diff --git a/lib/libc/sys/Symbol.map b/lib/libc/sys/Symbol.map
--- a/lib/libc/sys/Symbol.map
+++ b/lib/libc/sys/Symbol.map
@@ -71,3 +71,7 @@
 FBSD_1.7 {
 	_Fork;
 };
+
+FBSD_1.8 {
+	vfcntl;
+};
diff --git a/lib/libc/sys/fcntl.c b/lib/libc/sys/fcntl.c
--- a/lib/libc/sys/fcntl.c
+++ b/lib/libc/sys/fcntl.c
@@ -3,6 +3,7 @@
  *
  * Copyright (c) 2008 Isilon Inc http://www.isilon.com/
  * Authors: Doug Rabson <dfr@rabson.org>
+ * Authors: Alex Richardson <arichardson@freebsd.org>
  * Developed with Red Inc: Alfred Perlstein <alfred@freebsd.org>
  *
  * Copyright (c) 2014-2015 The FreeBSD Foundation.
@@ -38,16 +39,39 @@
 #include <sys/syscall.h>
 #include "libc_private.h"
 
+#pragma weak vfcntl
+int
+vfcntl(int fd, int cmd, va_list args)
+{
+	intptr_t arg;
+
+	/*
+	 * fctnl() uses varargs to implement optional arguments, so some calls
+	 * will not actually have an argument passed in this va_list.
+	 * While it is undefined behaviour to call va_arg when no argument
+	 * exists in the argument list, all currently supported architectures
+	 * will give us a result even if no argument/an int was passed instead
+	 * since va_arg() will generally read the next argument register or
+	 * arbitrary data from the stack. This is not true for all architectures
+	 * so once we support one that enforces checks on variadic args (e.g.
+	 * CHERI), we will need to avoid reading non-existent arguments.
+	 */
+	arg = va_arg(args, intptr_t);
+
+	return (INTERPOS_SYS(fcntl, fd, cmd, arg));
+}
+
 #pragma weak fcntl
 int
 fcntl(int fd, int cmd, ...)
 {
 	va_list args;
-	long arg;
+	int result;
 
 	va_start(args, cmd);
-	arg = va_arg(args, long);
+	result = vfcntl(fd, cmd, args);
 	va_end(args);
 
-	return (INTERPOS_SYS(fcntl, fd, cmd, arg));
+	return (result);
 }
+
diff --git a/lib/libsys/fcntl.2 b/lib/libsys/fcntl.2
--- a/lib/libsys/fcntl.2
+++ b/lib/libsys/fcntl.2
@@ -37,6 +37,9 @@
 .In fcntl.h
 .Ft int
 .Fn fcntl "int fd" "int cmd" "..."
+.In stdarg.h
+.Ft int
+.Fn vfcntl "int fd" "int cmd" "va_list ap"
 .Sh DESCRIPTION
 The
 .Fn fcntl
diff --git a/sys/sys/fcntl.h b/sys/sys/fcntl.h
--- a/sys/sys/fcntl.h
+++ b/sys/sys/fcntl.h
@@ -376,6 +376,7 @@
 int	creat(const char *, mode_t);
 int	fcntl(int, int, ...);
 #if __BSD_VISIBLE
+int	vfcntl(int, int, __va_list);
 int	flock(int, int);
 int	fspacectl(int, int, const struct spacectl_range *, int,
 	    struct spacectl_range *);