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PORace Condition with fork() and pipe()
 

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  1. PORace Condition with fork() and pipe()
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    <p>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.</p> <p>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.</p> <p>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: </p> <p><strong><a href="https://gist.github.com/chrisbenincasa/5101530" rel="nofollow">Gist with relevant code</a></strong></p> <p>Any help would be greatly appreciated</p> <p><strong>EDIT</strong> </p> <p>Here's the relevant source from the gist with the declarations added.</p> <pre><code>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 &lt; children; i++) { generate_board(&amp;(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]-&gt;finished) { // Read in move from parent printf("child[%d] about to read\n", getpid()); read(fd[i][0], &amp;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 &lt; 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 &lt; children; i++) { // Read move from child read(fd[i][2], &amp;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]-&gt;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]-&gt;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++; } } } </code></pre>
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    1. USChristian Benincasa
    1. USChristian Benincasa
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    1. COPlease post (relevant parts of) code directly in your question, not via a link. Thanks.
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      1. PORace Condition with fork() and pipe()
      1. USOliver Charlesworth
    2. COFirst thing that hits me is that you don't need those `dup` calls after creating the pipes. Especially since you don't close the original file descriptors. Depending on how many child processes you make, this might fill up the file descriptor table.
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      1. PORace Condition with fork() and pipe()
      1. USSome programmer dude
    3. CO*Why* would you link 100+ lines of code, and *not* include the opening function declaration at the top of the source file?
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      1. PORace Condition with fork() and pipe()
      1. USWhozCraig
 

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