CFFI User Manual: Pointers

7 Pointers

All C data in CFFI is referenced through pointers. This includes defined C variables that hold immediate values, and integers.

To see why this is, consider the case of the C integer. It is not only an arbitrary representation for an integer, congruent to Lisp’s fixnums; the C integer has a specific bit pattern in memory defined by the C ABI. Lisp has no such constraint on its fixnums; therefore, it only makes sense to think of fixnums as C integers if you assume that CFFI converts them when necessary, such as when storing one for use in a C function call, or as the value of a C variable. This requires defining an area of memory¹¹, represented through an effective address, and storing it there.

Due to this compartmentalization, it only makes sense to manipulate raw C data in Lisp through pointers to it. For example, while there may be a Lisp representation of a struct that is converted to C at store time, you may only manipulate its raw data through a pointer. The C compiler does this also, albeit informally.

Dictionary
• Basic Pointer Operations:
• Allocating Foreign Memory:
• Accessing Foreign Memory:
• foreign-free:
• foreign-alloc:
• foreign-symbol-pointer:
• inc-pointer:
• incf-pointer:
• make-pointer:
• mem-aptr:
• mem-aref:
• mem-ref:
• null-pointer:
• null-pointer-p:
• pointerp:
• pointer-address:
• pointer-eq:
• with-foreign-object:
• with-foreign-objects:
• with-foreign-pointer:

Footnotes

(11)

The definition of memory includes the CPU registers.