A deep understanding of the signals of inter-process communication and a deep understanding of Process Communication

A deep understanding of the signals of inter-process communication and a deep understanding of Process Communication
Signal and Signal Source

Signal nature

A signal is a simulation of the interrupt mechanism at the software level. In principle, a process receives a signal and the processor receives an interrupt request. The signal is asynchronous. A process does not have to wait for the signal to arrive through any operation. In fact, the process does not know when the signal will arrive.

Signals are the only asynchronous communication mechanism in the inter-process communication mechanism. They can be seen as asynchronous notifications to notify the processes that receive signals of what happened. After POSIX real-time expansion, the signal mechanism is more powerful. In addition to the basic notification function, it can also transmit additional information.

Signal Source

There are two sources for the occurrence of signal events: hardware sources (for example, we press the keyboard or other hardware faults); software sources, the most common system function for sending signals is kill, raise, alarm, setimer, and sigqueue functions. The software source also includes some illegal operations.

Process response to signals

A process can respond to a signal in three ways: (1) Ignore the signal, that is, no processing is performed on the signal. Two signals cannot be ignored: SIGKILL and SIGSTOP; (2) capture the signal. Define the signal processing function. When a signal occurs, execute the corresponding processing function. (3) perform the default operation. Linux specifies the default operation for each signal. Note that the default reaction of a process to a real-time signal is that the process is terminated.

In Linux, which of the above three methods is used to respond to signals depends on the parameters passed to the corresponding API functions.

SIGNAL INSTALLATION

If a process needs to process a signal, it must be installed in the process. The installation signal is mainly used to determine the ing between the signal value and the action of the process against the signal value, that is, the signal to be processed by the process, and the operation to be performed when the signal is transmitted to the process.

Linux mainly has two function implementation signals: signal () and sigaction (). Signal () is implemented based on reliable signal system calls and is a database function. It has only two parameters and does not support signal transmission information. It is mainly used for the installation of the first 32 non-real-time signals, while sigaction () is a newer function (called by two systems: sys_signal and sys_rt_sigaction) have three parameters that support signal transmission information and are mainly used in combination with sigqueue () system calls. Of course, sigaction () also supports non-real-time signal installation. Sigaction () is superior to signal () mainly because it supports signal parameters.

Signal

　　typedef void (*sighandler_t)(int);
　　sighandler_t signal(int signum, sighandler_t handler));

The first parameter specifies the signal value, and the second parameter specifies the processing of the previous signal value, which can be ignored (the parameter is set to SIG_IGN ); you can use the default signal processing method (the parameter is set to SIG_DFL), or you can implement the processing method by yourself (the parameter specifies a function address ).

IF signal () is called successfully, return the handler value when signal () is called for the last signum installation signal; If signal () fails, return SIG_ERR.

Sigaction

　　int sigaction(int signum,const struct sigaction *act,struct sigaction *oldact));

The sigaction function is used to change the behavior of a process after receiving a specific signal. The first parameter of this function is the signal value, which can be any specific valid signal Except SIGKILL and SIGSTOP (define your own processing function for these two signals, will cause a signal installation error ). The second parameter is a pointer to an instance of the sigaction structure. In the instance of the sigaction structure, the processing of specific signals is specified, which can be blank, the process processes the signal by default. The third parameter oldact points to the object to save the original processing of the corresponding signal. You can specify oldact as NULL. If the second and third parameters are set to NULL, this function can be used to check the signal validity.

The second parameter is the most important, including the processing of the specified signal, the information transmitted by the signal, and the functions to be shielded during the execution of the signal processing function.

Signal sending

Main functions for sending signals include: kill (), raise (), sigqueue (), alarm (), setitimer (), and abort ().

To be continued ......