[lwip-devel] [bug #26658] ping tool ICMP checksum error.

[David Empson]

Follow-up Comment #3, bug #26658 (project lwip):

hanhui's comment #1 refers to the issue that a macro parameter with side
effects (such as pre-increment or post-increment) will cause the side effect
to occur multiple times if the parameter is referenced more than once in the
macro.

In this case, assuming htons() is defined like this:

#define htons(x) LWIP_PLATFORM_HTONS(x)

#define LWIP_PLATFORM_HTONS(x) (u16_t)((((x) & 00ff) << 8) | 
(((x) & 0xff00) >> 8))

then the expression htons(++ping_seq_num) will expand to

(u16_t)((((++ping_seq_num) & 00ff) << 8) | 
(((++ping_seq_num) & 0xff00) >> 8))

As you can see, ping_seq_num is incremented twice.

This use of pre-increment or post-increment operators within the parameters
to htons() is only valid if htons() is required to be a function. If it is
ever implemented as a macro, the expression will have unexpected side
effects.

The question then is whether htons() etc. are supposed to be functions. If
this is a documented requirement, then this ping code is correct. If the port
is allowed to implement htons() as a macro, then this ping code is wrong, and
there may be other instances in the LWIP code where the same assumption is
made.


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