无法检测子进程是否已被杀死

Question

I been reading a countless time on stackoverflow that to determine if a process is running, one should use kill(pid,0) but I cant make it work

here is my code, , it cannot detect when the child process has been killed and I dont understand why

(sorry for the ident, but hte code escaping on stackoverflow is just a pain for long pieces of codes)

#include <sys/signal.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>

int main()
{
int pid = fork();

if (pid < 0)
{
    printf("Forking failed\n");
} 
else if (pid == 0) // child process
{ 
    do
    {
        printf("child process\n");  
        sleep(1);
    }while(1);
    //execvp("ls", NULL); //launches the application
} 
else //parent process
{ 
    printf("parent process\n");
    for(int i=0;i<10;i++)
    {
        if(i==5)
            kill(pid,SIGTERM); //kills the child process
        if(kill(pid, 0) == 0)
            printf("process child %d is runnig\n",pid);
        else if(kill(pid, 0) == -1)
        {
            switch(errno)
            {
                case ESRCH: printf("process child %d is not runnig\n",pid); break;
                case EPERM: printf("process child %d cannot be killed\n",pid); break;
                default: printf("%s\n",strerror(errno));    break;
            }
        }

        sleep(1); 
    }

} 
}

here is the output

parent process
process child 1918 is runnig
child process
process child 1918 is runnig
child process
process child 1918 is runnig
child process
process child 1918 is runnig
child process
process child 1918 is runnig
child process
process child 1918 is runnig
process child 1918 is runnig
process child 1918 is runnig
process child 1918 is runnig
process child 1918 is runnig

Answer 1

Until the killed child process gets reaped by wait() , its PID is still valid, and kill(pid, 0); will succeed, since the PID exists. Your code attempts to kill() the child process, but fails to wait() for it.

Note that even in the best case, kill(pid, 0); is not reliable. With only 32766 different PIDs, it doesn't take long before process IDs get recycled.

Answer 2

alright I finaly figured it out

kill(pid,SIGTERM); //kills the child process
do
    {
       pid = waitpid(-1, NULL, WNOHANG);
   if(pid == 0)
      sleep(1);
}while(pid<=0);