multi-threaded multibrot

Makefile

@@ -31,7 +31,7 @@ checkcheck: checkcheck.c
 	$(CC) $(CFLAGS) -o $@ $< -lcheck
 
 multibrot: multibrot.c
-	$(CC) $(CFLAGS) -o $@ $< -lm -lpng
+	$(CC) $(CFLAGS) -o $@ $< -lm -lpng -pthread
 
 multibrot.png: multibrot
-	./multibrot
+	./multibrot -j2

multibrot.c

@@ -1,8 +1,11 @@
+#include <assert.h>
 #include <complex.h>
+#include <getopt.h>
 #include <stdbool.h>
 #include <stdint.h>
 #include <stdlib.h>
 #include <png.h>
+#include <pthread.h>
 
 typedef uint32_t pixel_t;
 
@@ -17,8 +20,7 @@ const int max_it = 170;
 
 /* Compute the out-coloring based on the iteration counter. */
 pixel_t outcolor(int it) {
-  /* return 0x00010001 * ((it * 0xff) / max_it); */
-  return 0x00000001 * ((it * 0xff) / max_it);
+  return 0x00010001 * ((it * 0xff) / max_it);
 }
 
 /* "Compute" the in-coloring. */
@@ -26,12 +28,10 @@ pixel_t incolor() {
   return 0x00000000;            /* black */
 }
 
+/* Computes the visual representation of a mandelbrot set for a given line. */
+void compmandelbrot_line(bitmap_t* mbrot, size_t y) {
 
-void drawmandelbrot(bitmap_t * mbrot) {
-
-  for (size_t i = 0; i < mbrot->width * mbrot->height; i++) {
-    size_t y = i / mbrot->width;
-    size_t x = i % mbrot->width;
+  for (size_t x = 0; x < mbrot->width; x++) {
 
     float complex c = ((3.0f * x / mbrot->width) - 2.0f)
         + I * ((2.0f * y / mbrot->height) - 1.0f);
@@ -57,14 +57,42 @@ void drawmandelbrot(bitmap_t * mbrot) {
       color = incolor();
     }
 
+    size_t i = y * mbrot->width + x;
     mbrot->pixels[i] = color;
   }
 }
 
+/* Next line to compute. */
+size_t next_y = 0;
+
+/* Mutex for next_y. */
+pthread_mutex_t next_y_mutex;
+
+/* Thread to compute mandelbrot. */
+void *compmandelbrot(void *arg) {
+  bitmap_t* mbrot = (bitmap_t*) arg;
+
+  size_t y = 0;
+  while (y < mbrot->height) {
+    int ret;
+    ret = pthread_mutex_lock(&next_y_mutex);
+    assert(ret == 0);
+
+    y = next_y++;
+
+    ret = pthread_mutex_unlock(&next_y_mutex);
+    assert(ret == 0);
+
+    if (y < mbrot->height) {
+      compmandelbrot_line(mbrot, y);
+    }
+  }
+
+  pthread_exit(NULL);
+}
 
 /* Write "bitmap" to a PNG file specified by "path"; returns 0 on
    success, non-zero on error. */
-
 static int save_png_to_file(bitmap_t * bitmap, const char *path) {
   FILE *fp;
 
@@ -78,8 +106,6 @@ static int save_png_to_file(bitmap_t * bitmap, const char *path) {
    * is set to a value which means 'success'. */
   int status = -1;
 
-  /* FIXME The following number is set by trial and error only. I cannot see
-   * where it it is documented in the libpng manual.  */
   int pixel_size = 3;
   int depth = 8;
 
@@ -99,7 +125,7 @@ static int save_png_to_file(bitmap_t * bitmap, const char *path) {
     goto png_create_info_struct_failed;
   }
 
-  /*Set up error handling. */
+  /* Set up error handling. */
   if (setjmp(png_jmpbuf(png_ptr))) {
     goto png_failure;
   }
@@ -121,7 +147,6 @@ static int save_png_to_file(bitmap_t * bitmap, const char *path) {
         png_malloc(png_ptr, sizeof(uint8_t) * bitmap->width * pixel_size);
     row_pointers[y] = row;
     for (x = 0; x < bitmap->width; ++x) {
-      /* FIXME */
       pixel_t *pixel = bitmap->pixels + (y*bitmap->width + x);
       *row++ = (*pixel & 0x00ff0000) >> 16;
       *row++ = (*pixel & 0x0000ff00) >> 8;
@@ -144,7 +169,6 @@ static int save_png_to_file(bitmap_t * bitmap, const char *path) {
   png_free(png_ptr, row_pointers);
 
 png_failure:
-  /* FIXME info struct destroy? */
 png_create_info_struct_failed:
   png_destroy_write_struct(&png_ptr, &info_ptr);
 png_create_write_struct_failed:
@@ -154,21 +178,54 @@ fopen_failed:
 }
 
 int main(int argc, char *argv[]) {
-  bitmap_t mbrot;
-  mbrot.width = 640;
-  mbrot.height = 480; /* FIXME calculate */
-  mbrot.pixels = calloc(sizeof(pixel_t), mbrot.width * mbrot.height);
 
-  /* Do the mandelbrot. */
-  drawmandelbrot(&mbrot);
+  /* Get command line options. */
+  int num_threads = 1;
+  char* options = "j:";
+  if (getopt(argc, argv, options) == 'j') {
+    num_threads = (int) strtol(optarg, NULL, 10);
+    assert(num_threads != 0);
+  } else {
+    printf("Usage: multibrot -j <number_of_threads>\n");
+    exit(0);
+  }
+
+  /* Set up mandelbrot bitmap. */
+  bitmap_t mbrot;
+  mbrot.width = 1600;
+  mbrot.height = 1200;
+  mbrot.pixels = calloc(sizeof(pixel_t), mbrot.width * mbrot.height);
+  assert(mbrot.pixels != NULL);
+
+  /* Start computing threads. */
+  int ret;
+  pthread_t* threads;
+  threads = calloc(sizeof(pthread_t), num_threads);
+
+  ret = pthread_mutex_init(&next_y_mutex, NULL);
+  assert(ret == 0);
+
+  for (int t = 0; t < num_threads; t++) {
+    ret = pthread_create(&threads[t], NULL, compmandelbrot, (void *) &mbrot);
+    assert(ret == 0);
+  }
+
+  /* Wait for the threads to finish. */
+  for (int t = 0; t < num_threads; t++) {
+    ret = pthread_join(threads[t], NULL);
+    assert(ret == 0);
+  }
 
   /* Save PNG */
   char *file = "multibrot.png";
-  if (save_png_to_file(&mbrot, file) != 0) {
-    fprintf(stderr, "Could not save PNG to %s.\n", file);
-    exit(1);
-  }
+  ret = save_png_to_file(&mbrot, file);
+  assert(ret == 0);
 
   /* Quit. */
+  pthread_exit(NULL);
+  free(threads);
+  free(mbrot.pixels);
+  ret = pthread_mutex_destroy(&next_y_mutex);
+  assert(ret == 0);
   return 0;
 }