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进程组：一个或多个进程的集合，进程组id是一个正整数。
组长进程：进程组id == 进程id
组长进程可以创建一个进程组，创建该进程组的进程，终止了，只要进程组有一个进程存在，进程组就存在，与组长进程是否终止无关。
进程组生存期：进程组创建到最后一个进程离开（终止或转移到另一个进程组）
一个进程可以为自己或子进程设置进程组id
作用：
子进程退出时，不管父子进程同不同一个进程组，都会发SIGCHLD信号给父进程
当父子进程同进程组时，父进程应捕捉SIGCHLD信号，对子进程资源进程回收，防止僵尸进程的产生
当父子进程不同进程组时，比如设置子进程成为了一个新的进程组，这时候子进程退出，系统也会正常回收子进程的资源，不会产生僵尸进程的
相关函数
#include <unistd.h>
int setpgid(pid_t pid, pid_t pgid); //pid = 0, pgid = 0,让当前进程成为一个进程组，并且进程组id为当前进程的pid
pid_t getpgid(pid_t pid); //pid = 0,获取当前进程的进程组id
pid_t getpgrp(void); /* POSIX.1 version */
pid_t getpgrp(pid_t pid); /* BSD version */
int setpgrp(void); /* System V version */
int setpgrp(pid_t pid, pid_t pgid); /* BSD version */
demo
父子进程同一个进程组
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>void do_sigchld(int signo)
{pid_t pid;int status;printf("signo = %d\n", signo);while((pid = waitpid(0, &status, WNOHANG)) > 0) // 0:跟调用进程同组的子进程，WNOHANG：不阻塞，立即返回{if (WIFEXITED(status))printf("child %d exit status %d\n", pid, WEXITSTATUS(status));else if (WIFSIGNALED(status))printf("chid %d exit by signal %d\n", pid, WTERMSIG(status));}
}int main(int argc, char *argv[])
{pid_t pid;//阻塞SIGCHLD信号sigset_t set;sigemptyset(&set);sigaddset(&set, SIGCHLD);sigprocmask(SIG_BLOCK, &set, NULL);pid = fork();if (pid == 0){//in child//解除阻塞SIGCHLD信号sigprocmask(SIG_UNBLOCK, &set, NULL);//进程组printf("child pid = %d, process group id getpgid(0) = %d\n", getpid(), getpgid(0));printf("child pid = %d, process group id getpgid(getpid()) = %d\n", getpid(), getpgid(getpid()));printf("child pid = %d, process group id getpgrp() = %d\n", getpid(), getpgrp());/*setpgid(0, 0); //pid = 0, pgid = 0,让当前进程成为一个进程组，并且进程组id为当前进程的pidprintf("after setpgid(0, 0) child pid = %d, process group id getpgid(0) = %d\n", getpid(), getpgid(0));printf("after setpgid(0, 0) child pid = %d, process group id getpgid(getpid()) = %d\n", getpid(), getpgid(getpid()));printf("after setpgid(0, 0) child pid = %d, process group id getpgrp() = %d\n", getpid(), getpgrp());*/}else if (pid > 0){//in parent//先捕捉SIGCHLD信号struct sigaction act;act.sa_handler = do_sigchld;sigemptyset(&act.sa_mask);act.sa_flags = 0; //0:用sa_handler参数，SA_SIGINFO:用sa_sigaction参数sigaction(SIGCHLD, &act, NULL);//再解除阻塞SIGCHLD信号sigprocmask(SIG_UNBLOCK, &set, NULL);//进程组printf("parent pid = %d, process group id getpgid(0) = %d\n", getpid(), getpgid(0));printf("parent pid = %d, process group id getpgid(getpid()) = %d\n", getpid(), getpgid(getpid()));printf("parent pid = %d, process group id getpgrp() = %d\n", getpid(), getpgrp());sleep(1); //为了观察子进程退出时，父进程回收子进程资源}else{perror("fork");exit(1);}return 0;
}
子进程成为一个进程组时，上面的代码加入如下代码后
setpgid(0, 0); //pid = 0, pgid = 0,让当前进程成为一个进程组，并且进程组id为当前进程的pid
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>void do_sigchld(int signo)
{pid_t pid;int status;printf("signo = %d\n", signo);while((pid = waitpid(0, &status, WNOHANG)) > 0) // 0:跟调用进程同组的子进程，WNOHANG：不阻塞，立即返回{if (WIFEXITED(status))printf("child %d exit status %d\n", pid, WEXITSTATUS(status));else if (WIFSIGNALED(status))printf("chid %d exit by signal %d\n", pid, WTERMSIG(status));}
}int main(int argc, char *argv[])
{pid_t pid;//阻塞SIGCHLD信号sigset_t set;sigemptyset(&set);sigaddset(&set, SIGCHLD);sigprocmask(SIG_BLOCK, &set, NULL);pid = fork();if (pid == 0){//in child//解除阻塞SIGCHLD信号sigprocmask(SIG_UNBLOCK, &set, NULL);//进程组printf("child pid = %d, process group id getpgid(0) = %d\n", getpid(), getpgid(0));printf("child pid = %d, process group id getpgid(getpid()) = %d\n", getpid(), getpgid(getpid()));printf("child pid = %d, process group id getpgrp() = %d\n", getpid(), getpgrp());setpgid(0, 0); //pid = 0, pgid = 0,让当前进程成为一个进程组，并且进程组id为当前进程的pidprintf("after setpgid(0, 0) child pid = %d, process group id getpgid(0) = %d\n", getpid(), getpgid(0));printf("after setpgid(0, 0) child pid = %d, process group id getpgid(getpid()) = %d\n", getpid(), getpgid(getpid()));printf("after setpgid(0, 0) child pid = %d, process group id getpgrp() = %d\n", getpid(), getpgrp());}else if (pid > 0){//in parent//先捕捉SIGCHLD信号struct sigaction act;act.sa_handler = do_sigchld;sigemptyset(&act.sa_mask);act.sa_flags = 0; //0:用sa_handler参数，SA_SIGINFO:用sa_sigaction参数sigaction(SIGCHLD, &act, NULL);//再解除阻塞SIGCHLD信号sigprocmask(SIG_UNBLOCK, &set, NULL);//进程组printf("parent pid = %d, process group id getpgid(0) = %d\n", getpid(), getpgid(0));printf("parent pid = %d, process group id getpgid(getpid()) = %d\n", getpid(), getpgid(getpid()));printf("parent pid = %d, process group id getpgrp() = %d\n", getpid(), getpgrp());sleep(1); //为了观察子进程退出时，父进程回收子进程资源}else{perror("fork");exit(1);}return 0;
}