Re: is this correct C? (char*)spm += spm->partitionMapLength;

Spiros Bousbouras a écrit :
On Nov 30, 1:00 pm, O_TEXT <O_T...@xxxxxxxxx> wrote:
is line 11 valid?
if yes, is it equivalent to line 10?

1 typedef unsigned char uint8_t;
2 struct sparablePartitionMap
3 {
4 uint8_t partitionMapLength;
5 } __attribute__ ((packed));
6 void initialise(char *devicename)
7 {
8 struct sparablePartitionMap *spm;
9 spm += spm->partitionMapLength;
10 spm = ( struct sparablePartitionMap *) ((char*)spm +

spm->partitionMapLength);
11 (char*)spm += spm->partitionMapLength;
12 }

No , line 11 is not valid C because the expression
(char *)spm is not an lvalue.

gcc 4.1 gives this error message (not a lvalue).

I do not the requirment for a value to be a lvalue.
This is just a curiosity question.

Nevertheless I think
some compilers allow the construct as an extension
although I don't know its meaning.

Might be older gcc.

I note also that on line 9 you dereference the pointer
spm although it hasn't been initialised. That's undefined
behaviour.

Yes.
The code comes from udftools-1.0.0b3 .
I just removed line which was comiling okay and rewrittent line 11 to 9 and 10, just in order to understand compilation issue.

Furthermore if you post code with non standard constructs
like __attribute__ you should explain what they mean.

These are understood by gcc.
I assume the packed just mean there is no gap betwen struct fields.

Personally I would like to know whether lines 9 and 10 do
the same thing assuming that spm has been properly
initialised.

gcc answers no:

../a.out
spm : 0x2b3e202a8ca0
spm : 0x2b3e202a8ca5
spm : 0x2b3e202a8ccd

int main()
{
long *spm;
printf ("spm : %p \n", spm );
spm = ( long *) ((char*)spm + 5);
printf ("spm : %p \n", spm );
spm += 5;
printf ("spm : %p \n", spm );
return 0;
}
.

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