Re: [Chicken-users] FFI with callbacks for gnu readline

[Thomas Chust]

On Wed, 18 Jan 2006, LeviPearson wrote:

> The readline library expects to be able to call a generator function 
> with a string and an index and get back a newly-allocated string that it 
> can use and then free().  To accommodate this, I wrote a C function that 
> calls a define-external'd scheme function that returns a scheme-object 
> containing the string.  I found that returning a c-string would 
> sometimes give me some garbage characters as the strings were not always 
> null-terminated at the right spot.

Hello,

actually I doubt that the termination of strings with a '\0' character is 
broken. Maybe the problem is rather related to garbage collection 
somehow, if the string is used for a longer time by the readline 
library. But nevertheless your approach here looks sensible.

> [...] This all seems to work well, but after a bit of use of the 
> tab-completion functionality, the program segfaults or otherwise dies on 
> a call to the readline interface.

Well, this is no wonder, because your function gnu-readline is a 
foreign-lambda* and not a foreign-safe-lambda*. Callbacks into Scheme are 
only safe inside foreign-safe-lambda and friends, otherwise your stack may 
unexpectedly be sweeped clean by a minor garbage collection and your C 
code won't like that very much.

So just replace the foreign-lambda* by foreign-safe-lambda* in your 
readline.scm for every function that may eventually cause a callback intro 
Scheme and everything should be fine.

> [...]

cu,
Thomas