diff --git a/libs/protoutil/include/android/util/ProtoOutputStream.h b/libs/protoutil/include/android/util/ProtoOutputStream.h
index ce41849670261..faea9b21311dc 100644
--- a/libs/protoutil/include/android/util/ProtoOutputStream.h
+++ b/libs/protoutil/include/android/util/ProtoOutputStream.h
@@ -109,6 +109,12 @@ public:
     long long start(uint64_t fieldId);
     void end(long long token);
 
+    /**
+     * Returns how many bytes are buffered in ProtoOutputStream.
+     * Notice, this is not the actual(compact) size of the output data.
+     */
+    size_t bytesWritten();
+
     /**
      * Flushes the protobuf data out to given fd. When the following functions are called,
      * it is not able to write to ProtoOutputStream any more since the data is compact.
diff --git a/libs/protoutil/src/ProtoOutputStream.cpp b/libs/protoutil/src/ProtoOutputStream.cpp
index 9d8ee729a80dc..1904d40ea67ac 100644
--- a/libs/protoutil/src/ProtoOutputStream.cpp
+++ b/libs/protoutil/src/ProtoOutputStream.cpp
@@ -295,6 +295,12 @@ ProtoOutputStream::end(long long token)
     }
 }
 
+size_t
+ProtoOutputStream::bytesWritten()
+{
+    return mBuffer.size();
+}
+
 bool
 ProtoOutputStream::compact() {
     if (mCompact) return true;