CODEWITHSUNDEEP
cpipe
jkf
jkf

Reputation: 423

How does a process know when to read from pipe

A parent process writes integers from an array sequentially into a pipe.

...
close(thePipe[0]);
int array[]={1, 2, 5, 5, 5};
int j;

for(j=0; j<sizeof(array)/sizeof(int); j++){
  write(thePipe[1], &(array[j]), sizeof(int));
}
close(thePipe[1];
...

Its child process reads those integers and sums them up.

...
close(thePipe[1]);
int sum = 0;
int buffer;
while( 0 != read(thePipe[0], &buffer, sizeof(buffer)) ){
  sum = sum + buffer;
}
close(thePipe[0]);
...

How does the child know when to read from the pipe?

Even if the child gets a lot more CPU time it still will not read before the parent hasn't written to the pipe. How does that work?

Upvotes: 0

Views: 340

Answers (2)

Pankaj Andhale
Pankaj Andhale

Reputation: 403

Since there is nothing to read form pipe the child process will wait (blocked) until parent write something into the pipe.

Upvotes: 4

Thilo
Thilo

Reputation: 262814

The OS takes care of this. When you read from a pipe, execution will block until there is data available. Your program won't use CPU time while waiting that way.

Upvotes: 4

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