Theory,Processes fork() - c

Question

Goodmorning, i would like to ask 2 things..
1) what returns a fork() did on a child which has already a pid==0 ? if i continue to fork on every son, each of them will have 0 as pid ?? or not ?
2) this is my file Buffer.c and it runs on a single process.
At the beginning it forks() out some Producers who produce() and some Consumers who consume() ,but I am afraid that every producers enters in the next for cicle and it starts to produce himself other consumers!! because it write pid=-1 so...
I want that this piece of code produce only P producers and C consumers, but i need to know why every producer do not create other consumers!
Can you help me,maybe giving me a scheme of how many processes i will create with this code?
Maybe doing a scheme as this:
Father:
8 producers
-
-
-
...
each of them produces: 5 consumers
etc etc......
int main(int argc, char **argv) {
/....
pid_t pid;
pid_t cons_pid[C];
/* fork producers */
pid = -1;
for(i=0; i<P && pid!=0; i++)
pid=fork();
switch(pid) {
case -1:
...
case 0:
/* GENERIC PRODUCER i */
...
/* PRODUCE() */
printf("Producer %d exits\n",i);
...
return 0;
}
/* fork consumers */
pid = -1;
for (j=0; j<C && pid!=0; j++)
pid = cons_pid[j] = fork();
switch(pid) {
case -1:
....error
case 0:
/* GENERIC CONSUMER j */
CONSUME()....
}
return 0;
}

Answer 1

what returns a fork() did on a child which has already a pid==0
0 is not a valid PID, hence by definition there can't be an process with PID=0 and thus PID=0 is a perfectly well defined return for indicating child status.
if i continue to fork on every son, each of them will have 0 as pid
No process ever has PID=0. All PIDs are greater than zero! A zero is just the return value received by the newly forked process to indicate that it's the child. The actual PID a child process got is queried using the getpid function from the child process. However the parent process can't perform such a query, since in the time between fork and a assumed query function call, the child may already have terminated (race condition). So you want fork to return the PID to the parent directly.
BTW: The terminology is parent and child not father and son (processes are things not people, despite what the TRON movies depict).
Regarding your code snippet: A switch statement is the wrong choice here. You want to use an if statement.

Answer 2

fork() splits up the current process into a father and a child. The child will have a new PID, the father retains the old PID. In both processes fork() returns after the splitting. In the father the return value will be the PID of the child (to make it known), and in the child the return value will be 0.

Answer 3

1) The lowest possible process ID is 1, this is the ID of the init process from which all other processes are forked. Therefore, it is not possible for a child or for your parent process to "already have ID 0". Your child's process ID is necessarily greater than 1. Thus, the problem that you are afraid of cannot happen.
2) The confusion that you state is the reason why fork (which returns twice, once for the parent and once for the newly created child!) has a somewhat "weird" return value which can have so many different values:
it can be -1, then something went wrong, and no child was created.
it can be a positive value, then you are in the parent process, and the value is the child's process ID. It's as if you called any other "normal" function that just returned normally.
it can be 0, then your code knows it is now running in the child process.
You must examine the return value (if()) so you know what process you are in. Then no such thing as you decribe can happen (or, should happen, this presumes your code does not have any bugs).
EDIT:
The code can be rewritten slightly so it gets rid of the && pid!=0 inside the loop and thus looks a bit less scary overall:
int main()
{
int pid, i;
pid_t cons_pid[C];
for(int i=0; i<P; ++i)
{
pid=fork();
if(pid == -1) exit(1); /* fork error */
if(pid == 0) { producer(); return 0; }
}
for(i=0; i<C; ++i)
{
pid = fork();
if(pid == -1) /* fork error */
{ /* should do a kill_producers(); here */ exit(2); }
else if(pid == 0) /* consumer */
{ consumer(); return 0; }
else /* master process, remember all consumer pids */
{ cons_pid[j] = pid; }
}
/* ... */
return 0;
}