diff --git a/test/ReadFrames.cxx b/test/ReadFrames.cxx
new file mode 100644
index 000000000..2037134ff
--- /dev/null
+++ b/test/ReadFrames.cxx
@@ -0,0 +1,52 @@
+/*
+ * Copyright 2003-2021 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include "ReadFrames.hxx"
+#include "system/Error.hxx"
+#include "io/FileDescriptor.hxx"
+
+static size_t
+ReadOrThrow(FileDescriptor fd, void *buffer, size_t size)
+{
+	auto nbytes = fd.Read(buffer, size);
+	if (nbytes < 0)
+		throw MakeErrno("Read failed");
+
+	return nbytes;
+}
+
+std::size_t
+ReadFrames(FileDescriptor fd, void *_buffer, std::size_t size,
+	   std::size_t frame_size)
+{
+	auto buffer = (std::byte *)_buffer;
+
+	size = (size / frame_size) * frame_size;
+
+	size_t nbytes = ReadOrThrow(fd, buffer, size);
+
+	const size_t modulo = nbytes % frame_size;
+	if (modulo > 0) {
+		size_t rest = frame_size - modulo;
+		fd.FullRead(buffer + nbytes, rest);
+		nbytes += rest;
+	}
+
+	return nbytes;
+}
diff --git a/test/ReadFrames.hxx b/test/ReadFrames.hxx
new file mode 100644
index 000000000..ea851add1
--- /dev/null
+++ b/test/ReadFrames.hxx
@@ -0,0 +1,26 @@
+/*
+ * Copyright 2003-2021 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include <cstddef>
+
+class FileDescriptor;
+
+std::size_t
+ReadFrames(FileDescriptor fd, void *buffer, std::size_t size,
+	   std::size_t frame_size);
diff --git a/test/meson.build b/test/meson.build
index 75c3ff46f..3b9671410 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -520,6 +520,7 @@ test(
 executable(
   'run_filter',
   'run_filter.cxx',
+  'ReadFrames.cxx',
   include_directories: inc,
   dependencies: [
     filter_glue_dep,
diff --git a/test/run_filter.cxx b/test/run_filter.cxx
index 25c9a1786..04d35dfba 100644
--- a/test/run_filter.cxx
+++ b/test/run_filter.cxx
@@ -18,6 +18,7 @@
  */
 
 #include "ConfigGlue.hxx"
+#include "ReadFrames.hxx"
 #include "fs/Path.hxx"
 #include "fs/NarrowPath.hxx"
 #include "filter/LoadOne.hxx"
@@ -60,35 +61,6 @@ LoadFilter(const ConfigData &config, const char *name)
 	return filter_configured_new(*param);
 }
 
-static size_t
-ReadOrThrow(FileDescriptor fd, void *buffer, size_t size)
-{
-	auto nbytes = fd.Read(buffer, size);
-	if (nbytes < 0)
-		throw MakeErrno("Read failed");
-
-	return nbytes;
-}
-
-static size_t
-ReadFrames(FileDescriptor fd, void *_buffer, size_t size, size_t frame_size)
-{
-	auto buffer = (uint8_t *)_buffer;
-
-	size = (size / frame_size) * frame_size;
-
-	size_t nbytes = ReadOrThrow(fd, buffer, size);
-
-	const size_t modulo = nbytes % frame_size;
-	if (modulo > 0) {
-		size_t rest = frame_size - modulo;
-		fd.FullRead(buffer + nbytes, rest);
-		nbytes += rest;
-	}
-
-	return nbytes;
-}
-
 int main(int argc, char **argv)
 try {
 	if (argc < 3 || argc > 4) {