Variable size character array

ROOT
1

Hi,

I have a tree that contains some compressed data as a character buffer. The leaf with the data has the structure: “size/I:buf[size]/b”. How can I put this buffer into a string?

Thanks,
Vyacheslav

2

[quote]How can I put this buffer into a string? [/quote]What kind of strings?
Typically you should use:struct Holder { Int_t size; Char_t buf[MAX_BUFFER_SIZE]; }; ... Holder holder; tree->SetBranchAddress(name,&holder); tree->GetEntry(some_entry);and buf would contain the information you need (however the contain may or may not be null terminated depending on how it was filled).

Cheers,
Philippe.

3

Hi,

character buffers are automatically converted to python strings, and so it is assumed that they have a terminating \0. If not, then the approach that Philippe lays out needs to be followed. The Holder class can be made available to python by having CINT interpret it:[code]import ROOT

ROOT.gROOT.ProcessLine( “const int MAX_BUFFER_SIZE = 42;” )
ROOT.gROOT.ProcessLine( “struct Holder {
Int_t size;
Char_t buf[MAX_BUFFER_SIZE];
};” )

holder = ROOT.Holder()[/code]
Cheers,
Wim

4

The buffer in question is a png image stored as a char array, so not null terminated.

I followed the approach you suggested. After creating the holder struct and making it available in python, I set the branch address as follows:

mytree.SetBranchAddress(bname,AddressOf(holder,'buf'))
mytree.SetBranchAddress(bname,AddressOf(holder,'size'))

mytree.GetEntry(some_entry)

#save the image to a file
fil = open("image.png","wb")
fil.write(buffer(holder.buf,0,holder.size))
fil.close()

The problem is only the first 8 bytes from the buffer are saved. Is it not possible to handle raw binary data this way?

Thanks,
Vyachelsav

5

Vyachelsav,

well, the obvious first questions are: what is holder.size, and what is the (static) size of buf?

Cheers,
Wim

6

Hi Wim,

Checked those, no obvious problem there. The buffer size is set to 130000, while the the holder.size value prints to 123040.

7

Vyachelsav,

too many string conversions … and with that the Holder is not going to work. That leaves the python array:from array import array buf = array('c',['\0']*bufsize) mytree.SetBranchAddress(bname,buf)and you can then either write the array as you intended with the buffer object, or use the array.tofile() function (which should be a lot faster).

Cheers,
Wim