I recently received a poorly documented, poorly written DLL that I'm attempting to interface with perl. (Activestate, win32, 5.8.7). Obviously, there's a problem, the first one being that I don't understand XS very well yet... but I'm working on that. The gist of my question is if there's a good way to avoid an extra copy because there's a LOT of data. If there's no way to do so, I'll just use the straight-forward way.

Basically, one of the C routines will be looped over, and will easily return several thousand different 83-byte char buffers (IE, I pass in an 83-byte buffer, and it pushes data into it... several thousand times)

If I understand newSVpv correctly, that copies the message buffer into the new SV, which is an extra copy that I shouldn't need. What I'd like to do is something like 
AV*
GetTable()
PREINIT:
    AV *array;
    int max_rows, max_cols, row, col;
    char *message;
    SV *message_sv;
CODE:
    max_rows = 10;
    max_cols = 5;
    array = (AV*) sv_2mortal((SV*) newAV());
    for( row = 0; row < max_rows; row++) {
      for( col = 0; col < max_cols; col++) {
        message_sv = newSV(83);
        message = SvPV_nolen(message_sv);
        ExpensiveGetString(row, col, message, 83);
        av_push(array, message_sv);
      }
    }
    RETVAL = array;
OUTPUT:
    RETVAL

[download]
Where ExpensiveGetString is approximately equal to 
strcpy(message, "12345678901234567890123456789012345678901234567890123
+456789012345678901234567890123456789012345678901234567890123456789012
+\0");

[download]
which is exactly what I'm using to test it. I get an array of undefs back, which tells me that I'm missing something.

Why doesn't this work? Is there a better way to avoid copying the data twice (some of the datasets are in the 30k rows and 70 columns range)?

In reply to XS efficient copy by TheDauthi

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