< job search> process and signal

One 、 process

process ： An address space in which one or more threads are running and the system resources required by the threads .

ps command ：process status Check the status of the running process

PID：2-32768 1 Generally, it is a special process Init Retain ,init Processes are responsible for managing other processes .

1、 Create child process

Replacement process impression ：

exec Series of functions ： Switch the execution of a program from one program to another , After the new program starts , The original program is no longer executed .

Copy process impression ：

fork function ： Create a new process . The new process executes the same code as the original process , But it has its own data space 、 Environment and file descriptors .

Possible reasons for the failure of creating sub process ：

（1） When the number of child processes owned by the parent process exceeds the specified limit （CHILD_MAX）, It may cause the creation of child processes to fail .errno:EAGAIN

（2） There is not enough space in the process table to create new forms or there is not enough virtual memory .errno:ENOMEM

2、 Waiting process

（1）pid_t wait(int *stat_loc) Call... That returns the child process PID, If stat_loc It's not a null pointer , The status information of the child process will be written to the location it points to .

State information ：

WIFEXITED(stat_val)          If the subprocess ends normally , Return non 0 value

WEXITSTATUS(stat_val)         If WIFEXITED Not 0, Return the exit code of the child process

WIFSIGNALED(stat_val)        If the child process is terminated by an uncapped signal , Return non 0 value

WTERMSIG(stat_val)          If WIFSIGNALED Return non 0 value , Returns a signal code

WIFSTOPED(stat_val)          If the subprocess terminates unexpectedly , It returns non 0 value

WSTOPSIG(stat_val)        If WIFSTOPED Not 0, It returns a signal code

（2）pid_t waitpid(pid_t pid, int *stat_loc, int option) Wait for the specified process

3、 Zombie process

After the child process terminates , it The connection with the parent process will remain , Until the parent process also terminates normally or the negative process calls wait To terminate . therefore , Entries representing child processes in the process table will not be released immediately , Its exit code needs to be saved for the parent process in the future wait Use . At this time, it will become a zombie process or a dead process .

If the parent process terminates abnormally ,PID by 1 Of init The process will be the parent process .

Two 、 I / O redirection

ex： 

cat file.txt //  aaaaaa

./upper  < file.txt //AAAAAA

3、 ... and 、 The signal

A signal is an event generated by the system in response to certain conditions , The signal is generated by certain error conditions . If the executing process receives the signal but does not arrange to capture it in advance , The process will terminate .

Common signals ：

SIGABORT        Process aborted

SIGINT        Terminal interrupt （ctrl+c）

SIGKILL        Terminate the process （ This signal cannot be captured or ignored ）

SIGSEGV        Invalid memory segment access

SIGSTOP        Stop executing

SIGCHLD        The child process has stopped or exited , By default , It is ignored

SIGCONT        Continue the suspended process

If you want to send a signal to the process , But this process is not the current foreground process , Need to use Kill Command sending signal

1、 Sending signal

int kill(pid_t pid, int sig);

pase();// Suspend program execution , Until a signal appears .

2、 Signal processing functions

（1）void (*signal(int sig, void (*func)(int)))

sig: Processed signals   func: The function called after receiving the signal

signal The function returns the function pointer of the signal processing function .

Signal processing functions are also available SIG_IGV（ Ignore the signal ）、SIG_DFL（ Restore default behavior ）

#include <signal.h>
#include <stdio.h>
#include <unistd.h>
void A(int sig)
{
    printf("I got signal %d\n",sig);
    (void) signal(SIGINT, SIGDFL);// Restore default behavior 
}

int main()
{
    (void) signal(SIGINT, A);
    while(1)
    {
        printf("hh\n");
        sleep(1);
    }
}

Be careful , It is called in the signal processing function printf Functions are not safe .printf The internal implementation of needs to obtain the lock of the output first , Then modify the output , Finally, release the lock at the output . If the main program just gets the lock at the output , And the signal interrupts the main program , But the signal processing function is waiting for the main program to release the lock , This creates a deadlock .

（2） Robust signal interface

int sigaction(int sig,  const struct sigaction *act, struct sigaction *oact);

sigaction Structure contains ：

void (*) (int) sa_hander  //function, SIG_IGN or SIG_DFL

sigset_t sa_mask  // Signal set , Before calling the signal processing function , This signal set will be masked by the process

int sa_flags  // The signal processing function is not reset by default , If it needs to be reset , It can be set to SA_RESETHAND