linux中wait与waitpid

在linux中利用fork创建子进程，当子进程结束的时候，会产生僵尸进程，该僵尸进程会有保留一些内核的数据结构，供父进程查看子进程的状态；因此就有了wait与waitpid

用于父进程等待子进程，当子进程结束的时候，父进程才会被使用，相当于父进程被阻塞了；，同时其status用于父进程查询子进程当前是什么状态

wait获取staus后检测处理
宏定义  描述
WIFEXITED(status) 如果进程子进程正常结束，返回一个非零值
    WEXITSTATUS(status) 如果WIFEXITED非零，返回子进程退出码
WIFSIGNALED(status) 子进程因为捕获信号而终止，返回非零值
    WTERMSIG(status) 如果WIFSIGNALED非零，返回信号代码
WIFSTOPPED(status) 如果进程被暂停，返回一个非零值
    WSTOPSIG(status) 如果WIFSTOPPED非零，返回信号代码

用于等待指定的子进程，当子进程结束的时候，父进程才会被调用

• pid > 0; 不用管其他子进程，只要该pid还没有结束，父进程就不会结束；
• pid = -1; 等待任何一个子进程，此时waitpid和wait的作用一模一样
• 等待同一个进程组中的任何子进程，如果子进程已经加入了别的进程组，waitpid不会对它做任何理睬。
• pid<-1时，等待一个指定进程组中的任何子进程，这个进程组的ID等于pid的绝对值。
  options允许改变waitpid的行为，最有用的一个选项是WNOHANG，它的作用是防止waitpid把调用者的执行挂起
  返回值：成功返回等待子进程的pid，失败返回-1

#include 
#include 
#include 
#include 
#include 
using namespace std;
int main()
{
    int i = 0; 
    cout<<"before fork"< 
运行结果， 等待父进程结束才运行
 
before fork
after fork
after fork
fork sucess, this is son process
this is son process, i = 1
this is son process, i = 2
this is son process, i = 3
this is son process, i = 4
this is son process, i = 5
this is son process, i = 6
this is son process, i = 7
this is son process, i = 8
this is son process, i = 9
this is son process, i = 10
process destroy
fork sucess, this is father process, son process pid=5170
this is father process i=2
this is father process i=4
this is father process i=6
this is father process i=8
this is father process i=10
i:0flag:0p:5170
process destroy
 
waitpid 
先举个例子，当创建多个进程的时候，多个子进程与主进程同时运行
 
#include 
#include 
#include 
#include 
#include 
#include 
using namespace std;
pid_t create_fork()
{
    int i = 0;
    pid_t pid = fork();
    if(pid == 0) // 此时为子进程的内容
    {
        cout<<"fork sucess, this is son process, pid="< 
运行内容如下：
 
fork sucess, this is son process, pid=6662
this is son process, i = 1 pid = 6662
fork sucess, this is son process, pid=6663
this is son process, i = 1 pid = 6663
this is father process i=2
fork sucess, this is son process, pid=6664
this is son process, i = 1 pid = 6664
fork sucess, this is son process, pid=6665
this is son process, i = 1 pid = 6665
this is son process, i = 2 pid = 6662
this is son process, i = 2 pid = 6663
this is son process, i = 2 pid = 6664
this is son process, i = 2 pid = 6665
this is father process i=4
this is father process i=6
this is father process i=8
this is father process i=10
process destroy
 
此时没有使用waitpid，接下来修改相应的内容，当指定的子进程结束之后主进程才会运行。
 
修改每一个进程的睡眠时间，该内容让主进程保持第一个进程的pid，只有当第一个进程结束之后，主进程才会可以运行；
 
#include 
#include 
#include 
#include 
#include 
#include 
using namespace std;
pid_t create_fork(int time)
{
    int i = 0;
    pid_t pid = fork();
    if(pid == 0) // 此时为子进程的内容
    {
        cout<<"fork sucess, this is son process, pid="< 
结果如下，可以发现第一个经常destroy之后，主进程就开始了运行；
 
fork sucess, this is son process, pid=6769
this is son process, i = 1 pid = 6769
this is son process, i = 2 pid = 6769
fork destroy pid = 6769
fork sucess, this is son process, pid=6770
this is son process, i = 1 pid = 6770
fork sucess, this is son process, pid=6771
this is son process, i = 1 pid = 6771
this is father process i=2
fork sucess, this is son process, pid=6772
this is son process, i = 1 pid = 6772
fork sucess, this is son process, pid=6773
this is son process, i = 1 pid = 6773
this is son process, i = 2 pid = 6770
fork destroy pid = 6770
this is son process, i = 2 pid = 6771
this is father process i=4
this is son process, i = 2 pid = 6772
fork destroy pid = 6771
this is son process, i = 2 pid = 6773
this is father process i=6
fork destroy pid = 6772
this is father process i=8
fork destroy pid = 6773
this is father process i=10
process destroy
 
接下来修改代码，也就是说当最后一个进程执行完之后，主进程才会执行；
 
if(i == 4 ) // 将i 该为4
{
    p = temp;
}
 
运行结果如下
 
fork sucess, this is son process, pid=6823
this is son process, i = 1 pid = 6823
this is son process, i = 2 pid = 6823
fork destroy pid = 6823
fork sucess, this is son process, pid=6824
this is son process, i = 1 pid = 6824
fork sucess, this is son process, pid=6825
this is son process, i = 1 pid = 6825
fork sucess, this is son process, pid=6826
this is son process, i = 1 pid = 6826
fork sucess, this is son process, pid=6827
this is son process, i = 1 pid = 6827
this is son process, i = 2 pid = 6824
this is son process, i = 2 pid = 6825
fork destroy pid = 6824
this is son process, i = 2 pid = 6826
fork destroy pid = 6825
this is son process, i = 2 pid = 6827
fork destroy pid = 6826
fork destroy pid = 6827
this is father process i=2
this is father process i=4
this is father process i=6
this is father process i=8
this is father process i=10
process destroy
 
其中参数的使用 
int temp1 = WIFEXITED(flag);
cout<<"WIFEXITED: 如果子进程正确结束返回一个非0值"< 
可以加长子进程的工作时间，利用正常结束进程或者是
  kill -9 pid
 杀死相应的子进程来查看状态