CFFI User Manual - Common Lisp.net

Chapter 4: An Introduction to Foreign Interfaces and CFFI 12
(define-curl-option-setter set-curl-option-errorbuffer
curl-option :errorbuffer :pointer)
(define-curl-option-setter set-curl-option-url
curl-option :url :pointer)
’curl-option)
Macroexpanding one of the new define-curl-option-setter forms yields the following:
(progn
(defcfun ("curl_easy_setopt" set-curl-option-nosignal) curl-code
(easy-handle easy-handle)
(option curl-option)
(new-value :long))
(curry-curl-option-setter ’set-curl-option-nosignal ’:nosignal))
Finally, let’s try this out:
cffi-user> (set-curl-option-nosignal *easy-handle* 1)
⇒ 0
Looks like it works just as well. This interface is now reasonably high-level to wash out
some of the ugliness of the thinnest possible curl_easy_setopt FFI, without obscuring the
remaining C bookkeeping details we will explore.
4.8 Memory management
According to the documentation for curl_easy_setopt, the type of the third argument
when option is CURLOPT_ERRORBUFFER is char*. Above, we’ve defined set-curl-optionerrorbuffer
to accept a :pointer as the new option value. However, there is a CFFI type
:string, which translates Lisp strings to C strings when passed as arguments to foreign
function calls. Why not, then, use :string as the CFFI type of the third argument? There
are two reasons, both related to the necessity of breaking abstraction described in Section 4.6
[Breaking the abstraction], page 9.
The first reason also applies to CURLOPT_URL, which we will use to illustrate the point.
Assuming we have changed the type of the third argument underlying set-curl-optionurl
to :string, look at these two equivalent forms.
(set-curl-option-url *easy-handle* "http://www.cliki.net/CFFI")
≡ (with-foreign-string (url "http://www.cliki.net/CFFI")
(foreign-funcall "curl_easy_setopt" easy-handle *easy-handle*
curl-option :url :pointer url curl-code))
The latter, in fact, is mostly equivalent to what a foreign function call’s macroexpansion
actually does. As you can see, the Lisp string "http://www.cliki.net/CFFI" is copied
into a char array and null-terminated; the pointer to beginning of this array, now a C
string, is passed as a CFFI :pointer to the foreign function.
Unfortunately, the C abstraction has failed us, and we must break it. While :string
works well for many char* arguments, it does not for cases like this. As the curl_easy_
setopt documentation explains, “The string must remain present until curl no longer needs
it, as it doesn’t copy the string.” The C string created by with-foreign-string, however,
only has dynamic extent: it is “deallocated” when the body (above containing the foreignfuncall
form) exits.