+/*
+ * Copyright (c) 2014 Nicira, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at:
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#include <config.h>
+#include "daemon.h"
+#include <errno.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include "vlog.h"
+
+VLOG_DEFINE_THIS_MODULE(daemon);
+
+/* For each of the standard file descriptors, whether to replace it by
+ * /dev/null (if false) or keep it for the daemon to use (if true). */
+static bool save_fds[3];
+
+/* A daemon doesn't normally have any use for the file descriptors for stdin,
+ * stdout, and stderr after it detaches. To keep these file descriptors from
+ * e.g. holding an SSH session open, by default detaching replaces each of
+ * these file descriptors by /dev/null. But a few daemons expect the user to
+ * redirect stdout or stderr to a file, in which case it is desirable to keep
+ * these file descriptors. This function, therefore, disables replacing 'fd'
+ * by /dev/null when the daemon detaches. */
+void
+daemon_save_fd(int fd)
+{
+ ovs_assert(fd == STDIN_FILENO ||
+ fd == STDOUT_FILENO ||
+ fd == STDERR_FILENO);
+ save_fds[fd] = true;
+}
+
+/* Returns a readable and writable fd for /dev/null, if successful, otherwise
+ * a negative errno value. The caller must not close the returned fd (because
+ * the same fd will be handed out to subsequent callers). */
+static int
+get_null_fd(void)
+{
+ static int null_fd;
+#ifndef _WIN32
+ char *device = "/dev/null";
+#else
+ char *device = "nul";
+#endif
+
+ if (!null_fd) {
+ null_fd = open(device, O_RDWR);
+ if (null_fd < 0) {
+ int error = errno;
+ VLOG_ERR("could not open %s: %s", device, ovs_strerror(error));
+ null_fd = -error;
+ }
+ }
+
+ return null_fd;
+}
+
+/* Close standard file descriptors (except any that the client has requested we
+ * leave open by calling daemon_save_fd()). If we're started from e.g. an SSH
+ * session, then this keeps us from holding that session open artificially. */
+void
+close_standard_fds(void)
+{
+ int null_fd = get_null_fd();
+ if (null_fd >= 0) {
+ int fd;
+
+ for (fd = 0; fd < 3; fd++) {
+ if (!save_fds[fd]) {
+ dup2(null_fd, fd);
+ }
+ }
+ }
+
+ /* Disable logging to stderr to avoid wasting CPU time. */
+ vlog_set_levels(NULL, VLF_CONSOLE, VLL_OFF);
+}