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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