reading perldoc perlguts isn't is what you need: 
    You can get and set the current length of the string stored in an 
+SV
    with the following macros:

        SvCUR(SV*)
        SvCUR_set(SV*, I32 val)

    You can also get a pointer to the end of the string stored in the 
+SV
    with the macro:

        SvEND(SV*)

    But note that these last three macros are valid only if "SvPOK()" 
+is
    true.

[download]
also, search for SvCUR in perldoc perlguts

And, if all else fails, one can always do RTFS :):):)

In reply to Re: XS Prepending space to an SVs PV by vkon
in thread XS Prepending space to an SVs PV by BrowserUk

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