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Date:      Tue, 9 Apr 2002 16:02:01 -0700 (PDT)
From:      Julian Elischer <julian@elischer.org>
To:        net@freebsd.org
Subject:   IP checksup update
Message-ID:  <Pine.BSF.4.21.0204091557570.57350-100000@InterJet.elischer.org>
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Several years ago I wrte the following macro
to update a checksum when changing a word
in an IP packet.

here's the code...


  #define FIXSUM16(c, op, np)                                 \
    do {                                                      \
      (c) -= (u_int16_t) ~*((u_int16_t *) (op));              \
      if ((c) < 0) {                                          \
        (c) += 0xffff;                                        \
      }                                                       \
      (c) -= (u_int16_t)  *((u_int16_t *) (np));              \
      if ((c) < 0) {                                          \
        (c) += 0xffff;                                        \
      }                                                       \
    } while (0)



The question is:
apparently there are several "tricks" with this checksum
and + and - 0. Does anyone know how to test if this macro gets it
right?




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