Extern in .cpp files is a code smell.

fpdfsdk/src/fpdf_dataavail.cpp

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #include "../../public/fpdf_dataavail.h"
 #include "../include/fsdk_define.h"
 
 extern void ProcessParseError(FX_DWORD err_code);
@@ skipping 77 matching lines @@
 }
 
 DLLEXPORT int STDCALL FPDFAvail_IsDocAvail(FPDF_AVAIL avail,
                                            FX_DOWNLOADHINTS* hints) {
   if (avail == NULL || hints == NULL)
     return 0;
   CFPDF_DownloadHintsWrap hints_wrap(hints);
   return ((CFPDF_DataAvail*)avail)->m_pDataAvail->IsDocAvail(&hints_wrap);
 }
 
-extern void CheckUnSupportError(CPDF_Document* pDoc, FX_DWORD err_code);
-
 DLLEXPORT FPDF_DOCUMENT STDCALL
 FPDFAvail_GetDocument(FPDF_AVAIL avail, FPDF_BYTESTRING password) {
   if (avail == NULL)
     return NULL;
   CPDF_Parser* pParser = new CPDF_Parser;
   pParser->SetPassword(password);
 
   FX_DWORD err_code = pParser->StartAsynParse(
       ((CFPDF_DataAvail*)avail)->m_pDataAvail->GetFileRead());
   if (err_code) {
@@ skipping 29 matching lines @@
     return -1;
   CFPDF_DownloadHintsWrap hints_wrap(hints);
   return ((CFPDF_DataAvail*)avail)->m_pDataAvail->IsFormAvail(&hints_wrap);
 }
 
 DLLEXPORT FPDF_BOOL STDCALL FPDFAvail_IsLinearized(FPDF_AVAIL avail) {
   if (avail == NULL)
     return -1;
   return ((CFPDF_DataAvail*)avail)->m_pDataAvail->IsLinearizedPDF();
 }