►The Racket Foreign Interface
▼5 Derived Utilities
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allocator
deallocator
releaser
retainer
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5.5 Allocation and Finalization

 (require ffi/unsafe/alloc)
The ffi/unsafe/alloc library provides utilities for ensuring that values allocated through foreign functions are reliably deallocated.

((allocator dealloc) alloc) → procedure?
  dealloc : (any/c . -> . any)
  alloc : procedure?
Produces a procedure that behaves like alloc, but the result of alloc is given a finalizer that calls dealloc on the result if it is not otherwise freed through a deallocator (as designated with deallocator). In addition, alloc is called in atomic mode (see start-atomic); its result is received and registered in atomic mode, so that the result is reliably freed.

The dealloc procedure itself need not be specifically designated a deallocator (via deallocator). If a deallocator is called explicitly, it need not be the same as dealloc.

((deallocator [get-arg]) dealloc) → procedure?
  get-arg : (list? . -> . any/c) = car
  dealloc : procedure?
((releaser [get-arg]) dealloc) → procedure?
  get-arg : (list? . -> . any/c) = car
  dealloc : procedure?
Produces a procedure that behaves like dealloc. The dealloc procedure is called in atomic mode (see start-atomic), and the reference count on one of its arguments is decremented; if the reference count reaches zero, no finalizer associated by an allocator- or referencer-wrapped procedure is invoked when the value becomes inaccessible.

The optional get-arg procedure determines which of dealloc’s arguments correspond to the released object; get-arg receives a list of arguments passed to dealloc, so the default car selects the first one.

The releaser procedure is a synonym for deallocator.

((retainer release [get-arg]) retain) → procedure?
  release : (any/c . -> . any)
  get-arg : (list? . -> . any/c) = car
  retain : procedure?
Produces a procedure that behaves like retain. The procedure is called in atomic mode (see start-atomic), and the reference count on one of its arguments is incremented, with release recorded as the corresponding release procedure to be called by the finalizer on the retained object (unless some deallocator, as wrapped by deallocate, is explicitly called first).

The optional get-arg procedure determines which of retain’s arguments correspond to the retained object; get-arg receives a list of arguments passed to retain, so the default car selects the first one.

The release procedure itself need not be specifically designated a deallocator (via deallocator). If a deallocator is called explicitly, it need not be the same as release.

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