#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>
#define INSET(member,set) ((1<<(member))&(set))
#define INCL(set,member) ((set) |= 1<<(member))
int main() {
   const int size = 8; /* square size of the board */
   struct { int row, col; } pos[size]; /* queen positions */
   /* a queen on (row, col) threatens a row, a column,
      and 2 diagonals;
      rows and columns are characterized by their number (0..n-1),
      the diagonals by row-col+n-1 and row+col,
      (n is a shorthand for the square size of the board)
   */
   int rows = 0, cols = 0; /* bitmaps of [0..n-1] */
   int diags1 = 0; /* bitmap of [0..2*(n-1)] used for row-col+n-1 */
   int diags2 = 0; /* bitmap of [0..2*(n-1)] used for row+col */
   int row = 0; int col;

   setnextqueen: for (col = 0; col < size; ++col) {
      if (INSET(row, rows)) continue;
      if (INSET(col, cols)) continue;
      if (INSET(row - col + size - 1, diags1)) continue;
      if (INSET(row + col, diags2)) continue;
      int child = fork();
      if (child == -1) {
         perror("fork"); exit(255);
      }
      if (child == 0) {
         INCL(rows, row); INCL(cols, col);
         INCL(diags1, row - col + size - 1);
         INCL(diags2, row + col);
         pos[row].row = row; pos[row].col = col;
         ++row; /* set next queen in next row */
         if (row == size) exit(1); /* solution found */
         goto setnextqueen;
      }
   }

   /* count the results of all children */
   int stat; pid_t child; int nofsolutions = 0;
   while ((child = wait(&stat)) > 0) {
      if (!WIFEXITED(stat)) continue;
      int exitval = WEXITSTATUS(stat);
      if (exitval == 255) exit(255);
      nofsolutions += exitval;
   }
   exit(nofsolutions);
}