Openmp lastprivate confusing result

Question

I am getting following results roughly 20% of the execution with the given code and lastprivate clause is not working as expected. Pls note that end value of b should be 50 however at times it is not updated by last thread. Can anyone guide me what am I doing wrong? thanks.

#include <stdio.h> 
#include <omp.h> 
#include <unistd.h>
int main(int argc, char** argv){
   const int N=10;
   int a = 50;
   int b = 10;
   const int k=0;
   omp_set_num_threads(4);
   #pragma omp parallel for firstprivate(a) schedule(dynamic,2) lastprivate(b)
   for(int i=0;i<N;i++){
      b = a+i;
      printf("%d, %d, a=%d, b=%d\n",omp_get_thread_num(),i, a, b);
   }
   printf("a=%d b=%d (expected a = 50 b=59)\n",a,b);
   return 0;
}

occasional output

3, 8, a=50, b=58
3, 9, a=50, b=59
3, 2, a=50, b=52
3, 3, a=50, b=53
1, 4, a=50, b=54
0, 0, a=50, b=50
2, 6, a=50, b=56
2, 7, a=50, b=57
0, 1, a=50, b=51
1, 5, a=50, b=55
a=50 b=10 (expected a = 50 b=59)

Which compiler and which version do you use? Your expectation seems valid, so this looks like a compiler bug. I could not reproduce your result with different compilers I tested on godbolt. — Joachim
– Joachim, Commented Mar 22 at 13:22
Your output does not correspond to your code: the word "expected" is not there. — Victor Eijkhout
– Victor Eijkhout, Commented Mar 22 at 13:48
To be nitpicking: the code also does not compile, because of missing includes for printf and openmp functions. — Joachim
– Joachim, Commented Mar 22 at 14:05
Please update (edit) the question with such information, in order to have the right information ready when reading the question. — Joachim
– Joachim, Commented Mar 24 at 6:40
Code in comments is unreadable. Please edit the original qustion. — Victor Eijkhout
– Victor Eijkhout, Commented Mar 24 at 11:59

Joachim · Accepted Answer · 2025-03-24 12:57:57Z

3

The behavior you observe is a result of a compiler bug, which is fixed in newer versions of Clang. I can reproduce the behavior with clang 16 on Linux/x86. Clang 17 produces the correct and expected results. Your best option is to switch to an up-to-date compiler.

answered Mar 24 at 12:57

Joachim

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