appropriate. SIGINT is nearly identical to SIGTERM.
The SIGTSTP signal is sent to a process by its controlling terminal to request it to stop (terminal stop). It is commonly initiated by the user pressing Ctrl+Z. Unlike SIGSTOP, the process can register a signal handler for or ignore the signal.
SIGTTIN and SIGTTOU
The SIGTTIN and SIGTTOU signals are sent to a process when it attempts to read in or write out respectively from the tty while in the background. Typically, these signals are received only by processes under job control; daemons do not have controlling terminals and, therefore, should never receive these signals.
The SIGTRAP signal is sent to a process when an exception (or trap) occurs: a condition that a debugger has requested to be informed of — for example, when a particular function is executed, or when a particular variable changes value.
The SIGURG signal is sent to a process when a socket has urgent or out-of-band data available to read.
SIGUSR1 and SIGUSR2
The SIGUSR1 and SIGUSR2 signals are sent to a process to indicate user-defined conditions.
The SIGXCPU signal is sent to a process when it has used up the CPU for a duration that exceeds a certain predetermined user-settable value. The arrival of a SIGXCPU signal provides the receiving process a chance to quickly save any intermediate results and to exit gracefully, before it is terminated by the operating system using the SIGKILL signal.
The SIGXFSZ signal is sent to a process when it grows a file larger than the maximum allowed size.