Output line-buffered or packet-buffered

“-l“ option is used to specify output line-buffered. On Unix-like Operating Systems, setvbuf() or setlinebuf() will be used (code is here): 

  1.         ......
  2.         case 'l':
  3. #ifdef _WIN32
  4.             /*
  5.              * _IOLBF is the same as _IOFBF in Microsoft's C
  6.              * libraries; the only alternative they offer
  7.              * is _IONBF.
  8.              *
  9.              * XXX - this should really be checking for MSVC++,
  10.              * not _WIN32, if, for example, MinGW has its own
  11.              * C library that is more UNIX-compatible.
  12.              */
  13.             setvbuf(stdout, NULL, _IONBF, 0);
  14. #else /* _WIN32 */
  15. #ifdef HAVE_SETLINEBUF
  16.             setlinebuf(stdout);
  17. #else
  18.             setvbuf(stdout, NULL, _IOLBF, 0);
  19. #endif
  20. #endif /* _WIN32 */
  21.             break;
  22.         ......

tcpdump can also output packet-buffered through “-U/--packet-buffered“ option and call pcap_dump_flush API.