diff --git a/Makefile b/Makefile
index 4484d67..cecc5f2 100644
--- a/Makefile
+++ b/Makefile
@@ -7,12 +7,14 @@ HDR = util.h arg.h ubase.h
 LIB = \
 	$(OS)/grabmntinfo.o \
 	$(OS)/syslog.o      \
+	$(OS)/umount.o      \
 	util/eprintf.o      \
 	util/estrtol.o
 
 SRC = \
 	df.c                \
-	dmesg.c
+	dmesg.c             \
+	umount.c
 
 OBJ = $(SRC:.c=.o) $(LIB)
 BIN = $(SRC:.c=)
diff --git a/linux/umount.c b/linux/umount.c
new file mode 100644
index 0000000..1eea1e0
--- /dev/null
+++ b/linux/umount.c
@@ -0,0 +1,14 @@
+#include <sys/mount.h>
+#include <stdio.h>
+#include "../ubase.h"
+#include "../util.h"
+
+int
+do_umount(const char *target, int opts)
+{
+	int flags = 0;
+
+	if (opts & UBASE_MNT_FORCE)
+		flags |= MNT_FORCE;
+	return umount2(target, flags);
+}
diff --git a/openbsd/umount.c b/openbsd/umount.c
new file mode 100644
index 0000000..660642e
--- /dev/null
+++ b/openbsd/umount.c
@@ -0,0 +1,15 @@
+#include <sys/param.h>
+#include <sys/mount.h>
+#include <stdio.h>
+#include "../ubase.h"
+#include "../util.h"
+
+int
+do_umount(const char *target, int opts)
+{
+	int flags = 0;
+
+	if (opts & UBASE_MNT_FORCE)
+		flags |= MNT_FORCE;
+	return unmount(target, flags);
+}
diff --git a/ubase.h b/ubase.h
index 5e6a692..48da769 100644
--- a/ubase.h
+++ b/ubase.h
@@ -11,3 +11,10 @@ int grabmntinfo(struct mntinfo **minfo);
 /* syslog.c */
 int syslog_size(void);
 int syslog_read(void *buf, size_t n);
+
+/* umount.c */
+enum {
+	UBASE_MNT_FORCE = 1 << 0
+};
+
+int do_umount(const char *target, int opts);
diff --git a/umount.c b/umount.c
new file mode 100644
index 0000000..cee553b
--- /dev/null
+++ b/umount.c
@@ -0,0 +1,32 @@
+#include <stdio.h>
+#include "ubase.h"
+#include "util.h"
+
+static void
+usage(void)
+{
+	eprintf("usage: %s [-f] target\n", argv0);
+}
+
+int
+main(int argc, char *argv[]) {
+	int i;
+	int fflag = 0;
+	int ret = 0;
+
+	ARGBEGIN {
+	case 'f':
+		fflag = UBASE_MNT_FORCE;
+		break;
+	default:
+		usage();
+	} ARGEND;
+	if (argc < 1)
+		usage();
+	for (i = 0; i < argc; i++) {
+		if (do_umount(argv[i], fflag) < 0)
+			eprintf("do_umount:");
+		ret = 1;
+	}
+	return ret;
+}