CODEWITHSUNDEEP
cloopsfor-loopparallel-processingopenmp
Digitful
Digitful

Reputation: 63

How to use OMP in for loops (C Programming)

What am I doing wrong below? The code pegs CPU at 100% and does not end. Seems to be an infinite loop. Help appreciated.

#include <stdio.h>

int main(void)
{
  
  unsigned long number2;
  unsigned long number = 1;
   
  #pragma omp parallel for  
  for (number = 1; number <= 10000; number++)
  {
    unsigned long max = 0, count = 0;    
    number2 = number;

    while (number2 != 1)
    {
      if (number2 > max)
        max = number2;
      if (number2 % 2 == 0)
        number2 /= 2;
      else
        number2 = 3 * number2 + 1;
      count++;
    }    
  }

}

Update: The following works.

Not sure what the root cause was. It could've been

  1. data type was overflowing its boundaries, i.e. int -> unsigned int
  2. number2 had to be made private, etc.
#include <omp.h>

int main(int argc, char **argv)
{
  unsigned int i;
  #pragma omp parallel for
  for (i = 1; i < 10000000; i++)
  {
    unsigned int number2 = i; unsigned int max; unsigned int count = 0;
    while (number2 != 1)
    {
      if (number2 > max)
        max = number2;
      if (number2 % 2 == 0)
        number2 /= 2;
      else
        number2 = 3 * number2 + 1;
      count++;
    }
  }
}```

Upvotes: 0

Views: 134

Answers (2)

Laci
Laci

Reputation: 2818

In the first version of your code number2 is shared, all threads access/modify it simultaneously. It is a data race ---> undefined behavior.

Upvotes: 2

Digitful
Digitful

Reputation: 63

Resolved.

Not sure what the root cause was. It could've been

data type was overflowing its boundaries, i.e. int -> unsigned int

or number2 had to be made private, etc.

Upvotes: 0

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