Add openat() to compat.

compat/openat.c

@@ -0,0 +1,63 @@
+/* $Id$ */
+
+/*
+ * Copyright (c) 2013 Nicholas Marriott <nicm@users.sourceforge.net>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF MIND, USE, DATA OR PROFITS, WHETHER
+ * IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING
+ * OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <errno.h>
+#include <fcntl.h>
+#include <stdarg.h>
+#include <unistd.h>
+
+#include "tmux.h"
+
+int
+openat(int fd, const char *path, int flags, ...)
+{
+	mode_t	mode;
+	va_list ap;
+	int	dotfd, retval, saved_errno;
+
+	if (flags & O_CREAT) {
+		va_start(ap, flags);
+		mode = va_arg(ap, mode_t);
+		va_end(ap);
+	} else
+		mode = 0;
+
+	dotfd = -1;
+	if (fd != AT_FDCWD) {
+		dotfd = open(".", O_RDONLY);
+		if (dotfd == -1)
+			return (-1);
+		if (fchdir(fd) != 0)
+			return (-1);
+	}
+
+	retval = open(path, flags, mode);
+
+	if (dotfd != -1) {
+		if (fchdir(dotfd) != 0) {
+			saved_errno = errno;
+			close(retval);
+			close(dotfd);
+			errno = saved_errno;
+			return (-1);
+		}
+		close(dotfd);
+	}
+
+	return (retval);
+}