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E(and of)0 36 Q(fers it to each re)-.25 E(gistered compression handler)
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(The one with the highest bid is initialized with the \214rst block.)0
48 Q(Similarly)0 60 Q 2.5(,t)-.65 G
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/F0 10/Times-Roman@0 SF(\(Prior to 2.4.0, the format bidders were in)0
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(ry).15 E(.\))-.65 E(<h4 id="I/O Layer and Client Callbacks">)36 36 Q
(I/O Layer and Client Callbacks)36 48 Q(</h4>)36 60 Q
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/F0 10/Times-Roman@0 SF(As a result, there are fe)0 12 Q 2.5(wr)-.25 G
(estrictions on the beha)-2.5 E(vior of)-.2 E(the client callbacks.)0 24
Q(<p>)36 36 Q(The client read callback is e)0 48 Q(xpected to pro)-.15 E
(vide a block)-.15 E(of data on each call.)0 60 Q 2.5(Az)0 72 S
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/F0 10/Times-Roman@0 SF(blocks may be as small as one byte or as lar)0
12 Q(ge as the entire \214le.)-.18 E(In particular)0 24 Q 2.5(,b)-.4 G
(locks may be of dif)-2.5 E(ferent sizes.)-.25 E(<p>)36 36 Q
(The client skip callback returns the number of bytes actually)0 48 Q
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(The only requirement is that the skip not be lar)0 72 Q(ger)-.18 E(.)
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/F0 10/Times-Roman@0 SF(In particular)0 12 Q 2.5(,c)-.4 G
(lients are allo)-2.5 E(wed to return zero for an)-.25 E(y)-.15 E
(skip that the)0 24 Q 2.5(yd)-.15 G(on')-2.5 E 2.5(tw)-.18 G
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(<p>)36 48 Q -.25(Ke)0 60 S
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(clients reading from netw)0 72 Q(orks may pro)-.1 E(vide dif)-.15 E
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/F0 10/Times-Roman@0 SF(blocks on e)0 12 Q -.15(ve)-.25 G
(ry request and cannot skip at all;).15 E(adv)0 24 Q
(anced clients may use)-.25 E
(<a href="../html2/mmap.html">mmap\(2\)</a>)0 36 Q
(to read the entire \214le into memory at once and return the)0 48 Q
(entire \214le to libarchi)0 60 Q .3 -.15(ve a)-.25 H 2.5(sas).15 G
(ingle block;)-2.5 E(other clients may be)0 72 Q
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/F0 10/Times-Roman@0 SF(ne)0 12 Q(xt block on each request.)-.15 E
(<h4 id="Decompresssion Layer">)36 24 Q(Decompresssion Layer)36 36 Q
(</h4>)36 48 Q(The decompression layer not only handles decompression,)0
60 Q(it also b)0 72 Q(uf)-.2 E
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/F0 10/Times-Roman@0 SF(much nicer I/O model.)0 12 Q
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(tage peek/consume model.)-2.5 E 2.5(Ar)0 36 S
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(the decompression layer must pro)0 48 Q(vide a pointer to at least)-.15
E(that much data.)0 60 Q(If more data is immediately a)0 72 Q -.25(va)
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/F0 10/Times-Roman@0 SF(the format layer handles b)0 12 Q
(ulk data reads by asking for a minimum)-.2 E(of one byte and then cop)0
24 Q(ying as much data as is a)-.1 E -.25(va)-.2 G(ilable.).25 E(<p>)36
36 Q 2.5(As)0 48 S(ubsequent call to the)-2.5 E
(<b>consume</b>\(<code></code>\))0 60 Q(function adv)0 72 Q
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/F0 10/Times-Roman@0 SF(Note that data returned from a)0 12 Q
(<b>read_ahead</b>\(<code></code>\))0 24 Q
(call is guaranteed to remain in place until)0 36 Q(the ne)0 48 Q
(xt call to)-.15 E(<b>read_ahead</b>\(<code></code>\).)0 60 Q(Interv)0
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/F0 10/Times-Roman@0 SF(<b>consume</b>\(<code></code>\))0 12 Q
