c_vec.rs

/*
Module: c_vec

Library to interface with chunks of memory allocated in C.

It is often desirable to safely interface with memory allocated from C,
encapsulating the unsafety into allocation and destruction time.  Indeed,
allocating memory externally is currently the only way to give Rust shared
mutable state with C programs that keep their own references; vectors are
unsuitable because they could be reallocated or moved at any time, and
importing C memory into a vector takes a one-time snapshot of the memory.

This module simplifies the usage of such external blocks of memory.  Memory
is encapsulated into an opaque object after creation; the lifecycle of the
memory can be optionally managed by Rust, if an appropriate destructor
closure is provided.  Safety is ensured by bounds-checking accesses, which
are marshalled through get and set functions.

There are three unsafe functions: the two introduction forms, and the
pointer elimination form.  The introduction forms are unsafe for the obvious
reason (they act on a pointer that cannot be checked inside the method), but
the elimination form is somewhat more subtle in its unsafety.  By using a
pointer taken from a c_vec::t without keeping a reference to the c_vec::t
itself around, the c_vec could be garbage collected, and the memory within
could be destroyed.  There are legitimate uses for the pointer elimination
form -- for instance, to pass memory back into C -- but great care must be
taken to ensure that a reference to the c_vec::t is still held if needed.

 */

export t;
export create, create_with_dtor;
export get, set;
export len;
export ptr;

/*
 Type: t

 The type representing a native chunk of memory.  Wrapped in a enum for
 opacity; FIXME #818 when it is possible to have truly opaque types, this
 should be revisited.
 */

enum t<T> {
    t({ base: *mutable T, len: uint, rsrc: @dtor_res})
}

resource dtor_res(dtor: option<fn@()>) {
    alt dtor {
      option::none { }
      option::some(f) { f(); }
    }
}

/*
 Section: Introduction forms
 */

/*
Function: create

Create a c_vec::t from a native buffer with a given length.

Parameters:

base - A native pointer to a buffer
len - The number of elements in the buffer
*/
unsafe fn create<T>(base: *mutable T, len: uint) -> t<T> {
    ret t({base: base,
           len: len,
           rsrc: @dtor_res(option::none)
          });
}

/*
Function: create_with_dtor

Create a c_vec::t from a native buffer, with a given length,
and a function to run upon destruction.

Parameters:

base - A native pointer to a buffer
len - The number of elements in the buffer
dtor - A function to run when the value is destructed, useful
       for freeing the buffer, etc.
*/
unsafe fn create_with_dtor<T>(base: *mutable T, len: uint, dtor: fn@())
  -> t<T> {
    ret t({base: base,
           len: len,
           rsrc: @dtor_res(option::some(dtor))
          });
}

/*
 Section: Operations
 */

/*
Function: get

Retrieves an element at a given index

Failure:

If `ofs` is greater or equal to the length of the vector
*/
fn get<T: copy>(t: t<T>, ofs: uint) -> T {
    assert ofs < len(t);
    ret unsafe { *ptr::mut_offset((*t).base, ofs) };
}

/*
Function: set

Sets the value of an element at a given index

Failure:

If `ofs` is greater or equal to the length of the vector
*/
fn set<T: copy>(t: t<T>, ofs: uint, v: T) {
    assert ofs < len(t);
    unsafe { *ptr::mut_offset((*t).base, ofs) = v };
}

/*
 Section: Elimination forms
 */

/*
Function: len

Returns the length of the vector
*/
fn len<T>(t: t<T>) -> uint {
    ret (*t).len;
}

/*
Function: ptr

Returns a pointer to the first element of the vector
*/
unsafe fn ptr<T>(t: t<T>) -> *mutable T {
    ret (*t).base;
}

#[cfg(test)]
mod tests {
    import ctypes::*;

    #[nolink]
    #[abi = "cdecl"]
    native mod libc {
        fn malloc(n: size_t) -> *mutable u8;
        fn free(m: *mutable u8);
    }

    fn malloc(n: size_t) -> t<u8> {
        let mem = libc::malloc(n);

        assert mem as int != 0;

        ret unsafe { create_with_dtor(mem, n, bind libc::free(mem)) };
    }

    #[test]
    fn test_basic() {
        let cv = malloc(16u);

        set(cv, 3u, 8u8);
        set(cv, 4u, 9u8);
        assert get(cv, 3u) == 8u8;
        assert get(cv, 4u) == 9u8;
        assert len(cv) == 16u;
    }

    #[test]
    #[should_fail]
    #[ignore(cfg(target_os = "win32"))]
    fn test_overrun_get() {
        let cv = malloc(16u);

        get(cv, 17u);
    }

    #[test]
    #[should_fail]
    #[ignore(cfg(target_os = "win32"))]
    fn test_overrun_set() {
        let cv = malloc(16u);

        set(cv, 17u, 0u8);
    }

    #[test]
    fn test_and_I_mean_it() {
        let cv = malloc(16u);
        let p = unsafe { ptr(cv) };

        set(cv, 0u, 32u8);
        set(cv, 1u, 33u8);
        assert unsafe { *p } == 32u8;
        set(cv, 2u, 34u8); /* safety */
    }

}