[cffi-devel] Safe errno collection

[John Fremlin]

There are many ways to get errno.

With the very cool cffi-wrappers you can use 

(include "errno.h")

(defwrapper* "get_errno" :int ()
  "return errno;")

which finesses the point about whether or not errno is a real variable
or a thread-local variable or something else.

However, there is no guarantee that between calling your function and
getting the errno, the Lisp environment will not call a C function that
resets errno.

(progn
 (open ...)
 ;; something can happen here 
 (get-errno))

In practice, we have observed this problem.

Each Lisp might have a different way of handing it.

In Allegro CL it is possible to build the wrapper for the foreign
function so it collects the errno in a safe way, using def-foreign-call
with :ERROR-VALUE :ERRNO.

In SBCL there is a native get-errno function to get the errno.

Is there any plan to add a semi-portable wrapper to this functionality
for CFFI?

If not, would you accept a patch for it? I guess the obvious way is to
modify (cffi:defcfun ...) to take a :after-collect-value argument, so
that it could work not only for errno

On most Lisps I guess it might translate to something like this

(defmacro defcfun-with-collect-value ((name &key after-collect-value) ...)
`(without-interrupts
        (values 
                (call-foreign-function ,name ...) 
                (funcall ,after-collect-value))))

Suggestions for better names and interfaces much appreciated.