Extern in .cpp files is a code smell.

fpdfsdk/src/fpdfview.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 "../../core/include/fxcodec/fx_codec.h"
 #include "../../core/include/fxcrt/fx_safe_types.h"
 #include "../../public/fpdf_ext.h"
 #include "../../public/fpdf_progressive.h"
@@ skipping 164 matching lines @@
 
   FX_DWORD err_code = pParser->StartParse(pFileAccess);
   if (err_code) {
     delete pParser;
     ProcessParseError(err_code);
     return NULL;
   }
   return pParser->GetDocument();
 }
 
-extern void CheckUnSupportError(CPDF_Document* pDoc, FX_DWORD err_code);
-
 class CMemFile final : public IFX_FileRead {
  public:
   CMemFile(uint8_t* pBuf, FX_FILESIZE size) : m_pBuf(pBuf), m_size(size) {}
 
   virtual void Release() { delete this; }
   virtual FX_FILESIZE GetSize() { return m_size; }
   virtual FX_BOOL ReadBlock(void* buffer, FX_FILESIZE offset, size_t size) {
     if (offset < 0) {
       return FALSE;
     }
@@ skipping 718 matching lines @@
   if (!buffer) {
     *buflen = len;
   } else if (*buflen >= len) {
     memcpy(buffer, utf16Name.c_str(), len);
     *buflen = len;
   } else {
     *buflen = -1;
   }
   return (FPDF_DEST)pDestObj;
 }