Writing to aMappedFile
m.Data(2).x
ans =
35330
4902
31861
16877
23791
61500
52748
16841
51314
58795
16860
43523
8957
5182
16864
60110
18415
16871
59373
61001
52007
16875
26374
28570
16783
4356
52847
53977
16858
38427
16067
33318
65372
48883
53612
16861
18882
39824
61529
16869
4-37
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4
Memory-MappingDataFiles
DeletingaMemoryMap
Itisnotnecessarytoexplicitlycalladestructormethodtocleara
memmapfile
objectfrommemorywhenyounolongerneedit.MATLABcallsthedestructor
foryouwheneveryoudoanyofthefollowing:
• Reassignanothervaluetothe
memmapfile
object’svariable
• Cleartheobject’svariablefrommemory
• Exitthefunctionscopeinwhichtheobjectwascreated
TheEffectofSharedDataCopiesOnPerformance
Whenyouassignthe
Data
fieldofthe
memmapfile
objecttoavariable,
MATLABmakesashareddatacopyofthemappeddata.Thisisveryefficient
asnomemoryactuallygetscopied.Inthefollowingstatement,
memdat
isa
shareddatacopyofthedatamappedfromthefile:
memdat = m.Data;
Whenyoufinishusingthemappeddata,makesuretoclearanyvariables
thatshareddatawiththemappedfilebeforeclearingtheobjectitself.Ifyou
cleartheobjectfirst,thenthesharingofdatabetweenthefileanddependent
variablesisbroken,andthedataassignedtosuchvariablesmustbecopied
intomemorybeforetheobjectisdestroyed.Ifaccesstothemappedfilewas
overanetwork,thencopyingthisdatatolocalmemorycantakeconsiderable
time.So,ifthestatementshownaboveassignsdatatothevariable
memdat
,
youshouldbesuretoclear
memdat
beforeclearing
m
whenyouarefinished
withtheobject.
Note Keepinmindthatthe
memmapfile
objectcanbeclearedinanyofthe
threewaysdescribedunder“DeletingaMemoryMap”onpage4-38.
4-38
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Memory-MappingDemo
Memory-MappingDemo
Inthis section...
“Introduction”onpage4-39
“ThesendFunction”onpage4-39
“TheanswerFunction”onpage4-41
“RunningtheDemo”onpage4-42
Introduction
Inthisdemonstration,twoseparateMATLABprocessescommunicatewith
eachotherbywritingandreadingfromasharedfile. Theysharethefileby
mappingpartoftheirmemoryspacetoacommonlocationinthefile.Awrite
operationtothememorymapbelongingtothefirstprocesscanbereadfrom
themapbelongingtothesecond,andviceversa.
OneMATLABprocess(running
send.m
)writesamessagetothefileviaits
memorymap.Italsowritesthelengthofthemessagetobyte1inthefile,
whichservesasameansofnotifyingtheotherprocessthatamessageis
available.Thesecondprocess(running
answer.m
)monitorsbyte1and,upon
seeingitset,displaysthereceivedmessage,putsitintouppercase,andechoes
themessagebacktothesender.
The sendFunction
Thisfunctionpromptsyoutoenterastringandthen,usingmemory-mapping,
passesthestringtoanotherinstanceofMATLABthatisrunningthe
answer
function.
Copythe
send
and
answer
functionstofiles
send.m
and
answer.m
inyour
currentworkingdirectory.Beginthedemonstrationbycalling
send
withno
inputs.Next,startasecondMATLABsessiononthesamemachine,andcall
the
answer
functioninthissession.Toexit,pressEnter.
function send
% Interactively y send a a message to o ANSWER using g memmapfile e class.
filename = fullfile(tempdir, 'talk_answer.dat');
4-39
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Memory-MappingDataFiles
% Create e the communications s file e if f it is s not already there.
if ~exist(filename, 'file')
[f, msg] = = fopen(filename, 'wb');
if f ~= -1
fwrite(f, zeros(1,256), , 'uint8');
fclose(f);
else
error('MATLAB:demo:send:cannotOpenFile', ...
'Cannot open file "%s": %s.', , filename, , msg);
end
end
% Memory y map the file.
m = = memmapfile(filename, 'Writable', true, 'Format', 'uint8');
while true
% Set t first t byte to zero, indicating a message is not
% yet t ready.
m.Data(1) = 0;
str = = input('Enter send string g (or r RETURN N to end): ', 's');
len = = length(str);
if (len == = 0)
disp('Terminating SEND D function.')
break;
end
str = = str(1:min(len, , 255));
% Message limited to 255 chars.
% Update the file via the memory map.
m.Data(2:len+1) = str;
m.Data(1)=len;
% Wait until the first byte is set t back to o zero,
% indicating that a response is s available.
while (m.Data(1) ) ~= 0)
pause(.25);
end
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Memory-MappingDemo
% Display the e response.
disp('response from ANSWER R is:')
disp(char(m.Data(2:len+1))')
end
The answerFunction
The
answer
functionstartsaserverthat,usingmemory-mapping,watches
foramessagefrom
send
.Whenthemessageisreceived,
answer
replacesthe
messagewithanuppercaseversionofit,andsendsthisnewmessageback
to
send
.
