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Multiplying 2D arrays using fork()

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# 1  
Old 03-13-2012
Multiplying 2D arrays using fork()
HI,
i am trying to multiply 2 2D arrays (a[2][3],b[3][4]) using fork.
The answer will be at c[2][4].
Each child have to calculate 1 row of c[2][4].
The code is right, as i think of it, with no errors but i dont get the correct c array...
I think there is maybe a mistake in i dimension ...
Anyway, here is the code:
Code:
//multiply a[2][3] * b[3][4] = c[2][4]
//with fork
//each child completes one c[2][4]'s row

#include<stdio.h>
#include<stdlib.h>

main()
{
  int a[2][3]={1,2,3, 
	       4,5,6};
  int b[3][4]={1,2,3,4,
	       5,6,7,8,
	       9,10,11,12};
  int c[2][4];
  int i,j;
  int m;	//for the calculation
  int n;	//common 3
  int pid;	//fork
  
  for(i=0;i<2;i++)
  {
    pid=fork();
    if(pid==-1)
    {
      printf("Can't fork\n");
    }
    if(pid==0)//child
    {
      for(j=0;j<4;j++)
      {
	for(n=0;n<3;n++)
	{
	  m=m+a[i][n]*b[n][j];
	}
	c[i][j]=m;
	m=0;
      }
      exit(EXIT_SUCCESS);
    }
    if(pid>0)//parent
    {
      wait(0);
    }
  }
  printf("C:\n");
  for(i=0;i<2;i++)
  {
    for(j=0;j<4;j++)
    {
      printf("%d\t",c[i][j]);
    }
    printf("\n");
  }	       
}

Any help, please ?
# 2  
Old 03-13-2012
First, move your initialisation of m to the start of the loop:


Code:
for(j=0;j<4;j++)
{    
    m = 0;
    for(n=0;n<3;n++)
    {
        m=m+a[i][n]*b[n][j];
    }   
    c[i][j]=m;
}

That would only affect your [0][0] value though.


The reason you're not seeing the result you expect is that you're not passing back the computed value from the child to the parent. With fork(), the child receives a copy of the process, but any manipulations to the data segment are local to the child. If you memset() your result array to -99 or something, you'll notice that it's still -99 at the end.


To pass back values from the child, have a look at creating a pipe that the child can write to and the parent can read from. See the man(2) page for pipe which will have all the details that you'll need.
This User Gave Thanks to agama For This Post:
giampoul
# 3  
Old 03-13-2012
Are you expecting child processes storing numbers in their copy of array c to update array c in the parent?
# 4  
Old 03-13-2012
Quote:
Originally Posted by Skaperen
Are you expecting child processes storing numbers in their copy of array c to update array c in the parent?
I didnt knew that child processes use a copy of the variables i use. i should re-check the man fork(). Yes i want to update array c in the parent. Looking pipes ...

---------- Post updated at 07:01 PM ---------- Previous update was at 07:00 PM ----------

Quote:
Originally Posted by agama
The reason you're not seeing the result you expect is that you're not passing back the computed value from the child to the parent. With fork(), the child receives a copy of the process, but any manipulations to the data segment are local to the child. If you memset() your result array to -99 or something, you'll notice that it's still -99 at the end.


To pass back values from the child, have a look at creating a pipe that the child can write to and the parent can read from. See the man(2) page for pipe which will have all the details that you'll need.
Initiliazation corrected. Just looking for the use of pipes...

---------- Post updated at 08:40 PM ---------- Previous update was at 07:01 PM ----------

Ok, i read about pipe.
Trying to figure out how it works, i made an easy example.
The program makes a multiply of 2 integers (a and b) and saves the answer to c...
Code:
#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>

main()
{
  int a,b,c;
  int pid;
  int pipefd[2];
  char buff[10];
  char str[100+1]={'\0'};
  int w,r;
  a=3;
  b=10;
  
  pipe(pipefd);
    if(pipefd<0)
      printf("pipe error\n");
    
  pid=fork();
  if(pid==-1)
    printf("Fork error\n");
  if(pid==0)
  {
    //child
    c=a*b;
    printf("child says answer %d\n",c);
    
    sprintf(str,"%d",c);
    w=write(pipefd[1],str,c);
    if(w!=c)
      printf("write error");
    exit(0);
  }
  if(pid>0)
  {
    //parent
    printf("father says answer %d\n",c);
    wait(0);
    
    r=read(pipefd[0], buff, sizeof(buff));
    if(r<=0)
      printf("read error\n");
    c=atoi(buff);
    printf("child told father answer is %d\n",c);
    
  } 
}

It works with no errors or notifications... Smilie
But when i tried with the arrays, troubles appeared again...
Any help?
Based on my first code and adding pipes i have this :
Code:
//multiply a[2][3] * b[3][4] = c[2][4]
//with fork
//each child completes one c[2][4]'s row

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>

main()
{
  int a[2][3]={1,2,3, 
	       4,5,6};
  int b[3][4]={1,2,3,4,
	       5,6,7,8,
	       9,10,11,12};
  int c[2][4];
  int i,j;
  int m[4];	//for the calculation. Array is just an idea, dont know if works
  int n;	//common 3
  int pid;	//fork
  int pipefd[2];
  int w,r;    //for write() and read()
  char buff[100];
  char str[100+1]={'\0'};
   
  for(i=0;i<2;i++)
  {
    pipe(pipefd);
      if(pipefd<0)
	printf("Pipe error\n");
    pid=fork();
    if(pid==-1)
    {
      printf("Can't fork\n");
    }
    if(pid==0)//child
    {
      for(j=0;j<4;j++)
      {
	//m initialize
	for(n=0;n<3;n++)
	{
	  m[j]=m[j]+a[i][n]*b[n][j];
	}
      }
      //what !?
      sprintf(str,"%d",m[j]);
      w=write(pipefd[1],"%d",m);
      if(w!=m)
	printf("write error\n");
      //m=0;
      exit(0);
    }
    if(pid>0)//parent
    {
      wait(0);
      r=read(pipefd[0], buff,sizeof(buff));
      if(r<=0)
	printf("read error\n");
    }
  }
  printf("C:\n");
  for(i=0;i<2;i++)
  {
    for(j=0;j<4;j++)
    {
      printf("%d\t",c[i][j]);
    }
    printf("\n");
  }	       
}

Of course, it doesnt works. Its unfinished. But i want help...
I will update it when i change something...
# 5  
Old 03-14-2012
Only have time for a quick look, but this is what jumped out at me:

Write is not like printf(), it takes a buffer of data and a length and doesn't do any formatting. You had the right idea in your first test (without arrays), but you need to give write the length -- you gave it 'c' which probably caused write to grab too many bytes depending on the value of c.

Code:
data_len = snprintf( str, sizeof( str ), "%d\n", m[j]);
write( pipe[1], str, data_len );

I added a new line -- you'll need some form of space between the data that the parent process will be reading.

Hope this gets you a bit further.
This User Gave Thanks to agama For This Post:
giampoul
# 6  
Old 03-14-2012
There are many things wrong with the pipe and array code...not initalizing m[4] to 0 before using it in the addition and in the parent you are not reading the value into c[i][j] but instead putting it into buff.
This User Gave Thanks to shamrock For This Post:
giampoul
# 7  
Old 03-14-2012
Looking for it and will reply back!
Thanks Smilie
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