why pid of child and its process are different? - c++

Question

Here my code:
extern int errno;
pid_t system1(const char * command)
{
pid_t pid;
pid = fork();
cout<<"PID in child "<<(int)pid<<endl;
if (pid < 0) {
return pid;
} else if (pid == 0) {
execl("/bin/sh", "sh", "-c", command, (char*)NULL);
_exit(1);
}
int stat_val;
pid_t child_pid;
cout << "Hello1" << endl;
child_pid = wait(&stat_val);
cout << "child_pid = " <<(int)child_pid<< endl;//LINE 1
if(WIFEXITED(stat_val))
printf("Child has terminated with exit code %d\n", WIFEXITED(stat_val));
else
printf("Child has existed abnormally\n");
return child_pid;
}
int main( )
{
int pid_1 = system1("setup.csh &");;
struct stat status;
sleep(10);
cout<<"errno = "<<errno<<endl;
int i = kill(pid_1,0);
cout<<"Pid id = "<<pid_1<<endl;
cout<<"i === "<<i<<endl;
cout<<"errno = "<<errno<<endl;
if(errno == ESRCH)
{
cout<<"process does not exist";
}
else
{
cout<<"process exist"<<endl;
}
return 0;
}
In above code I am getting different PID of child at LINE 1 and PID of process setup.csh . Can anybody please help me out. I want to get PID of my process setup.csh.
I am looking other PID value using ps -u user | grep setup.csh in console.

Answer 1

When you run:
sh -c 'setup.csh &'
the original shell process forks another child process to run csh. The process hierarchy is:
YourProgram
sh -c 'setup.csh &'
csh setup.csh
There's no way to get this PID directly in the original program. Why don't you run setup.csh directly from your program, instead of going through the shell?
Actually, there's a way you can accomplish this. If you use the exec shell command, it runs the specified command in its own process, rather than forking a child:
system1('exec setup.csh &');