Touse
answer
,callitwithnoinputs:
function answer
% Respond to o SEND D using g memmapfile class.
disp('ANSWER server is s awaiting g message');
filename = fullfile(tempdir, 'talk_answer.dat');
% Create e the communications s file e if f it is s not already there.
if ~exist(filename, 'file')
[f, msg] = = fopen(filename, 'wb');
if f ~= -1
fwrite(f, zeros(1,256), , 'uint8');
fclose(f);
else
error('MATLAB:demo:answer:cannotOpenFile', ...
'Cannot open file "%s": %s.', , filename, , msg);
end
end
% Memory y map the file.
m = = memmapfile(filename, 'Writable', true, 'Format', 'uint8');
while true
% Wait till first byte is not zero.
while m.Data(1) ) == = 0
pause(.25);
4-41
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4
Memory-MappingDataFiles
end
% The e first t byte now contains the length of the e message.
% Get t it t from m m.
msg = = char(m.Data(2:1+m.Data(1)))';
% Display the e message.
disp('Received message from SEND:')
disp(msg)
% Transform the message e to all l uppercase.
m.Data(2:1+m.Data(1)) = = upper(msg);
% Signal to SEND that the response e is s ready.
m.Data(1) = 0;
end
RunningtheDemo
Toseewhatthedemonstrationlookslikewhenitisrun,first,starttwo
separateMATLABsessionsonthesamecomputersystem. Callthe
send
functioninoneandthe
answer
functionintheothertocreateamapineachof
theprocesses’memorytothecommonfile:
% Run n SEND in the e first t MATLAB B session.
send
Enter send string g (or r RETURN to end):
% Run n ANSWER in the second MATLAB session.
answer
ANSWER server is awaiting message
Next,enteramessageatthepromptdisplayedbythe
send
function.MATLAB
writesthemessagetothesharedfile.ThesecondMATLABsession,running
the
answer
function,loopsonbyte1ofthesharedfileand,whenthebyteis
writtenby
send
,
answer
readsthemessagefromthefileviaitsmemorymap.
The
answer
functionthenputsthemessageintouppercaseandwritesitback
tothefile,and
send
(waitingforareply)readsthemessageanddisplaysit:
% SEND D writes a message and d reads s the uppercase reply.
4-42
Memory-MappingDemo
Hello. Is s there e anybody out t there?
response from ANSWER is:
HELLO.
IS THERE ANYBODY OUT THERE?
Enter send string g (or r RETURN to end):
% ANSWER R reads s the e message from SEND.
Received message from SEND:
Hello.
Is there anybody out there?
send
writesasecondmessagetothefile.
answer
readsit,putitinto
uppercase,andthenwritesthemessagetothefile:
% SEND D writes a second d message to o the shared file.
I received d your reply.
response from ANSWER is:
I RECEIVED D YOUR REPLY.
Enter send string g (or r RETURN to end): <Enter>
Terminating SEND function.
% ANSWER R reads s the e second message.
Received message from SEND:
I received d your reply.
4-43
4
Memory-MappingDataFiles
4-44
5
InternetFileAccess
MATLABsoftwareprovidesfunctionsforexchangingfilesovertheInternet.
Youcanexchangefilesusingcommonprotocols,suchasFileTransferProtocol
(FTP),SimpleMailTransportProtocol(SMTP),andHyperTextTransfer
Protocol(HTTP).Inaddition,youcancreateziparchivestominimizethe
transmittedfilesize,andalsosaveandworkwithWebpages.
• “DownloadingWebContentandFiles”onpage5-2
• “CreatingandDecompressingZipArchives”onpage5-4
• “SendingEmail”onpage5-5
• “PerformingFTPFileOperations”onpage5-8
5
InternetFileAccess
DownloadingWebContentandFiles
MATLABprovidestwofunctionsfordownloadingWebpagesandfilesusing
HTTP:
urlread
and
urlwrite
. Withthe
urlread
function,youcanread
andsavethecontentsofaWebpagetoastringvariableintheMATLAB
workspace. Withthe
urlwrite
function,youcansaveaWebpage’scontent
toafile.
Becauseitcreatesastringvariableintheworkspace,the
urlread
functionis
usefulforworkingwiththecontentsofWebpagesinMATLAB.The
urlwrite
functionisusefulforsavingWebpagestoalocalfolder.
NoteWhenusing
urlread
,rememberthatonlytheHTMLinthatspecific
Webpageisretrieved. Thehyperlinktargets,images,andsoonarenot
retrieved.
IfyouneedtopassparameterstoaWebpage,the
urlread
and
urlwrite
functionsletyouuseHTTP
post
and
get
methods.Formoreinformation,see
the
urlread
and
urlwrite
referencepages.
Example—UsingtheurlreadFunction
ThefollowingproceduredemonstrateshowtoretrievethecontentsoftheWeb
pagelistingthefilessubmittedtotheMATLABCentralFileExchange,. It
assignstheresultstoastringvariable,
fullList
:
filex = 'http://www.mathworks.com/matlabcentral/fileexchange/';
fullList = urlread(filex);
Topassarguments,youcanincludethemmanuallyusingtheURL,orpass
parametersusingstandardHTTPmethods,including
post
and
get
.
Forexample,topassargumentsaspartoftheURL,andretrieveonlythefiles
uploadedtotheCentralFileExchangewithinthepast7daysthatcontainthe
word
Simulink
:
filex = sprintf('%s%s',...
'http://www.mathworks.com/matlabcentral/fileexchange/',...
5-2
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