From: Eric Hodel <drbrain@...7.net>
Date: 2011-02-05T05:20:51+09:00
Subject: Re: Does RSTRING_PTR() return a '\0' terminated C?

On Feb 4, 2011, at 12:07 PM, Eric Wong wrote:
> Iņaki Baz Castillo <ibc@aliax.net> wrote:
>> Hi, in a Ruby C extension I need to get a '\0' terminated string given
>> a Ruby string. I'm just interesed in Ruby 1.9.
>> 
>> I'm testing that RSTRING_PTR(str) does returns a '\0' terminated
>> string, ot it seems so, but I'm not sure. Source code says:
>> 
>> #define RSTRING_PTR(str) \
>>    (!(RBASIC(str)->flags & RSTRING_NOEMBED) ? \
>>     RSTRING(str)->as.ary : \
>>     RSTRING(str)->as.heap.ptr)
>> 
>> Honestly, no idea about what it does :)
> 
> It's one of my favorite things about Ruby 1.9: small strings (3 words -
> 1 byte) are embedded directly inside the object to avoid malloc()
> overhead.
> 
> RSTRING_PTR() checks for the embed flag, and it'll return the embedded C
> string, otherwise it'll return the pointer allocated from malloc().
> 
> The same thing is done with RArray and RStruct, looking at the structure
> definitions will be useful.
> 
>> So, can somebody ensure me that RSTRING_PTR() returns a pointer to C
>> char finished with '\0'?
> 
> It's always '\0'-terminated in Ruby[1], it makes life much easier for
> interacting with existing C functions that assume C strings (e.g.
> *printf("%s"), open(), rename(), unlink(), etc...)
> 
> [1] - unless there's a bug somewhere, or somebody hell bent on using the
>      extra byte :)

I don't think this is true, use RSTRING_LEN with RSTRING_PTR to get the length since the string may contain embedded NULLs.

If you want a safe C string use StringValueCStr().

PS: See also README.EXT section 1.3