About string header

About string header

[Xue Fuqiao]

In (info "(elisp) Garbage Collection"):

 -- Command: garbage-collect
[...]
So an overall result is:
          ((`conses' CONS-SIZE USED-CONSES FREE-CONSES)
           (`symbols' SYMBOL-SIZE USED-SYMBOLS FREE-SYMBOLS)
           (`miscs' MISC-SIZE USED-MISCS FREE-MISCS)
           (`strings' STRING-SIZE USED-STRINGS FREE-STRINGS)
           (`string-bytes' BYTE-SIZE USED-BYTES)
           (`vectors' VECTOR-SIZE USED-VECTORS)
           (`vector-slots' SLOT-SIZE USED-SLOTS FREE-SLOTS)
           (`floats' FLOAT-SIZE USED-FLOATS FREE-FLOATS)
           (`intervals' INTERVAL-SIZE USED-INTERVALS FREE-INTERVALS)
           (`buffers' BUFFER-SIZE USED-BUFFERS)
           (`heap' UNIT-SIZE TOTAL-SIZE FREE-SIZE))
[...]
    STRING-SIZE
          Internal size of a string header, i.e., `sizeof (struct
          Lisp_String)'.

What does the "string header" mean here?  I had searched the manuals and source files in src/ but I didn't find anything useful.

-- 
Best regards, Xue Fuqiao.
http://www.emacswiki.org/emacs/XueFuqiao

Re: About string header

[Stefan Monnier]

> What does the "string header" mean here?  I had searched the manuals
> and source files in src/ but I didn't find anything useful.

Every string is made of 2 parts:
The first part is a "Lisp_String" struct of fixed size which holds:
- the size of the string (in bytes and in chars).
- a pointer to the string's text-properties (stored in an "intervals tree").
- a pointer to the second part.
The second part are the actual bytes of the string, which are stored
elsewhere (and can be relocated so as to compact the memory).
The "string header" is the first part.


        Stefan

Re: About string header

[Xue Fuqiao]

On Thu, 28 Feb 2013 08:47:54 -0500
Stefan Monnier <monnier@iro.umontreal.ca> wrote:

> > What does the "string header" mean here?

> Every string is made of 2 parts:
> The first part is a "Lisp_String" struct of fixed size which holds:
> - the size of the string (in bytes and in chars).
> - a pointer to the string's text-properties (stored in an "intervals tree").
> - a pointer to the second part.
> The second part are the actual bytes of the string, which are stored
> elsewhere (and can be relocated so as to compact the memory).
> The "string header" is the first part.

Thanks for your explanation.  Can this be in the elisp manual?

>         Stefan

-- 
Best regards, Xue Fuqiao.
http://www.emacswiki.org/emacs/XueFuqiao