diff --git a/extent_mapper.h b/extent_mapper.h
index 8805393..e03ef21 100644
--- a/extent_mapper.h
+++ b/extent_mapper.h
@@ -22,7 +22,16 @@
 // the blocksize of a filesystem is often 4096 bytes, that is not always
 // the case, so one should consult GetFilesystemBlockSize(), too.
 // Returns true on success.
+//
+// ExtentsForFileChunkFibmap gets the blocks starting from
+// |chunk_offset|. |chunk_offset| must be a multiple of the block size. If
+// |chunk_size| is not -1, only blocks covering up to |chunk_size| bytes are
+// returned.
 bool ExtentsForFileFibmap(const std::string& path, std::vector<Extent>* out);
+bool ExtentsForFileChunkFibmap(const std::string& path,
+                               off_t chunk_offset,
+                               off_t chunk_size,
+                               std::vector<Extent>* out);
 
 // Puts the blocksize of the filesystem, as used by the FIBMAP ioctl, into
 // out_blocksize by using the FIGETBSZ ioctl. Returns true on success.