1. 关于fork

fork()函数:

   用于创建一个进程,所创建的进程复制父进程的代码段/数据段/BSS段/堆/栈等所有用户空间信息;在内核中操作系统重新为其申请了一个PCB,并使用父进程的PCB进行初始化;

#include <iostream>
#include <unistd.h>
using namespace std;
int val = 10;
int main(int argc, char *argv[])
{
    pid_t pid;
    int lval = 20;
​
    pid = fork();
​
    if(pid == 0){
        val += 2;
        lval += 5;
    }else{
        val -= 2;
        lval += 5;
    }
​
    if(pid == 0){
        cout << "val:" << val << ", lval = " << lval << endl;
    }else{
        cout << "val:" << val << ", lval = " << lval << endl;
    }
    return 0;
}

对于父进程而言,fork()函数返回子进程的ID(子进程的PID);而对于子进程而言,fork函数返回0。

僵尸进程

  父进程创建子进程后,子进程运行到终止时刻(例如,调用exit()函数,或者运行到main中的return语句时,都会将返回的值传递给 操作系统),此时如果父进程还在运行,子进程并不会立即被销毁,直到这些值传到了产生该子进程的父进程。也就是说,如果父进程没有主动要求获得子进程的结束状态值,操作系统就会一直保存该进程的相关信息,并让子进程长时间处于僵尸状态,例如下面程序:

int main(){
    pid_t pid = fork();
    if(pid == 0){
        cout << "I am a Child Process." <<endl;
    }else{
        cout << "I am a Father Process and Child Process is " << pid << endl;
        sleep(30);  //让父进程休眠30秒,此时便于观察子进程的状态
    }
    if(pid == 0){
        cout << " Child Process exits " << endl;
    }else{
        cout << "Father Process exits " << endl;
    }
    return 0;
}

此时,运行该程序,查看后台进程可知(test16是该测试程序的名称,defunct表示僵尸进程):

gqx@gqx-Lenovo-Product:~$ ps -au
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root       923  0.6  0.9 480840 159824 tty7    Ssl+ 4月09  36:07 /usr/lib/xorg/
root      1351  0.0  0.0  17676  1768 tty1     Ss+  4月09   0:00 /sbin/agetty -
...
gqx      24856  0.0  0.0      0     0 pts/11   Z+   11:03   0:00 [tes16] <defunct>
gqx      24859  0.0  0.0  39104  3300 pts/3    R+   11:03   0:00 ps -au

僵尸进程的消除

方法一:调用wait()函数:

/* Wait for a child to die.  When one does, put its status in *STAT_LOC
   and return its process ID.  For errors, return (pid_t) -1.
​
   This function is a cancellation point and therefore not marked with
   __THROW.  */
extern __pid_t wait (__WAIT_STATUS __stat_loc);

成功返回终止的进程ID,失败返回-1;子进程的最终返回值将指向该函数参数所指向的内存空间,但函数所指向的内存单元总还含有其他的信息,需要使用宏进行分离。

# define WIFEXITED(status)  __WIFEXITED (__WAIT_INT (status))  //子进程正常终止返回"true"
# define WEXITSTATUS(status)    __WEXITSTATUS (__WAIT_INT (status)) //返回子进程的返回值

要注意的是:如果没有已终止的子进程,那么程序将被阻塞,直到有子进程终止。

方法二:调用waitpid()函数

/* Wait for a child matching PID to die.
   If PID is greater than 0, match any process whose process ID is PID.
   If PID is (pid_t) -1, match any process.
   If PID is (pid_t) 0, match any process with the
   same process group as the current process.
   If PID is less than -1, match any process whose
   process group is the absolute value of PID.
   If the WNOHANG bit is set in OPTIONS, and that child
   is not already dead, return (pid_t) 0.  If successful,
   return PID and store the dead child's status in STAT_LOC.
   Return (pid_t) -1 for errors.  If the WUNTRACED bit is
   set in OPTIONS, return status for stopped children; otherwise don't.
​
   This function is a cancellation point and therefore not marked with
   __THROW.  */
extern __pid_t waitpid (__pid_t __pid, int *__stat_loc, int __options);

第一个参数:如果__pid的值是-1,则与wait()函数相同,可以等待任意的子程序终止;如果是0,则等待进程组识别码与目前进程相同的任何子进程;如果pid>0,则等待任何子进程识别码为 pid 的子进程。

第二个参数:与前一个函数wait()的参数意义相同。

第三个参数:常用WNOHANG——若pid指定的子进程没有结束,则waitpid()函数返回0,不予以等待。若结束,则返回该子进程的ID。

示例程序如下:

#include <iostream>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <sys/wait.h>
using namespace std;
​
void read_childproc(int sig){
    int status;
    pid_t id = waitpid(-1, &status, WNOHANG);
    if(WIFEXITED(status)){
        printf("Remove proc id: %d \n", id);
        printf("Child send: %d \n", WEXITSTATUS(status));
    }
}
​
int main(){
    pid_t pid;
    struct sigaction act;
    act.sa_handler = read_childproc;
    sigemptyset(&act.sa_mask);
    act.sa_flags = 0;
    sigaction(SIGCHLD, &act, 0);
​
    pid = fork();
​
    if(pid == 0){
        puts("Hi, I am a child process!");
        sleep(6);
        return 12;
    }else{
        printf("Child proc id: %d \n", pid);
        pid = fork();
        if(pid == 0){
            puts("Hi, I am a child process!");
            sleep(13);
            exit(24);
        }else{
            int i;
            printf("Child proc id: %d \n", pid);
            for(i  = 0; i < 4; i++){
                puts("wait...");
                sleep(5);
            }
        }
    }
    return 0;
}

  

 

06-29 14:00