(should not cause the data to mo)0 24 Q -.15(ve)-.15 G(.).15 E(<p>)36 36
Q(Skip requests must al)0 48 Q -.1(wa)-.1 G(ys be handled e).1 E(xactly)
-.15 E(.)-.65 E(Decompression handlers that cannot seek forw)0 60 Q
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/F0 10/Times-Roman@0 SF(the API layer \214lls in a generic skip handler\
 that reads and discards data.)0 12 Q(<p>)36 24 Q 2.5(Ad)0 36 S
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(<dl compact>)0 48 Q(<dt>Re)0 60 Q(gistration/Con\214guration<dd>)-.15 E
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/F0 10/Times-Roman@0 SF(the decompression handler in)0 12 Q -.2(vo)-.4 G
-.1(ke).2 G 2.5(st).1 G(he internal)-2.5 E(<b>__archi)0 24 Q -.15(ve)
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(function to pro)0 36 Q(vide bid and initialization functions.)-.15 E
(This function returns)0 48 Q(<b></b><b>NULL</b>)0 60 Q
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/F0 10/Times-Roman@0 SF(<b></b><b>struct</b><b> decompressor_t</b>.)0 12
Q(This structure contains a)0 24 Q(<i></i><i>v)0 36 Q
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(slot that can be used for storing an)0 48 Q 2.5(yc)-.15 G
(ustomization information.)-2.5 E(<dt>Bid<dd>)0 60 Q
(The bid function is in)0 72 Q -.2(vo)-.4 G -.1(ke).2 G 2.5(dw).1 G
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/F0 10/Times-Roman@0 SF(The decompressor can access its con\214g data)0
12 Q(through the)0 24 Q(<i></i><i>decompressor</i>)0 36 Q
(element of the)0 48 Q(<b></b><b>archi)0 60 Q -.15(ve)-.25 G(_read</b>)
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/F0 10/Times-Roman@0 SF(The bid function is otherwise stateless.)0 12 Q
(In particular)0 24 Q 2.5(,i)-.4 G 2.5(tm)-2.5 G(ust not perform an)-2.5
E 2.5(yI)-.15 G(/O operations.)-2.5 E(<p>)36 36 Q(The v)0 48 Q
(alue returned by the bid function indicates its suitability)-.25 E
(for handling this data stream.)0 60 Q 2.5(Ab)0 72 S
(id of zero will ensure that this decompressor is ne)-2.5 E -.15(ve)-.25
G 2.5(ri).15 G -1.9 -.4(nv o)-2.5 H -.1(ke).4 G(d.).1 E 0 Cg EP
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/F0 10/Times-Roman@0 SF(Return zero if magic number checks f)0 12 Q
(ail.)-.1 E(Otherwise, your initial implementation should return the nu\
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(re).15 G(xample, if you v)-2.65 E(erify tw)-.15 E 2.5(of)-.1 G
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/F0 10/Times-Roman@0 SF(be careful to only inspect the data you are gi)0
12 Q -.15(ve)-.25 G(n.).15 E(\(The current decompressors require tw)0 24
Q 2.5(ob)-.1 G(ytes for correct bidding.\))-2.5 E(<dt>Initialize<dd>)0
36 Q(The winning bidder will ha)0 48 Q .3 -.15(ve i)-.2 H
(ts init function called.).15 E
(This function should initialize the remaining slots of the)0 60 Q
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/F0 10/Times-Roman@0 SF(object pointed to by the)0 12 Q
(<i></i><i>decompressor</i>)0 24 Q(element of the)0 36 Q
(<i></i><i>archi)0 48 Q -.15(ve)-.25 G(_read</i>).15 E(object.)0 60 Q
(In particular)0 72 Q 2.5(,i)-.4 G 2.5(ts)-2.5 G(hould allocate an)-2.5
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/F0 10/Times-Roman@0 SF(in the)0 12 Q(<i></i><i>data</i>)0 24 Q
(slot of that structure.)0 36 Q
(The init function is called with the block of data that)0 48 Q -.1(wa)0
60 S 2.5(su).1 G(sed for tasting.)-2.5 E
(At this point, the decompressor is responsible for all I/O)0 72 Q 0 Cg
EP
%%Page: 36 36
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/F0 10/Times-Roman@0 SF(requests to the client callbacks.)0 12 Q
(The decompressor is free to read more data as and when)0 24 Q
(necessary)0 36 Q(.)-.65 E(<dt>Satisfy I/O requests<dd>)0 48 Q
