diff --git a/lib/fs.c b/lib/fs.c
index 7e3ad331..5fb14cc3 100644
--- a/lib/fs.c
+++ b/lib/fs.c
@@ -778,6 +778,61 @@ uc_fs_ftruncate(uc_vm_t *vm, size_t nargs)
 	return ucv_boolean_new(true);
 }
 
+/**
+ * Locks or unlocks a file.
+ *
+ * The mode argument specifies lock/unlock operation flags.
+ *
+ * | Flag    | Description                  |
+ * |---------|------------------------------|
+ * | "s"     | shared lock                  |
+ * | "x"     | exclusive lock               |
+ * | "n"     | don't block when locking     |
+ * | "u"     | unlock                       |
+ *
+ * Returns `true` if the file was successfully locked/unlocked.
+ *
+ * Returns `null` if an error occurred.
+ *
+ * @function module:fs#flock
+ *
+ * @param {number} [fd]
+ * The file descriptor.
+ *
+ * @param {string} [op]
+ * The lock operation flags
+ *
+ * @returns {?boolean}
+ */
+static uc_value_t *
+uc_fs_flock(uc_vm_t *vm, size_t nargs)
+{
+	uc_value_t *fd = uc_fn_arg(0);
+	uc_value_t *mode = uc_fn_arg(1);
+	int i, op = 0;
+	char *m;
+
+	if (ucv_type(fd) != UC_INTEGER ||
+	    ucv_type(mode) != UC_STRING)
+		err_return(EINVAL);
+
+	m = ucv_string_get(mode);
+	for (i = 0; m[i]; i++) {
+		switch (m[i]) {
+		case 's': op |= LOCK_SH; break;
+		case 'x': op |= LOCK_EX; break;
+		case 'n': op |= LOCK_NB; break;
+		case 'u': op |= LOCK_UN; break;
+		default: err_return(EINVAL);
+		}
+	}
+
+	if (flock(ucv_int64_get(fd), op) < 0)
+		err_return(errno);
+
+	return ucv_boolean_new(true);
+}
+
 /**
  * Obtain current read position.
  *
@@ -2650,6 +2705,7 @@ static const uc_function_list_t global_fns[] = {
 	{ "realpath",	uc_fs_realpath },
 	{ "pipe",		uc_fs_pipe },
 	{ "ftruncate",	uc_fs_ftruncate },
+	{ "flock",		uc_fs_flock },
 };