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stringmemory-managementforth

Copy statically allocated string into dynamically allocated one

I was following an example, trying to explain to myself what it does:

: place                                   \ ptr len ptr2
    2dup                                  \ ptr len ptr2 len ptr2
    >r >r                                 \ ptr len ptr2
    char+                                 \ ptr len (ptr2 + 1)
    swap                                  \ ptr (ptr2 + 1) len
    chars                                 \ ptr (ptr2 + 1) (len * char)
    cmove                                 \ --
                                          \ from to        how-many 
    r> r>                                 \ ptr2 len
    c! ;                                  \ len = ptr2 ???
\ s" Hello! " name place

It all made sense, until the last instruction... where did I go wrong?

EDIT:

I've added some tracing:

: place                    \ ptr len ptr2                |
    2dup    cr .s          \ ptr len ptr2 len ptr2       | <5> 16490736 5 2126333248 5 2126333248  
    >r >r   cr .s          \ ptr len ptr2                | <3> 16490736 5 2126333248               
    char+   cr .s          \ ptr len (ptr2 + 1)          | <3> 16490736 5 2126333249               
    swap    cr .s          \ ptr (ptr2 + 1) len          | <3> 16490736 2126333249 5               
    chars   cr .s          \ ptr (ptr2 + 1) (len * char) | <3> 16490736 2126333249 5               
    cmove   cr .s          \ --                          | <0>                                     
                           \ from to        how-many     |
    r> r>   cr .s          \ ptr2 len                    | <2> 5 2126333248  ok
    c! ;                   \ ptr2 = len ???              |
\ s" Hello! " name place
Solution

  • I think the first part of Will Hartung's answer is correct.

    The string representation that is being used is as he described, namely a character count and then the actual string.

    So in your example c! is storing the length of the string in the the first cell of the memory that starts with ptr2.

    So if you wanted to retrieve your string you would only need to know the address, you could then fetch from that address to get the length n, and the fetch n chars starting at the address + 1.