Bug 1:

You are including the NUL in the buffer, and you are falsely claiming there is a trailing NUL after the buffer. 

sv_usepvn_flags(x, dx, 8, SV_HAS_TRAILING_NUL);

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should be 
sv_usepvn_flags(x, dx, 7, SV_HAS_TRAILING_NUL);

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and it should be written as 
sv_usepvn_flags(x, dx, strlen(dx), SV_HAS_TRAILING_NUL);

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Bug 2:

The buffer can't be modified, but you offer no protection. For example, $x =~ s/./a/s; will result in undefined behaviour (which fortunately segfaults on my machine).

Add the following: 

SvREADONLY_on(x);

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I found you can't enlarge the SV containing C string by using .=, no any error throw though.

I already mentioned that. Or rather, I mentioned that it causes the buffer to get replaced with a new one that contains a copy.

Before $x .= "a";: 

SV = PV(0x3e1a1785e0) at 0x3e1a197528
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x35260c8bbd5 "dddddd"       <-- Pointer into C static storage
  CUR = 6
  LEN = 0                           <-- Doesn't belong to Perl.

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After $x .= "a";: 

SV = PV(0x3e1a1785e0) at 0x3e1a197528
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x3e1a19fe50 "dddddda"\0     <-- New buffer (copy + "a")
  CUR = 7
  LEN = 17                          <-- Belongs to Perl.

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However, SvREADONLY_on(x) will prevent this. So if you want to allow this, you'll need to remove the READONLY flag, which means you'll need to make a copy of the constant string.

In reply to Re^2: What difference between malloc and Newx, how attach a C string to SV directly? by ikegami
in thread What difference between malloc and Newx, how attach a C string to SV directly? by xiaoyafeng

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