(The format handler will in)0 60 Q -.2(vo)-.4 G .2 -.1(ke t).2 H(he).1 E
(<i></i><i>read_ahead</i>,)0 72 Q 0 Cg EP
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/F0 10/Times-Roman@0 SF(<i></i><i>consume</i>,)0 12 Q(and)0 24 Q
(<i></i><i>skip</i>)0 36 Q(functions as needed.)0 48 Q(<dt>Finish<dd>)0
60 Q(The \214nish method is called only once when the archi)0 72 Q .3
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/F0 10/Times-Roman@0 SF(It should release an)0 12 Q
(ything stored in the)-.15 E(<i></i><i>data</i>)0 24 Q(and)0 36 Q
(<i></i><i>con\214g</i>)0 48 Q(slots of the)0 60 Q
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/F0 10/Times-Roman@0 SF(object.)0 12 Q(It should not in)0 24 Q -.2(vo)
-.4 G .2 -.1(ke t).2 H(he client close callback.).1 E(</dl>)0 36 Q
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/F0 10/Times-Roman@0 SF(The read formats ha)0 12 Q .3 -.15(ve a s)-.2 H
(imilar lifec).15 E(ycle to the decompression handlers:)-.15 E
(<dl compact>)0 24 Q(<dt>Re)0 36 Q(gistration<dd>)-.15 E
(Allocate your pri)0 48 Q -.25(va)-.25 G
(te data and initialize your pointers.).25 E(<dt>Bid<dd>)0 60 Q -.15(Fo)
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/F0 10/Times-Roman@0 SF(<b>read_ahead</b>\(<code></code>\))0 12 Q
(decompression method b)0 24 Q(ut not calling the)-.2 E
(<b>consume</b>\(<code></code>\))0 36 Q(method.)0 48 Q(This allo)0 60 Q
(ws each bidder to look ahead in the input stream.)-.25 E
(Bidders should not look further ahead than necessary)0 72 Q 2.5(,a)-.65
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/F0 10/Times-Roman@0 SF
(look aheads put pressure on the decompression layer to b)0 12 Q(uf)-.2
E(fer)-.25 E(lots of data.)0 24 Q(Most formats only require a fe)0 36 Q
2.5(wh)-.25 G(undred bytes of look ahead;)-2.5 E(look aheads of a fe)0
48 Q 2.5(wk)-.25 G(ilobytes are reasonable.)-2.5 E
(\(The ISO9660 reader sometimes looks ahead by 48k, which)0 60 Q
(should be considered an upper limit.\))0 72 Q 0 Cg EP
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/F0 10/Times-Roman@0 SF(<dt>Read header<dd>)0 12 Q
(The header read is usually the most comple)0 24 Q 2.5(xp)-.15 G
(art of an)-2.5 E 2.5(yf)-.15 G(ormat.)-2.5 E(There are a fe)0 36 Q 2.5
(ws)-.25 G(trate)-2.5 E(gies w)-.15 E(orth mentioning:)-.1 E -.15(Fo)0
48 S 2.5(rf).15 G
(ormats such as tar or cpio, reading and parsing the header is)-2.5 E
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/F0 10/Times-Roman@0 SF(the \214rst header read request may ha)0 12 Q .3
-.15(ve t)-.2 H 2.5(or).15 G(ead all headers into)-2.5 E
(memory and store that data, sorted by the location of the \214le)0 24 Q
(data.)0 36 Q(Subsequent header read requests will skip forw)0 48 Q
(ard to the)-.1 E(be)0 60 Q
(ginning of the \214le data and return the corresponding header)-.15 E
(.)-.55 E(<dt>Read Data<dd>)0 72 Q 0 Cg EP
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/F0 10/Times-Roman@0 SF(The read data interf)0 12 Q
(ace supports sparse \214les; this requires that)-.1 E
(each call return a block of data specifying the \214le of)0 24 Q
(fset and)-.25 E(size.)0 36 Q
(This may require you to carefully track the location so that you)0 48 Q
(can return accurate \214le of)0 60 Q(fsets for each read.)-.25 E
(Remember that the decompressor will return as much data as it has.)0 72
Q 0 Cg EP
%%Page: 46 46
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/F0 10/Times-Roman@0 SF(Generally)0 12 Q 2.5(,y)-.65 G(ou will w)-2.5 E
(ant to request one byte,)-.1 E -.15(ex)0 24 S(amine the return v).15 E
(alue to see ho)-.25 E 2.5(wm)-.25 G(uch data is a)-2.5 E -.25(va)-.2 G
(ilable, and).25 E(possibly trim that to the amount you can use.)0 36 Q
-1.1(Yo)0 48 S 2.5(us)1.1 G(hould in)-2.5 E -.2(vo)-.4 G .2 -.1(ke c).2
