Pointers and arrays

Question

Please can somebody explain how the answer is 3 not 6??

Comments

Since p1 and p2 are integers pointers. If it was char pointers answer would have been 6.

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Assigning character pointer inside an integer pointer will not give an error?

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@srestha : Formal argument is itself integer ! look at the function.

Answer 1

You should not use an integer pointer to point to a character or anything other than int) or else the behaviour is undefined in C.

Comments

yes, the code is wrong on many aspects. Just using an int pointer to point to a char itself is not valid in C -- because it can break the alignment usage and give unexpected results. Then, assuming this works; the code is using integer pointer to point to character array "GATECS" and incrementing the pointer with int size as 2. So, first comparison will be "TE" == 0, then "CS" == 0 and then "\0..." == 0, here the last "..." is junk -- it may or may not be 0 and is unallocated memory. This is another reason for the undefined behaviour.

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alignment usage means data alignment and data structure padding , Here problem when we do padding by.pointer conversion from char to int ?

why is it comparing with o and lastly 0 is comparing with junk bits?

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A data object 'X' is aligned means X always start in memory from some multiple of the alignment. For structure, this can happen and that involves structure padding. This can also happen for integers where an integer always starts from say a multiple of 4. 

while (*++p1);

Here, ++p1 happens first. It does p1 = p1+1, which numerically becomes p1 = p1 + sizeof(*p1) as p1 is a pointer. 

The next part is what is missed in the question - *. '*' returns the value and p1 being an int pointer '*' returns 4 bytes (or 2 as given in the question). But only one byte for '\0' is allocated (this is done implicitly by the compiler for any string literal).