diff -Naur mp4v2-trunk-r355/src/mp4atom.cpp mp4v2-trunk-r355/src/mp4atom.cpp --- mp4v2-trunk-r355/src/mp4atom.cpp 2012-02-21 19:40:03.000000000 +0100 +++ mp4v2-trunk-r355/src/mp4atom.cpp 2012-02-21 19:36:38.000000000 +0100 @@ -543,6 +543,10 @@ m_pFile->SetPosition(m_start + 8); m_pFile->WriteUInt64(m_size); } else { + if (!(m_size <= (uint64_t)0xFFFFFFFF)) { + // Let the user know what the following assert is all about + fprintf(stderr, "MP4ERROR: File size exceeded 4 GB; output unplayable. Enable \"Large file size\" to fix it.\n"); + } ASSERT(m_size <= (uint64_t)0xFFFFFFFF); m_pFile->SetPosition(m_start); m_pFile->WriteUInt32(m_size);