Add new option: --monitor <cmd>

Option lets user specify a command <cmd> that will get
    forked by cgminer on startup. cgminer's stderr output
    subsequently gets piped directly to this command.

    This permits smart batch monitoring of the miner by a
    baby-sitter log parser while retaining the full benefit
    of the curses interface, e.g. when running cgminer under
    screen.

    Doing the same thing is possible using advanced bash
    redirection paraphernalia, but it is somewhat of a pain,
    especially when running screen'd.

    This option makes things more straightforward.

main.c

@@ -231,6 +231,7 @@ static char datestamp[40];
 static char blocktime[30];
 
 static char *opt_kernel = NULL;
+static char *opt_stderr_cmd = NULL;
 
 enum cl_kernel chosen_kernel;
 
@@ -596,9 +597,13 @@ static struct opt_table opt_config_table[] = {
 			opt_set_bool, &use_syslog,
 			"Use system log for output messages (default: standard error)"),
 #endif
+
+	OPT_WITH_ARG("--monitor|-m",
+		    opt_set_charp, NULL, &opt_stderr_cmd,
+                    "Use custom pipe cmd for output messages"),
 	OPT_WITHOUT_ARG("--text-only|-T",
-			opt_set_invbool, &use_curses,
-			"Disable ncurses formatted screen output"),
+                    opt_set_invbool, &use_curses,
+                    "Disable ncurses formatted screen output"),
 	OPT_WITH_ARG("--url|-o",
 		     set_url, opt_show_charp, &trpc_url,
 		     "URL for bitcoin JSON-RPC server"),
@@ -3713,6 +3718,71 @@ out:
 	return ret;
 }
 
+static void fork_monitor()
+{
+        // Make a pipe: [readFD, writeFD]
+        int pfd[2];
+        int r = pipe(pfd);
+        if (r<0) {
+                perror("pipe");
+                exit(1);
+        }
+
+        // Make stderr write end of pipe
+        fflush(stderr);
+        r = dup2(pfd[1], 2);
+        if (r<0) {
+                perror("dup2");
+                exit(1);
+        }
+        r = close(pfd[1]);
+        if (r<0) {
+                perror("close");
+                exit(1);
+        }
+
+        // Don't let a dying monitor kill the main process
+        sighandler_t sr = signal(SIGPIPE, SIG_IGN);
+        if (SIG_ERR==sr) {
+                perror("signal");
+                exit(1);
+        }
+
+        // Fork a child process
+        r = fork();
+        if (r<0) {
+                perror("fork");
+                exit(1);
+        }
+
+        // In child, launch command
+        if (0==r) {
+                // Make stdin read end of pipe
+                r = dup2(pfd[0], 0);
+                if (r<0) {
+                        perror("dup2");
+                        exit(1);
+                }
+                close(pfd[0]);
+                if (r<0) {
+                        perror("close");
+                        exit(1);
+                }
+
+                // Launch user specified command
+                execl("/bin/bash", "/bin/bash", "-c", opt_stderr_cmd, (char*)NULL);
+                perror("execl");
+                exit(1);
+        }
+
+        // In parent, clean up unused fds
+        r = close(pfd[0]);
+        if (r<0) {
+                perror("close");
+                exit(1);
+        }
+}
+
 int main (int argc, char *argv[])
 {
 	unsigned int i, j = 0, x, y, pools_active = 0;
@@ -3866,6 +3936,9 @@ int main (int argc, char *argv[])
 		openlog(PROGRAM_NAME, LOG_PID, LOG_USER);
 #endif
 
+        if (opt_stderr_cmd)
+                fork_monitor();
+
 	mining_threads = opt_n_threads + gpu_threads;
 
 	total_threads = mining_threads + 5;