I would guess memory map the whole file, then do the equivalent of: memmove(beginning_of_file, beginning_of_file + 300_m_line_offset, file_size - 300_m_line_of

I would guess memory map the whole file, then do the equivalent of: memmove(beginning_of_file, beginning_of_file + 300_m_line_offset, file_size - 300_m_line_offset);