This - famous Perl guts illustrated, and $5, will make you a coffee.

I suspect you get traps because you substitute Perl's allocated memory with your own?

Allocating more place than needed for an SV is why SvGROW exists: 

   Although Perl will automatically grow strings for you, if you need 
+to
   force Perl to allocate more memory for your SV, you can use the mac
+ro

       SvGROW(SV*, STRLEN newlen)

[download]
Then, reading further in perlguts gives all your required manipulations: 
  Offsets
    Perl provides the function "sv_chop" to efficiently remove charact
+ers
    from the beginning of a string; you give it an SV and a pointer to
    somewhere inside the PV, and it discards everything before the poi
+nter.
    The efficiency comes by means of a little hack: instead of actuall
+y
    removing the characters, "sv_chop" sets the flag "OOK" (offset OK)
+ to
    signal to other functions that the offset hack is in effect, and i
+t puts
    the number of bytes chopped off into the IV field of the SV. It th
+en
    moves the PV pointer (called "SvPVX") forward that many bytes, and
    adjusts "SvCUR" and "SvLEN".

[download]

In reply to Re^3: XS Prepending space to an SVs PV by vkon
in thread XS Prepending space to an SVs PV by BrowserUk

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