Error when appending string from word or variable

I'm trying to append two strings in gforth, but I get some scary looking error messages.

While s" foo" s" bar" append type cr works fine, as soon as I start storing strings in variables or creating them from words, I get errors. For instance:

: make-string ( -- s )
    s" foo" ;

: append-print ( s s -- )
    append type cr ;

make-string s" bar" append-print

Running it produces the following error:

$ gforth prob1.fs -e bye
gforth(41572,0x7fff79cc2310) malloc: *** error for object 0x103a551a0: pointer being realloc'd was not allocated
*** set a breakpoint in malloc_error_break to debug

Abort trap: 6.

I'm well versed in C, so it seems pretty clear that I'm using Forth incorrectly! I suppose I need to learn something very basic about memory management in Forth.

Can anyone please explain what goes wrong here, and what I should do?

I also run into problems when I try to append a string that is stored in a variable:

variable foo
s" foo" foo !

foo s" bar " append type cr

This ends in a loop that I have to break:

$ gforth prob2.fs
foo��^C
in file included from *OS command line*:-1
prob2.fs:4: User interrupt
foo s" bar " append >>>type<<< cr
Backtrace:
$10C7C2E90 write-file

For reference, I'm using gforth 0.7.2 on Mac OS X. I would be very grateful for some good explanations on what's going on.

Update

I can see the definition of append:

see append
: append
  >l >l >l >l @local0 @local1 @local3 + dup >l resize throw >l @local4 @local0 @local3 + @local5
  move @local0 @local1 lp+!# 48 ; ok

So, it would seem I need to manage memory myself in Forth? If so, how?

Solution

Andreas Bombe provides the clue below. The final program that works would be

: make-string ( -- s )
  s" foo" ;

: append-print
  s+ type cr ;

make-string s" bar" append-print

Output is

$ gforth b.fs -e bye
foobar

Solution

append uses resize on the first string make space to append the second string. This requires that the string be allocated on the heap.

When you compile a string with s" into a word, it gets allocated in the dictionary. If you try resize (directly or indirectly through append) on that pointer you will get the error you see.

Normally s" has undefined interpretation semantics. Gforth defines its interpretation semantics for convenience as allocating the string on the heap. That's why it works (in gforth) as long as you don't compile it.

Edit:

I've found the definition of append, it's part of libcc.fs (a foreign function interface builder as it seems) and not a standard word. This is the definition in the source, more readable than the see decompile:

: append { addr1 u1 addr2 u2 -- addr u }
    addr1 u1 u2 + dup { u } resize throw { addr }
    addr2 addr u1 + u2 move
    addr u ;

Immediately before that is a definition of s+:

: s+ { addr1 u1 addr2 u2 -- addr u }
    u1 u2 + allocate throw { addr }
    addr1 addr u1 move
    addr2 addr u1 + u2 move
    addr u1 u2 +
;

As you can see this one allocates new memory space instead of resizing the first string and concatenates both strings into it. You could use this one instead. It is not a standard word however and just happens to be in your environment as an internal implementation detail of libcc.fs in gforth so you can't rely on it being available elsewhere.