#include <stdio.h>
#include <pthread.h>
#include <getopt.h>

#define MAX_DIMENSIONS 25 
#define MAX_THREADS    9

typedef struct {
   int size;
   int top;
   int matrix[MAX_DIMENSIONS][MAX_DIMENSIONS];
   int stack[MAX_DIMENSIONS];
} board_t;

void* create_start(void*);
void init(board_t*);
void scan(board_t*,int,int);
void mark_attacks(board_t*,int,int);
void print_board(board_t*);
void parse_options(int,char**,int*,int*,int*);

pthread_mutex_t SOL_MUTEX;      // for locking solution discovery
pthread_mutex_t JOB_MUTEX;      // for locking who does what

long SOLUTIONS         = 0;     // holds total number of solutions
int CURR_BOARD_INDEX   = -1;    // incr as each board is worked on
int THREADS            = 1;     // number of threads to create
int SIZE               = 4;     // dimension of board to solve
int QUIET              = 0;     // boolean for quite mode on/off

board_t *BOARD[MAX_DIMENSIONS];

int main(int argc, char **argv) {

   int i,rc;
   char s;

   pthread_t thread_id[MAX_THREADS];
   pthread_mutex_init(&SOL_MUTEX, NULL);
   pthread_mutex_init(&JOB_MUTEX, NULL);

   parse_options(argc, argv, &THREADS, &SIZE, &QUIET);

   // cheating?
   if (SIZE == 0) {
      SOLUTIONS = 1;
   }
   else {
      for (i = 1; i < THREADS; i++)
         rc = pthread_create(&thread_id[i-1],NULL,create_start,(void *
+)SIZE);

      create_start((void *)SIZE);
      for (i = 0; i < THREADS - 1; i++)
         pthread_join(thread_id[i],NULL);
   }

   //clean up
   pthread_mutex_destroy(&SOL_MUTEX);
   pthread_mutex_destroy(&JOB_MUTEX);

   for (i = 0; i < SIZE; i++)
      free(BOARD[i]);

   s = (SOLUTIONS == 1) ? '\0' : 's';
   printf("%d solution%c found\n",SOLUTIONS,s);

   return 0;
}

void* create_start(void* numb_rows) {

   int my_board_index;

   while (CURR_BOARD_INDEX < (int)numb_rows - 1) {

      pthread_mutex_lock(&JOB_MUTEX);
         my_board_index = ++CURR_BOARD_INDEX;
      pthread_mutex_unlock(&JOB_MUTEX);

      BOARD[my_board_index] = (board_t *)malloc(sizeof(board_t));
      BOARD[my_board_index]->size = (int)numb_rows;

      init(BOARD[my_board_index]);
      scan(BOARD[my_board_index],my_board_index,0);
   }

}

void scan(board_t* board, int start_row, int col) {
   int i,r,c;
   int row;
   int copy[MAX_DIMENSIONS][MAX_DIMENSIONS];

   // copy board
   for (r = 0; r < board->size; r++) {
      for (c = 0; c < board->size; c++)
         copy[r][c] = board->matrix[r][c];
   }

   if (col == board->size) {

      // found solution!

      pthread_mutex_lock(&SOL_MUTEX);
         SOLUTIONS++;

         if (!QUIET) {
            for (i = 0; i < board->size; i++)
               printf ("%d ",board->stack[i]);
            printf("\n");
         }
      pthread_mutex_unlock(&SOL_MUTEX);

      return;
   }

   for (row = start_row; row < board->size; row++) {
      
      if (board->matrix[row][col] == 1) {
         // found possible candidate - add to stack
         board->stack[board->top++] = row;

         mark_attacks(board,row,col);
         scan(board,0,col + 1);

         if (col == 0)
            // this thread is finished
            return;

         // restore board from copy
         for (r = 0; r < board->size; r++) {
            for (c = 0; c < board->size; c++)
               board->matrix[r][c] = copy[r][c];
         }

         board->top--;
      }
   }

   return;
}

void mark_attacks(board_t* board,int row, int col) {
   int up   = row;
   int down = row;
   int c;

   board->matrix[row][col] = 8;

   for(c = col + 1; c < board->size; c++) {
      --up;
      ++down;

      board->matrix[row][c]  = 0;

      if (up >= 0)
         board->matrix[up][c]   = 0;

      if (down < board->size)
         board->matrix[down][c] = 0;
   }
}

/* debugging purposes */
void print_board(board_t* board) {
   int row,col;
   for(row = 0; row < board->size; row++) {
      for(col = 0; col < board->size; col++)
         fprintf(stderr,"%d ",board->matrix[row][col]);
      fprintf(stderr,"\n");
   }
}

void init (board_t* board) {
   int row,col;
   for(row = 0; row < board->size; row++) {
      for(col = 0; col < board->size; col++)
         board->matrix[row][col] = 1;
   }
}

void parse_options(int argc, char** argv, int* t, int* n, int* q) {

   int c;

   while (c != -1) {
      c = getopt(argc, argv, "t:n:qh");
      switch (c) {
         case 't':
            *t = atoi(optarg);
            break;
         case 'n':
            *n = atoi(optarg);
            break;
         case 'q':
            *q = 1;
            break;
         case 'h':
            printf("USAGE: p1 [-t threads -n size -q]\n");
            printf("  -t threads (default 1, max 9)\n");
            printf("  -n size    (default 4, max 25)\n");
            printf("  -q         (only print # solutions)\n");
            exit(0);
      }
   }

   // make sure n is no larger than 25
   *n = *n > 25 ? 25 : *n;

   // make sure t is no larger than 9
   *t = *t > 9 ? 9 : *t;
}
[download]

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)