Race Condition with fork() and pipe() - c

Question

I am writing a program for an assignment where a parent plays connect four against N child processes. The program uses pipes to communicate game moves between processes.
I am having a problem fixing a race condition that is present in my program, however. There is a condition where the child process hangs one its read() call after the game has finished. This only seems to happen when there is more than one child process.
I've tried several things, such as named semaphores, but I'm still pretty new to forks, pipes, and IPC. I've posted a gist with relevant code (I tried to clean it up as best as I can for readability) here:
Gist with relevant code
Any help would be greatly appreciated
EDIT
Here's the relevant source from the gist with the declarations added.
int main (int argc, char const *argv[])
{
int dimension = 8, children = 2, i;
int child_play_to_win = 0;
int fd[children][4];
pid_t childpid[children];
Board** boards = (Board**) malloc(sizeof(Board*) * children);
GameMove* lastMove, *tmpMove;
char buf[80];
for(i = 0; i < children; i++) {
generate_board(&(boards[i]), dimension);
int tmp[2];
pipe(tmp);
// child read
fd[i][0] = dup(tmp[0]);
// parent write
fd[i][1] = dup(tmp[1]);
pipe(tmp);
// parent read
fd[i][2] = dup(tmp[0]);
// child write
fd[i][3] = dup(tmp[1]);
childpid[i] = fork();
if(childpid[i] == -1) {
perror("fork");
exit(1);
}
if(childpid[i] == 0) {
srand(getpid());
close(fd[i][1]);
close(fd[i][2]);
while(!boards[i]->finished) {
// Read in move from parent
printf("child[%d] about to read\n", getpid());
read(fd[i][0], &buf, sizeof(GameMove));
// repeat parent move on this board
if(gameNotFinished) {
// make child move
// write move back to parent
write(fd[i][3], lastMove, sizeof(GameMove));
// If the board is finished (there was a win),
if (!gameNotFinihsed) {
// Child wins
close(fd[i][0]);
close(fd[i][3]);
printf("child[%d] ending\n", getpid());
break;
}
}
else {
// Parent won
close(fd[i][0]);
close(fd[i][3]);
break;
}
}
dealloc(boards[i]);
exit(0);
}
}
// When this hits children amount, all games are done
int games_complete = 0;
// Make first move to all children
for (i = 0; i < children; i++) {
close(fd[i][0]);
close(fd[i][3]);
lastMove = placePieceAtBestPosition(boards[i], 1);
printf("parent writing to child[%d]\n", childpid[i]);
write(fd[i][1], lastMove, sizeof(GameMove));
}
while (games_complete != children) {
for (i = 0; i < children; i++) {
// Read move from child
read(fd[i][2], &buf, sizeof(GameMove));
// repeat child move
// Check for a child win...
if (!checkForWin(boards[i], 2)) {
// No win yet, place piece at best position
lastMove = placePieceAtBestPosition(boards[i], 1);
// check for win again
boards[i]->finished = checkForWin(boards[i], 1);
// Write move back to child
write(fd[i][1], lastMove, sizeof(GameMove));
// If we won, close everything up and increment
// the games_complete counter.
if(boards[i]->finished) {
close(fd[i][1]);
close(fd[i][2]);
games_complete++;
}
} else {
// write back child move if there was a win
write(fd[i][1], lastMove, sizeof(GameMove));
close(fd[i][1]);
close(fd[i][2]);
printf("Parent lost! ):\n");
games_complete++;
}
}
}

Answer 1

I think I know what your problem is. When you fork each child, you close the parent side of its pipes. however, each child still has open the parent side of the pipes for all of the previous children. Because of this, only the last-created child will have its parent-sides of its pipes closed.
Suggest you change:
close(fd[i][1]);
close(fd[i][2]);
to something like:
for (j = 0; j <=i; j++) {
close(fd[j][1]);
close(fd[j][2]);
}