H(onsume for each block just before you return it.).1 E
(<dt>Skip All Data<dd>)0 60 Q(The skip data call should skip o)0 72 Q
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0 Cg EP
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/F0 10/Times-Roman@0 SF
(This is called automatically by the API layer just before each)0 12 Q
(header read.)0 24 Q
(It is also called in response to the client calling the public)0 36 Q
(<b>data_skip</b>\(<code></code>\))0 48 Q(function.)0 60 Q
(<dt>Cleanup<dd>)0 72 Q 0 Cg EP
%%Page: 48 48
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/F0 10/Times-Roman@0 SF
(On cleanup, the format should release all of its allocated memory)0 12
Q(.)-.65 E(</dl>)0 24 Q(<h4 id="API Layer">)36 36 Q(API Layer)36 48 Q
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/F0 10/Times-Roman@0 SF(<h3 id="WRITE ARCHITECTURE">)36 12 Q
(WRITE ARCHITECTURE)36 24 Q(</h3>)36 36 Q
(The write API has a similar set of four layers:)0 48 Q(an API layer)0
60 Q 2.5(,af)-.4 G(ormat layer)-2.5 E 2.5(,ac)-.4 G(ompression layer)
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/F0 10/Times-Roman@0 SF(one format and one compression can be re)0 12 Q
(gistered at a time.)-.15 E(<h4 id="I/O Layer and Client Callbacks">)36
24 Q(I/O Layer and Client Callbacks)36 36 Q(</h4>)36 48 Q(XXX T)0 60 Q
2.5(ob)-.8 G 2.5(ew)-2.5 G(ritten XXX)-2.5 E
(<h4 id="Compression Layer">)36 72 Q 0 Cg EP
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/F0 10/Times-Roman@0 SF(Compression Layer)36 12 Q(</h4>)36 24 Q(XXX T)0
36 Q 2.5(ob)-.8 G 2.5(ew)-2.5 G(ritten XXX)-2.5 E(<h4 id="F)36 48 Q
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/F0 10/Times-Roman@0 SF(XXX T)0 12 Q 2.5(ob)-.8 G 2.5(ew)-2.5 G
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(<h3 id="WRITE_DISK ARCHITECTURE">)36 72 Q 0 Cg EP
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/F0 10/Times-Roman@0 SF(WRITE_DISK ARCHITECTURE)36 12 Q(</h3>)36 24 Q
(The write_disk API is intended to look just lik)0 36 Q 2.5(et)-.1 G
(he write API)-2.5 E(to clients.)0 48 Q
(Since it does not handle multiple formats or compression, it)0 60 Q
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/F0 10/Times-Roman@0 SF(<h3 id="GENERAL SER)36 12 Q(VICES">)-.8 E
(GENERAL SER)36 24 Q(VICES)-.8 E(</h3>)36 36 Q(The)0 48 Q(<b>archi)0 60
Q -.15(ve)-.25 G(_read</b>,).15 E(<b>archi)0 72 Q -.15(ve)-.25 G
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/F0 10/Times-Roman@0 SF(and)0 12 Q(<b>archi)0 24 Q -.15(ve)-.25 G
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48 Q -.15(ve)-.25 G(</b>).15 E(object which pro)0 60 Q
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(\(Recall that ANSI/ISO C90 guarantees that you can cast freely between)
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/F0 10/Times-Roman@0 SF 2.5(ap)0 12 S
(ointer to a structure and a pointer to the \214rst element of that)-2.5
E(structure.\))0 24 Q(The)0 36 Q(<b>archi)0 48 Q -.15(ve)-.25 G(</b>).15
E(object has a magic v)0 60 Q(alue that indicates which API this object)
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/F0 10/Times-Roman@0 SF(slots for storing error information,)0 12 Q
(and function pointers for virtualized API functions.)0 24 Q
(<h3 id="MISCELLANEOUS NO)36 36 Q(TES">)-.4 E(MISCELLANEOUS NO)36 48 Q
(TES)-.4 E(</h3>)36 60 Q(Connecting e)0 72 Q(xisting archi)-.15 E
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/F0 10/Times-Roman@0 SF(quite dif)0 12 Q(\214cult.)-.25 E(In particular)
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/F0 10/Times-Roman@0 SF(<p>)36 12 Q -.15(Fo)0 24 S 2.5(re).15 G
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/F0 10/Times-Roman@0 SF(header)0 12 Q 2.5(,c)-.4 G
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