一、多进程创建和回收
- 孤儿进程：父进程先退出了，子进程没有退出，成为孤儿进程，父进程变成1号进程。
- 僵尸进程：父进程没有退出，子进程退出了，但是父进程没有回收子进程资源，导致子进程变成僵尸进程。
1. fork()
创建子进程函数，一个进程可以创建多个子进程。
pid_t fork(void)
返回值小于0 创建失败等于0 是子进程大于0 是父进程示例：
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>int main()
{pid_t ret = fork();if (ret < 0){perror("fork error");return -1;}else if (ret > 0){printf("here is father,pid is [%d], child pid is [%d]\n", getpid(), ret);sleep(1);}else if (ret == 0){printf("here is child pid is [%d],father is [%d]\n", getpid(), getppid());}return 0;
}2. wait()
父进程回收子进程，其中分为wait()和waitpid()函数
pid_t wait(int *status);
返回值大于0 返回值为回收的进程id返回-1 回收失败参数 status 进程回收状态可以使用下面的宏来说明当前的回收状态WIFEXITED(wstatus) 如果正常返回，返回trueWEXITSTATUS(wstatus) 用于输出正常返回的状态 用 %d 格式化输出WIFSIGNALED(wstatus) 如果被信号杀死，返回trueWTERMSIG(wstatus) 用于输出被哪个信号杀死 用 %d 格式化输出WIFSTOPPED(wstatus) 如果子进程停止了，返回trueWSTOPSIG(wstatus) 用于输出进程停止是由于哪个信号WIFCONTINUED(wstatus) 如果进程被信号SIGCONT重启，返回trueWCOREDUMP(wstatus) 如果子进程发生核心转储，返回true示例：
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>// pid_t wait(int *status);int main()
{pid_t ret = fork();if (ret < 0){perror("fork error");return -1;}else if (ret > 0){int waitstatus;printf("here is father,pid is [%d], child pid is [%d]\n", getpid(), ret);pid_t waitid = wait(&waitstatus);if (waitid == -1){perror("wait error");return -1;}else if (waitid > 0){printf("child pid [%d] is over\n", waitid);}if (WIFEXITED(waitstatus)){printf("exited, status=%d\n", WEXITSTATUS(waitstatus));}else if (WIFSIGNALED(waitstatus)){printf("killed by signal %d\n", WTERMSIG(waitstatus));}else if (WIFSTOPPED(waitstatus)){printf("stopped by signal %d\n", WSTOPSIG(waitstatus));}else if (WIFCONTINUED(waitstatus)){printf("continued\n");}}else if (ret == 0){printf("here is child pid is [%d],father is [%d]\n", getpid(), getppid());}return 0;
}//输出
here is father,pid is [24290], child pid is [24291]
here is child pid is [24291],father is [24290]
child pid [24291] is over
exited, status=03. waitpid()
waitpid中的第三个参数可以让回收进程变成非阻塞的。
pid_t waitpid(pid_t pid, int *status, int options);
返回值大于0 成功，返回的是回收的进程id等于0 返回0是因为第三个参数设置为了 WNOHANG ，等待是非阻塞的，并且这个时候没有子进程需要被回收小于0 回收失败参数 pid 进程id小于-1 :等待该数字绝对值的所在组所有子进程，例如-21328，则等待21328组内所有子进程-1 :等待所有子进程0 :等待组ID等于调用进程的组ID的子进程大于0 :等待某个子进程
参数 status 进程回收状态同wait函数的
参数 options 回收选项0 :不添加选项WNOHANG :如果没有子进程需要被回收，就立即返回WUNTRACED :如果一个子进程被停止了，就返回WCONTINUED :如果一个子进程被SIGCONT从停止转变为运行，就返回示例：
创建多个子进程并回收它们
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>// pid_t waitpid(pid_t pid, int *status, int options);int main()
{int i = 0;for (i = 0; i < 3; i++){pid_t ret = fork();if (ret < 0){perror("fork error");return -1;}else if (ret > 0){printf("here is father,pid is [%d], child pid is [%d]\n", getpid(), ret);// pid_t waitid = wait(&waitstatus);}else if (ret == 0){sleep(3);printf("here is child [%d] pid is [%d],father is [%d]\n", i, getpid(), getppid());break;}}if (i == 3){int waitstatus;int num = 0;do{pid_t waitid = waitpid(-1, &waitstatus, WNOHANG);if (waitid == -1){perror("wait error");return -1;}else if (waitid > 0){printf("child pid [%d] is over\n", waitid);num++;}else if (waitid == 0){// 没有进程被回收，可以做其他事情sleep(1);continue;}if (WIFEXITED(waitstatus)){printf("[%d] exited, status=%d\n", waitid, WEXITSTATUS(waitstatus));}else if (WIFSIGNALED(waitstatus)){printf("[%d] killed by signal %d\n", waitid, WTERMSIG(waitstatus));}else if (WIFSTOPPED(waitstatus)){printf("[%d] stopped by signal %d\n", waitid, WSTOPSIG(waitstatus));}else if (WIFCONTINUED(waitstatus)){printf("[%d] continued\n", waitid);}if (num == i){break;}} while (1);}return 0;
}
//输出
here is father,pid is [24553], child pid is [24554]
here is father,pid is [24553], child pid is [24555]
here is father,pid is [24553], child pid is [24556]
here is child [0] pid is [24554],father is [24553]
here is child [2] pid is [24556],father is [24553]
here is child [1] pid is [24555],father is [24553]
child pid [24554] is over
[24554] exited, status=0
child pid [24555] is over
[24555] exited, status=0
child pid [24556] is over
[24556] exited, status=0
