How do I synchronize father and child processes using pipes? - c++

Question

I believe there's a way to synchronize two processes using pipes, but I'm unsure on how to implement it. In my code both the child and father process execute its code at the same time, I would like for one process to wait for the other. For example, using a pipe to block one process until the other process is done.
My code:
int main()
{
int one[2];
int two[2];
int x=0;
char messageRead[256], messageRead2[256], messageWrite[256], messageWrite2[256];
pipe(one);
pipe(two);
pid_t pid = fork();
if(pid == 0) //child process
{
while (x==0) //loop until condition is met (didn't write the condition yet, just testing)
{
std::cout << "Child process.\n\n";
std::cin >> messageWrite;
write(one[1], messageWrite, 256);
read(two[0], messageRead2, 256 );
}
}
else if(pid>0) //father process
{
while(x==0)
{
std::cout << "Father process.\n\n";
std::cin >> messageWrite;
write(two[1], messageWrite2, 256);
read(one[0], messageRead, 256);
}
}
}
Right now, both processes go back and forth, like I want but they both execute at the same time like this:
Father process.
Child process.
user input here
user input here
Father process.
Child process.
user input here
user input here
Child process.
Father process.
user input here
user input here
etc...

Answer 1

One method that can work rather well is to simply rely on pipes blocking if you try to read and there's nothing in them. With that (configured if necessary), you define one process to own the resource you're protecting and while it continues to own this resource, it uses it as it wishes (and the pipe is empty).
When the resource owner wishes to relinquish the resource, it writes a token into the pipe. This token can be anything that makes sense to you… a simple meaningless piece of data, or something that has meaning.
When either process wishes to own the resource (while it doesn't already), it reads the pipe… this blocks it until something is in there. When the read is successful, that process now owns the resource.
You can do this with non-blocking I/O also, as long as you keep track in each process if you own the resource or not.
The problem in the code you've provided in your question is that both processes write to the other's pipe right away, allowing the other end's read to be successful without any delay.