孤兒進(jìn)程與僵尸進(jìn)程

孤兒進(jìn)程:

? ?如果父進(jìn)程先退出，子進(jìn)程還沒(méi)退出那么子進(jìn)程的父進(jìn)程將變?yōu)閕nit進(jìn)程。（注：任何一個(gè)進(jìn)程都必須有父進(jìn)程）

//生成孤兒進(jìn)程 int main(int argc, char *argv[]) {pid_t pid = fork();if (pid < 0)err_exit("fork error");else if (pid > 0)exit(0);else{sleep(10);cout << "Child, ppid = " << getppid() << endl;}exit(0); }

僵尸進(jìn)程:

? ?如果子進(jìn)程先退出，父進(jìn)程還沒(méi)退出，那么子進(jìn)程必須等到父進(jìn)程捕獲到了子進(jìn)程的退出狀態(tài)才真正結(jié)束，否則這個(gè)時(shí)候子進(jìn)程就成為僵尸進(jìn)程。

//生成僵尸進(jìn)程 int main(int argc, char *argv[]) {pid_t pid = fork();if (pid < 0)err_exit("fork error");else if (pid == 0)exit(0);else{sleep(50);}exit(0); }

附-查詢父子進(jìn)程狀態(tài)

? ps?-le?|?grep?main

?

避免僵尸進(jìn)程

? ?signal(SIGCHLD,?SIG_IGN);

//示例: 避免僵尸進(jìn)程 int main(int argc, char *argv[]) {signal(SIGCHLD, SIG_IGN);pid_t pid = fork();if (pid < 0)err_exit("fork error");else if (pid == 0)exit(0);else{sleep(50);}exit(0); }

文件共享

? ?父進(jìn)程的所有文件描述符都被復(fù)制到子進(jìn)程中,?就好像調(diào)用了dup函數(shù),?父進(jìn)程和子進(jìn)程每個(gè)相同的打開文件描述符共享一個(gè)文件表項(xiàng)(因此,?父子進(jìn)程共享同一個(gè)文件偏移量);

//根據(jù)上圖: 理解下面這段程序和下圖的演示 int main(int argc, char *argv[]) {signal(SIGCHLD, SIG_IGN);int fd = open("test.txt", O_WRONLY|O_CREAT|O_TRUNC, 0666);if (fd == -1)err_exit("file open error");cout << "We Don`t flash memory\n";char buf[BUFSIZ];bzero(buf, sizeof(buf));pid_t pid = fork();if (pid < 0)err_exit("fork error");else if (pid > 0){strcpy(buf, "Parent...");write(fd, buf, strlen(buf));close(fd);cout << "fd = " << fd << endl;exit(0);}else if (pid == 0){strcpy(buf, "Child...");write(fd, buf, strlen(buf));close(fd);cout << "fd = " << fd << endl;exit(0);} }

fork?VS?vfork

? 在UNIX/Linux中的fork還沒(méi)實(shí)現(xiàn)copy?on?write(寫時(shí)復(fù)制)技術(shù)之前。Unix設(shè)計(jì)者很關(guān)心fork之后立刻執(zhí)行exec所造成的地址空間浪費(fèi)，所以引入了vfork系統(tǒng)調(diào)用。其中,vfork子進(jìn)程與父進(jìn)程共享數(shù)據(jù)段,并不真正復(fù)制父進(jìn)程內(nèi)存,因此在vfork之后執(zhí)行exec系列函數(shù),并不會(huì)導(dǎo)致地址空間浪費(fèi)以及無(wú)用的空間復(fù)制時(shí)間.而且,即使fork實(shí)現(xiàn)了copy?on?write，效率也沒(méi)有vfork高.

? 但是,vfork有個(gè)限制，子進(jìn)程必須立刻執(zhí)行_exit或者exec系列函數(shù)。因此我們不推薦使用vfork，因?yàn)閹缀趺恳粋€(gè)vfork的實(shí)現(xiàn)，都或多或少存在一定的問(wèn)題(可以嘗試在vfork之后的子進(jìn)程中既不執(zhí)行_exit,也不執(zhí)行exec函數(shù))。

?

fork與vfork的區(qū)別

1. fork子進(jìn)程拷貝父進(jìn)程的數(shù)據(jù)段(但是現(xiàn)在提供了寫時(shí)復(fù)制技術(shù),只有當(dāng)子進(jìn)程真正需要寫內(nèi)存時(shí),才復(fù)制出該內(nèi)存的一段副本),因此,在父進(jìn)程/子進(jìn)程中對(duì)全局變量所做的修改并不會(huì)影響子進(jìn)程/父進(jìn)程的數(shù)據(jù)內(nèi)容.

