Greetings everyone, I need a bit of help in solving the following problem:
I'm given an array of numbers and I have to compute the sum of the array elements using n processes, and the inter process communication has to be done with pipes(one pipe, to be exact).
I managed to solve the problem by waiting for a random child to finish, reading the sum sent by the child through the pipe, and so on for the following children, and incrementing the final sum.
Now I have to solve it by reading the partial sums in one fell swoop, after all the children have finished, and I'm not sure as to how to save the entire information sent by the kids. I tried to use a for statement, but it failed, since all the information would be passed in one single element of the array, and offering only the address of the first element in the array failed as well.
Any input would be much appreciated.
Here is the code that's bugging me:
Code:
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
void main () {
int size, fd [2];
printf ("enter the size of the number array: ");
scanf ("%d", &size);
int i, arr [size];
for(i=0; i < size; i++)
arr [i] = i + 1;
int nr_proc;
printf ("enter the number of processes that you want to use: ");
scanf ("%d", &nr_proc);
pid_t pid [nr_proc];
for (i = 0; i < nr_proc; i++) {
pipe (fd);
pid [i] = fork ();
if (pid [i] < 0)
printf ("error in fork.\n");
if (pid [i] == 0) {
close (fd [0]);
int j = 0, sum = 0;
for (j = i * size/nr_proc; j < (i+1) * size/nr_proc; j++) {
sum += arr[j];
}
printf ("test %d\n", sum);
write(fd [1], &sum, sizeof (int));
exit (1);
}
}
close (fd [1]);
for (i = 0; i < nr_proc; i++)
wait ();
int aux[nr_proc], test = 0;
read (fd [0], aux, sizeof(aux)); // this is where i need help
printf ("test aux %d\n", aux [1]);
int sum = 0;
for (i = 0; i < nr_proc; i++)
sum += aux [i];
printf ("result = %d\n", sum);
}
Sorry about that, I'm not a native english speaker. I'll try to give an example:
suppose we have this array:
arr[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
I want to compute the sum of it's elements using 2 child processes, to do so we'll split the array in two equal(or almost equal, if the number of elements is an odd number), and the first child will compute:
(1+2+3+4+5 = 15)
while the second will do:
(6+7+8+9+10 = 40).
Now, each child will send their computed sum to the parent through the pipe fd(2). The problem I'm having is that I don't know how to save both sums at the same time, that is, if I try to read the sums after all kids have finished, for example
int buffer[2];
read (fd (0), buffer, sizeof (buffer));
will first assign 15 to buffer [0], and then he'll assign 40 to buffer [0] , and i want it to assign 15 to buffer [0] and 40 to buffer [1]
Sorry about that, I'm not a native english speaker. I'll try to give an example:
suppose we have this array:
arr[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
I want to compute the sum of it's elements using 2 child processes, to do so we'll split the array in two equal(or almost equal, if the number of elements is an odd number), and the first child will compute:
(1+2+3+4+5 = 15)
while the second will do:
(6+7+8+9+10 = 40).
Now, each child will send their computed sum to the parent through the pipe fd(2). The problem I'm having is that I don't know how to save both sums at the same time, that is, if I try to read the sums after all kids have finished, for example
int buffer[2];
read (fd (0), buffer, sizeof (buffer));
will first assign 15 to buffer [0], and then he'll assign 40 to buffer [0] , and i want it to assign 15 to buffer [0] and 40 to buffer [1]
Then how about creating 2 pipes...this way one child's output is read into buffer[0] and the other child's output is read into buffer[1].
Now, each child will send their computed sum to the parent through the pipe fd(2). The problem I'm having is that I don't know how to save both sums at the same time, that is, if I try to read the sums after all kids have finished, for example
int buffer[2];
read (fd (0), buffer, sizeof (buffer));
will first assign 15 to buffer [0], and then he'll assign 40 to buffer [0] , and i want it to assign 15 to buffer [0] and 40 to buffer [1]
That doesn't happen, the child doesn't magically cause read() to rewind and overwrite array index 0. They don't control that.
Check how many bytes you actually read. You may have to read more than once.
You're not guaranteed to get them in order, either.
You're not even guaranteed to not get one number written inside another, unless you do advisory locking on the pipe in your children.
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11-07-2011
. I have to figure out a mechanism that allows me to read everything in the pipe at once, and store it, been googling for pipe I/O for a few hours now to no avail 
.