Question

I’m currently looking at the smallpt code by Keavin Beason. I compiled the code with what it says on the tin using g++ -O3 -fopenmp smallpt.cpp, and I’m running into what seems like either an infinite loop or a deadlock.

Compiling the code using just g++ -O3 smallpt.cpp produces the images seen on his page, but I can’t get the OpenMP parallelization to work at all.

For reference, I’m compiling on a Windows 7 64-bit machine using Cygwin with GCC 4.5.0. The author himself has stated he’s run the same exact code and has run into no issues whatsoever, but I can’t get the program to actually exit when it’s done tracing the image.

Could this be an issue with my particular compiler and environment, or am I doing something wrong here? Here’s the particular snippet of code that’s parallelized using OpenMP. I’ve only modified it with some minor formatting to make it more readable.

---

int main(int argc, char *argv[])
{
  int w=1024, h=768, samps = argc==2 ? atoi(argv[1])/4 : 1;

  Ray cam(Vec(50,52,295.6), Vec(0,-0.042612,-1).norm()); // cam pos, dir
  Vec cx=Vec(w*.5135/h);
  Vec cy=(cx%cam.d).norm()*.5135, r, *c=new Vec[w*h];

  #pragma omp parallel for schedule(dynamic, 1) private(r)       // OpenMP
  for (int y=0; y<h; y++)                       // Loop over image rows
  {
    fprintf(stderr,"\rRendering (%d spp) %5.2f%%",samps*4,100.*y/(h-1));
    for (unsigned short x=0, Xi[3]={0,0,y*y*y}; x<w; x++)   // Loop cols
    {
      for (int sy=0, i=(h-y-1)*w+x; sy<2; sy++)     // 2x2 subpixel rows
      {
        for (int sx=0; sx<2; sx++, r=Vec())        // 2x2 subpixel cols
        {
          for (int s=0; s<samps; s++)
          {
            double r1=2*erand48(Xi), dx=r1<1 ? sqrt(r1)-1: 1-sqrt(2-r1);
            double r2=2*erand48(Xi), dy=r2<1 ? sqrt(r2)-1: 1-sqrt(2-r2);
            Vec d = cx*( ( (sx+.5 + dx)/2 + x)/w - .5) +
                    cy*( ( (sy+.5 + dy)/2 + y)/h - .5) + cam.d;
            r = r + radiance(Ray(cam.o+d*140,d.norm()),0,Xi)*(1./samps);
          } // Camera rays are pushed ^^^^^ forward to start in interior
          c[i] = c[i] + Vec(clamp(r.x),clamp(r.y),clamp(r.z))*.25;
        }
      }
    }
  }

  /*  PROBLEM HERE!
      The code never seems to reach here
      PROBLEM HERE!
  */
  FILE *f = fopen("image.ppm", "w");         // Write image to PPM file.
  fprintf(f, "P3\n%d %d\n%d\n", w, h, 255);
  for (int i=0; i<w*h; i++)
    fprintf(f,"%d %d %d ", toInt(c[i].x), toInt(c[i].y), toInt(c[i].z));
}

Here’s the output that the program produces, when it runs to completion:

$ time ./a
Rendering (4 spp) 100.00%spp)  spp)   00..0026%%

---

The following is the most basic code that can reproduce the above behavior

#include <cstdio>
#include <cstdlib>
#include <cmath>

struct Vector
{
  double x, y, z;
  Vector() : x(0), y(0), z(0) {}
};

int toInt(double x)
{
  return (int)(255 * x);
}

double clamp(double x)
{
  if (x < 0) return 0;
  if (x > 1) return 1;
  return x;
}

int main(int argc, char *argv[])
{
  int w = 1024;
  int h = 768;
  int samples = 1;

  Vector r, *c = new Vector[w * h];

  #pragma omp parallel for schedule(dynamic, 1) private(r)
  for (int y = 0; y < h; y++)
  {
    fprintf(stderr,"\rRendering (%d spp) %5.2f%%",samples * 4, 100. * y / (h - 1));
    for (unsigned short x = 0, Xi[3]= {0, 0, y*y*y}; x < w; x++)
    {
      for (int sy = 0, i = (h - y - 1) * w + x; sy < 2; sy++)
      {
        for (int sx = 0; sx < 2; sx++, r = Vector())
        {
          for (int s = 0; s < samples; s++)
          {
            double r1 = 2 * erand48(Xi), dx = r1 < 1 ? sqrt(r1) - 1 : 1 - sqrt(2 - r1);
            double r2 = 2 * erand48(Xi), dy = r2 < 1 ? sqrt(r2) - 1 : 1 - sqrt(2 - r2);
            r.x += r1;
            r.y += r2;
          }

          c[i].x += clamp(r.x) / 4;
          c[i].y += clamp(r.y) / 4;
        }
      }
    }
  }

  FILE *f = fopen("image.ppm", "w");         // Write image to PPM file.
  fprintf(f, "P3\n%d %d\n%d\n", w, h, 255);
  for (int i=0; i<w*h; i++)
    fprintf(f,"%d %d %d ", toInt(c[i].x), toInt(c[i].y), toInt(c[i].z));
}

This is the output obtained from the following sample program:

$ g++ test.cpp
$ ./a
Rendering (4 spp) 100.00%

$ g++ test.cpp -fopenmp
$ ./a
Rendering (4 spp) 100.00%spp)  spp)   00..0052%%

Answer 1

fprintf is not guarded by a critical section or a #pragma omp single/master. I wouldn’t be surprised if on Windows this thing messes up the console.