Please help me understand this bit-wise manipulation of pointer values

Question

I am not able to understand why is addr being typecasted to long, and then complemented with expression.. basically the whole line involving the calculation of peekAddr

void *addr;
char *peekAddr ;
peekAddr = (char *) ((long)addr & ~(sizeof(long) - 1 ) ) ;
peekWord = ptrace( PTRACE_PEEKDATA, pid, peekAddr, NULL ) ;

Answer 1

sizeof (long)     = (0)00000100
sizeof(long)-1    = (0)00000011
~(sizeof(long)-1) = (1)11111100

so 2 bits set to 0 make the address aligned to 4 bytes. additionally it's mostly used when the address was already incremented by sizeof(long)-1

Answer 2

It's cast to long because (1) you can't do any operations on a void* except cast it and (2) on the author's platform, a void* value just so happens to fit in a long. He should really have used uintptr_t or size_t instead.

What the piece of code does:

sizeof(long) - 1

is most likely either 3 or 7, depending on the platform.

~(sizeof(long) - 1)

is a bitmask that selects all but the last few bits.

((long)addr & ~(sizeof(long) - 1))

is addr rounded down/aligned to address a long-sized chunk. Rounding occurs because the last lg(3) or lg(7) bits are zeros while the rest is copied from addr (where lg is integer binary logarithm).