Errors

Every error cl-ffi signals is an ffi-error, and each says what was wrong and what to write instead.

Condition	Signalled when	Readers
load-foreign-library-error	no file of a library's definition or spec could be loaded, or no clause fits the platform	load-foreign-library-error-library, the name or spec; load-foreign-library-error-attempts, a list of (file reason)
encoding-error	a string holds a character its encoding cannot represent	encoding-error-encoding, encoding-error-character
decoding-error	foreign bytes are no valid string in the encoding asked for	decoding-error-encoding, decoding-error-position, the byte where it goes wrong
ffi-error	anything else: an unknown type, struct slot, enum keyword, callback or library; a struct read as a scalar; memory malloc cannot give

Misuse that the types say at compile time is refused there: mem-ref of a struct type, foreign-slot-value of an array slot, a struct passed by value. SBCL warns when it compiles such code, and signals the error when it runs.

An argument of the wrong kind is a type-error, not an ffi-error. cl-ffi's own functions declaim the full domain of their arguments: the encodings for :encoding, the shape of a type specifier, of a struct or union specifier and of a library's spec, the name of a defined type for the enum and bitfield functions. SBCL checks them when the function is called, before cl-ffi looks at the argument, and warns of a constant one when it compiles the call. A malformed type specifier, such as (:string :size 4), or a spec within (:or ...) is still an ffi-error, from cl-ffi as it reads it.

A defcfun function's arguments are checked the same way against the ftype it declaims, which signals a type-error for a value of the wrong type before cl-ffi's own conversion would.