fork和僵尸进程
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；
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#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
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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：
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#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
", id); printf("Child send: %d
", 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
", 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
", pid); for(i = 0; i < 4; i++){ puts("wait..."); sleep(5); } } } return 0;} |