Dear Rene,
thank you very much for your help. Your samples work perfectly. I
found an interesting bug/feature of the rootd server. If I change
in your samples TFile to TNetFile and file to
root://server/junk.root
where server the name of the computer with the running rootd. Write
part works perfectly, but the read part says:
"Error in <TNetFile::TNetFile> file already opened in write mode".
When I use mounted path via nfs everything is OK. Is it possible to
use rootd in a DAQ/Viewer way how it's shown in your examples?
Best regards, Yuriy.
RB> You will find below two small scripts treew.C and treer.C
RB> Run treew.C in a Root session. This will update the file at regular
RB> intervals.
RB> In a separate Root session, run treer.C. This will update an histogram
RB> reading fresh data only.
RB> You can combine this example with the new producer/consumer example
RB> in $ROOTSYS/tutorials/spyserv.C and spy.C (version 3.03 only in CVS)
RB> void treew() {
RB> TFile f("junk.root","recreate");
RB> TNtuple *ntuple = new TNtuple("ntuple","Demo ntuple","px:py:pz:random:i");
RB> Float_t px, py, pz;
RB> for ( Int_t i=0; i<1000000; i++) {
RB> gRandom->Rannor(px,py);
RB> pz = px*px + py*py;
RB> Float_t random = gRandom->Rndm(1);
RB> ntuple->Fill(px,py,pz,random,i);
RB> if (i%1000 == 1) {
RB> ntuple->AutoSave();
RB> f.SaveSelf();
RB> }
RB> }
RB> }
RB> void treer() {
RB> TFile f("junk.root");
RB> TCanvas c1;
RB> Int_t first = 0;
RB> while(1) {
RB> f.ReadKeys();
RB> TTree *ntuple = (TTree*)f.Get("ntuple");
RB> if (first == 0) ntuple->Draw("px>>hpx","","",10000000,first);
RB> else ntuple->Draw("px>>+hpx","","",10000000,first);
RB> first = (Int_t)ntuple->GetEntries();
RB> delete ntuple;
RB> c1.Update();
RB> gSystem->Sleep(1000); //sleep 1 second
RB> }
RB> }
RB> Rene Brun
RB> Yuriy Prokazov wrote:
>>
>> Dear ROOTers,
>> I'd like to make work 2 independent processes where one of them
>> writes information into the file and other reads it. If it is
>> possible I would like to use TNetFile (if these processes are
>> running on a different computers) or via TFile if (they are in
>> the single box). I faced with following problem: from the WRITE
>> process I create file using TFile and create TTree object with
>> the single branch of a simple type. Every second WRITE process
>> fills the branch and saves some amount of data. In the READ
>> part I open the file using TFile::Open().
>>
>> The problem is that in the READ call tree->GetEntries() always
>> returns the same number. From the ROOT source code it is
>> obviosly clear that it could return only the one value that have
>> been saved during initialization.
>>
>> But the same time file->GetFileSize() show that the file is
>> growing.
>>
>> Is there any way how to make independed reading/writing and say
>> to READ process that the file was changed and it is required to
>> read new data?
>>
>> Thank you.
>> Best regards,
>> Yuriy.
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