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; }