offset a cast?
Hi
I have a uint8_t array that I am casting to a struct:
However the first two bytes in the buffer are not required / not wanted – is there a way I can offset the cast to start working from the 3rd byte in on the buffer?Code:void processe(uint8_t *buffer, int length) { AppConfigData_t* configData; configData = (AppConfigData_t *) buffer; } // processe
Many thanks
David
Sure:
Code:configData = (AppConfigData_t *) buffer + 2;
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Hi - thanks for the reply - sadly if i do that the data within the strut get mangled, do I need to specify its the pointer I want to increment or something?
David
> do I need to specify its the pointer I want to increment or something?
That's what it does...
Perhaps you want (buffer + 2) so that the 2 is added before the cast - I'm not sure of the order precedence for casts.Originally Posted by DavidDobson
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Mats
Compilers can produce warnings - make the compiler programmers happy: Use them!
Please don't PM me for help - and no, I don't do help over instant messengers.
> I'm not sure of the order precedence for casts.
Same as +... but they have right-left associativity.
cast has a preceedence, so I think it should be
configData = (AppConfigData_t *) (buffer + 2);
All problems in computer science can be solved by another level of indirection,
except for the problem of too many layers of indirection.
– David J. Wheeler
same as unary +-
All problems in computer science can be solved by another level of indirection,
except for the problem of too many layers of indirection.
– David J. Wheeler
Ideally it should be...
Code:configData = (AppConfigData_t *) (buffer + sizeof(the only member he wants to skip (all of or part of the first)));
ah! that did it - thanks everyone
But don't rely on (aka, it's wrong) your AppConfigData_t pointer being meaningfully aligned for your architecture.
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
If at first you don't succeed, try writing your phone number on the exam paper.
I do agree that this type of "changing the pointer" may well cause unaligned data structures. But it does depend (a lot) on the content of AppConfigData_t's actual alignment requirements. If it consists solely of char arrays, any alignment should work OK. If it has integers in it, it may need to be aligned to an even 4 bytes, so if buffer in itself is aligned to 4 bytes, then the new pointer will not be aligned correctly - which can lead to a crash due to access of unaligned data.
--
Mats
Compilers can produce warnings - make the compiler programmers happy: Use them!
Please don't PM me for help - and no, I don't do help over instant messengers.
