/*?Find?the?first?occurrence?of?C?in?S.??*/
char?*
strchr?（s?c_in）
?????const?char?*s;
?????int?c_in;
{
??const?unsigned?char?*char_ptr;
??const?unsigned?long?int?*longword_ptr;
??unsigned?long?int?longword?magic_bits?charmask;
??unsigned?reg_char?c;

??c?=?（unsigned?char）?c_in;

??/*?Handle?the?first?few?characters?by?reading?one?character?at?a?time.
?????Do?this?until?CHAR_PTR?is?aligned?on?a?longword?boundary.??*/
??for?（char_ptr?=?（const?unsigned?char?*）?s;
???????（（unsigned?long?int）?char_ptr?&?（sizeof?（longword）?-?1））?!=?0;
???????++char_ptr）
????if?（*char_ptr?==?c）
??????return?（void?*）?char_ptr;
????else?if?（*char_ptr?==?‘/0‘）
??????return?NULL;

??/*?All?these?elucidatory?comments?refer?to?4-byte?longwords
?????but?the?theory?applies?equally?well?to?8-byte?longwords.??*/

??longword_ptr?=?（unsigned?long?int?*）?char_ptr;

??/*?Bits?31?24?16?and?8?of?this?number?are?zero.??Call?these?bits
?????the?“holes.“??Note?that?there?is?a?hole?just?to?the?left?of
?????each?byte?with?an?extra?at?the?end:

?????bits:??01111110?11111110?11111110?11111111
?????bytes:?AAAAAAAA?BBBBBBBB?CCCCCCCC?DDDDDDDD

?????The?1-bits?make?sure?that?carries?propagate?to?the?next?0-bit.
?????The?0-bits?provide?holes?for?carries?to?fall?into.??*/
??switch?（sizeof?（longword））
????{
????case?4:?magic_bits?=?0x7efefeffL;?break;
????case?8:?magic_bits?=?（（0x7efefefeL?<????default:
??????abort?（）;
????}

??/*?Set?up?a?longword?each?of?whose?bytes?is?C.??*/
??charmask?=?c?|?（c?<??charmask?|=?charmask?<??if?（sizeof?（longword）?>?4）
????/*?Do?the?shift?in?two?steps?to?avoid?a?warning?if?long?has?32?bits.??*/
????charmask?|=?（charmask?<??if?（sizeof?（longword）?>?8）
????abort?（）;

??/*?Instead?of?the?traditional?loop?which?tests?each?character
?????we?will?test?a?longword?at?a?time.??The?tricky?part?is?testing
?????if?*any?of?the?four*?bytes?in?the?longword?in?question?are?zero.??*/
??for?（;;）
????{
??????/*?We?tentatively?exit?the?loop?if?adding?MAGIC_BITS?to
	?LONGWORD?fails?to?change?any?of?the?hole?bits?of?LONGWORD.

	?1）?Is?this?safe???Will?it?catch?all?the?zero?bytes?
	?Suppose?there?is?a?byte?with?all?zeros.??Any?carry?bits
	?propagating?from?its?left?will?fall?into?the?hole?at?its
	?least?significant?bit?and?stop.??Since?there?will?be?no
	?carry?from?its?most?significant?bit?the?LSB?of?the
	?byte?to?the?left?will?be?unchanged?and?the?zero?will?be
	?detected.

	?2）?