How do i use wait ? It just baffles me to no end. I fork a tree of procs with recursion and now the children have to pause(wait/sleep) while I run pstree so I can print the proc tree.
Should i use
int status;
wait(&status);
or rather
wait(NULL)
and where should i put this? in the parent if(pid > 0) or in the children if(pid==0)? Maybe at the end of ifs, so I store all the pids in array and then run a for over them and use wait?
my code template:
void ProcRec(int index)
{
pid_t pid;
int noChild = getNChild(index);
int i= 0;
for(i = 0; i < noChild; i++)
{
pid = fork();
if (pid > 0)
{
/* parent process */
}
else if (pid == 0)
{
/* child process. */
createProc(index+1);
}
else
{
/* error */
exit(EXIT_FAILURE);
}
}
if(getpid() == root)
{
sleep(1);
pid = fork();
if(pid == 0)
execl("/usr/bin/pstree", "pstree", getppid(), 0);
}
}
1 Answer
The wait system-call puts the process to sleep and waits for a child-process to end. It then fills in the argument with the exit code of the child-process (if the argument is not NULL).
So if in the parent process you have
int status;
if (wait(&status) >= 0)
{
if (WEXITED(status))
{
/* Child process exited normally, through `return` or `exit` */
printf("Child process exited with %d status\n", WEXITSTATUS(status));
}
}
And in the child process you do e.g. exit(1), then the above code will print
Child process exited with 1 status
Also note that it's important to wait for all child processes. Child processes that you don't wait for will be in a so-called zombie state while the parent process is still running, and once the parent process exits the child processes will be orphaned and made children of process 1.
