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POUsing poll function with buffered streams
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copied!<p>I am trying to implement a client-server type of communication system using the <a href="http://man7.org/linux/man-pages/man2/poll.2.html" rel="nofollow">poll</a> function in C. The flow is as follows:</p> <ol> <li>Main program forks a sub-process</li> <li>Child process calls the <code>exec</code> function to execute <code>some_binary</code></li> <li>Parent and child send messages to each other alternately, each message that is sent depends on the last message received.</li> </ol> <p>I tried to implement this using <code>poll</code>, but ran into problems because the child process buffers its output, causing my <code>poll</code> calls to timeout. Here's my code:</p> <pre><code>int main() { char *buffer = (char *) malloc(1000); int n; pid_t pid; /* pid of child process */ int rpipe[2]; /* pipe used to read from child process */ int wpipe[2]; /* pipe used to write to child process */ pipe(rpipe); pipe(wpipe); pid = fork(); if (pid == (pid_t) 0) { /* child */ dup2(wpipe[0], STDIN_FILENO); dup2(rpipe[1], STDOUT_FILENO); close(wpipe[0]); close(rpipe[0]); close(wpipe[1]); close(rpipe[1]); if (execl("./server", "./server", (char *) NULL) == -1) { fprintf(stderr, "exec failed\n"); return EXIT_FAILURE; } return EXIT_SUCCESS; } else { /* parent */ /* close the other ends */ close(wpipe[0]); close(rpipe[1]); /* poll to check if write is good to go This poll succeeds, write goes through */ struct pollfd pfds[1]; pfds[0].fd = wpipe[1]; pfds[0].events = POLLIN | POLLOUT; int pres = poll(pfds, (nfds_t) 1, 1000); if (pres > 0) { if (pfds[0].revents & POLLOUT) { printf("Writing data...\n"); write(wpipe[1], "hello\n", 6); } } /* poll to check if there's something to read. This poll times out because the child buffers its stdout stream. */ pfds[0].fd = rpipe[0]; pfds[0].events = POLLIN | POLLOUT; pres = poll(pfds, (nfds_t) 1, 1000); if (pres > 0) { if (pfds[0].revents & POLLIN) { printf("Reading data...\n"); int n = read(rpipe[0], buffer, 1000); buffer[n] = '\0'; printf("child says:\n%s\n", buffer); } } kill(pid, SIGTERM); return EXIT_SUCCESS; } } </code></pre> <p>The server code is simply:</p> <pre><code>int main() { char *buffer = (char *) malloc(1000); while (scanf("%s", buffer) != EOF) { printf("I received %s\n", buffer); } return 0; } </code></pre> <p>How do I prevent <code>poll</code> calls from timing out because of buffering?</p> <p>EDIT:</p> <p>I would like the program to work even when the <code>exec</code>ed binary is external, i.e., I have no control over the code - like a unix command, e.g., <code>cat</code> or <code>ls</code>.</p>

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