Hi folks, I am writing a simple program to understand how fork() and waitpid works, but it doesn't seem that is working like I wanted.
Code:
if(fork()==0){
//el hijo
pid1=getpid();
printf("\nSoy el hijo %d",pid1);
}else {
//el padre
if (fork()==0) {
//el hijo
pid2=getpid();
printf("\nSoy el hijo %d",pid2);
}
else {
.... more forks....the father (last else)
else{
// el padre
// espera el hijo 1
waitpid(pid1,status1,0);
hijos--;
printf("\nacaba de finalizar mi hijo con PID %d",pid1);
//espera al hijo 2
waitpid(pid2,status2,0);
hijos--;
printf("\nacaba de finalizar mi hijo con PID %d",pid2);
One of the problems I have encountered is that the printed PID by the father does not correspond to any of the childs (sometimes 0 is printed).
The other one is that the fater doesn't wait for the childs, the message "acaba de finalizar..." will pop out first than " soy el hijo...", and what I want is for the father to wait for the child to finish, and once is finished, print that the child with PID X has finished.
You're not quitting in your child code, so they finish executing the child code then continue blithely on into the parent code. Since you haven't posted your complete code I can't tell what they'd be doing, but it's possible that some of your children may be fork()-ing and ending up printing like the parent should. That would explain some of the strange PID values.
Instead of using a 9-long if/else chain for 9 pid's, use a loop that can handle any number of PID's with the same code:
You should put \n at the end of the line, not the beginning, because printf only actually prints AFTER it receives a \n. printf("\nthis is a string"); will leave stuff in the buffer that might be printed twice (since fork() copies the buffer along with the rest of the process).
I'm not sure what your status1, status2, ...variables are since you didn't post that code, but since you're not using & anywhere, you're probably doing it wrong. You should give waitpid() a pointer to an integer -- not an integer, and not a bare pointer.
Code:
int main(void)
{
int n;
pid_t pids[10];
for(n=0; n<10; n++)
{
pid[n]=fork();
if(pid[n] == 0) // child code
{
printf("My PID is %d\n", getpid());
exit(0); // DON'T FORGET THIS or child will keep executing!
}
}
for(n=0; n<10; n++)
{
int s;
waitpid(pid[n], &s, 0); // It needs a pointer TO an integer -- an address pointing to a valid 'int'.
printf("Child %d has completed\n", pid[n]);
}
return(0);
}
Last edited by Corona688; 12-09-2011 at 04:44 PM..
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--------------
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About this code
for (i = 1; i < n; i++)
if ((childpid = fork()) <= 0)
break;
I really can't understand the output .
and the way fork () return the value .
how about the process Id ,the child process Id and the parent ID
in this case
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12-09-2011
