> What is the internal representation of string? Is it basically a C-string 
> [with or without terminating '\0'] plus integer storing its size? Or is it 
> something more sophisticated?

Like all heap blocks, strings contain a header defining the size of
the string in machine words.  The actual block contents are:
- the characters of the string
- padding bytes to align the block on a word boundary.  
  The padding is one of
    00
    00 01
    00 00 02
    00 00 00 03
  on a 32-bit machine, and up to 00 00 .... 07 on a 64-bit machine.

Thus, the string is always zero-terminated, and its length can be
computed as follows:

    number_of_words_in_block * sizeof(word) + last_byte_of_block - 1

The null-termination comes handy when passing a string to C, but is
not relied upon to compute the length (in Caml), allowing the string
to contain nulls.

> Also, do functions like String.sub implement 
> copy-on-write mechanism or do they copy when they are called?

They copy when they are called.  Caml strings really behave like
compactly-represented character arrays.

Hope this answers your question,

- Xavier Leroy
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