????vfork子進(jìn)程與父進(jìn)程共享數(shù)據(jù)段,因此父子進(jìn)程對(duì)數(shù)據(jù)的更新是同步的；

2. fork父、子進(jìn)程的執(zhí)行次序是未知的,取決于操作系統(tǒng)的調(diào)度算法

? ? vfork：子進(jìn)程先運(yùn)行，父進(jìn)程后運(yùn)行；

//示例1:vfork出錯(cuò)情況 //在Linux 2.6內(nèi)核上會(huì)持續(xù)執(zhí)行,不會(huì)退出 //而在Linux 3.13內(nèi)核上, 則會(huì)引發(fā)core dump int main() {int iNumber = 0;pid_t pid = vfork();if (pid == -1){perror("fork");return -1;}else if (pid > 0){cout << "In Parent Program..." << endl;cout << "iNumber = " << iNumber << endl;cout << "pid = " << static_cast<int>(getpid());cout << "\t ppid = " << static_cast<int>(getppid()) << endl;//_exit(0);}else if (pid == 0){iNumber ++;cout << "In Child Program..." << endl;cout << "iNumber = " << iNumber << endl;cout << "pid = " << static_cast<int>(getpid());cout << "\t ppid = " << static_cast<int>(getppid()) << endl;//_exit(0);}return 0; }

//示例2: 父進(jìn)程/子進(jìn)程修改全局?jǐn)?shù)據(jù)的情況 int main() {int iNumber = 10;cout << "Before vfork, pid = " << getpid() << endl;//對(duì)比f(wàn)ork()pid_t pid = vfork();if (pid == -1)err_exit("fork");else if (pid > 0){sleep(4);cout << "Parent, iNumber: " << iNumber << endl;}else if (pid == 0){++ iNumber;cout << "Child, iNumber = " << iNumber << endl;_exit(0);}return 0; } //示例3:用vfork執(zhí)行當(dāng)前目錄下的hello程序 int main() {int iNumber = 0;pid_t pid = vfork();if (pid == -1){perror("fork");return -1;}else if (pid > 0){cout << "In Parent Program..." << endl;cout << "iNumber = " << iNumber << endl;cout << "pid = " << static_cast<int>(getpid());cout << "\t ppid = " << static_cast<int>(getppid()) << endl;}else if (pid == 0){iNumber ++;cout << "In Child Program..." << endl;cout << "iNumber = " << iNumber << endl;cout << "pid = " << static_cast<int>(getpid());cout << "\t ppid = " << static_cast<int>(getppid()) << endl;//將自己寫的程序啟動(dòng)起來(lái)execve("./hello",NULL,NULL);_exit(0);}return 0; }

//測(cè)試4,用vfork執(zhí)行系統(tǒng)命令 int main() {int iNumber = 0;pid_t pid = vfork();if (pid == -1){perror("fork");return -1;}else if (pid > 0){cout << "In Parent Program..." << endl;cout << "iNumber = " << iNumber << endl;cout << "pid = " << static_cast<int>(getpid());cout << "\t ppid = " << static_cast<int>(getppid()) << endl;}else if (pid == 0){iNumber ++;cout << "In Child Program..." << endl;cout << "iNumber = " << iNumber << endl;cout << "pid = " << static_cast<int>(getpid());cout << "\t ppid = " << static_cast<int>(getppid()) << endl;//將ls命令啟動(dòng)起來(lái),注意:由于C++嚴(yán)格的類型轉(zhuǎn)換機(jī)制,需要在字符串前加(char*)char *const args[] = {(char *)"/bin/ls", (char *)"-l", NULL};int res = execve("/bin/ls",args,NULL);if (res == -1){perror("execve");_exit(1);}_exit(0);}return 0; }

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