# This file is a part of Julia. License is MIT: https://julialang.org/license

"""
    Ptr{T}

A memory address referring to data of type `T`.  However, there is no guarantee that the
memory is actually valid, or that it actually represents data of the specified type.
"""
Ptr

## converting pointers to an appropriate unsigned ##

"""
    C_NULL

The C null pointer constant, sometimes used when calling external code.
"""
const C_NULL = bitcast(Ptr{Cvoid}, 0)

# TODO: deprecate these conversions. C doesn't even allow them.

# pointer to integer
convert(::Type{T}, x::Ptr) where {T<:Integer} = T(UInt(x))

# integer to pointer
convert(::Type{Ptr{T}}, x::Union{Int,UInt}) where {T} = Ptr{T}(x)

# pointer to pointer
convert(::Type{Ptr{T}}, p::Ptr{T}) where {T} = p
convert(::Type{Ptr{T}}, p::Ptr) where {T} = bitcast(Ptr{T}, p)

# object to pointer (when used with ccall)

"""
    unsafe_convert(T, x)

Convert `x` to a C argument of type `T`
where the input `x` must be the return value of `cconvert(T, ...)`.

In cases where [`convert`](@ref) would need to take a Julia object
and turn it into a `Ptr`, this function should be used to define and perform
that conversion.

Be careful to ensure that a Julia reference to `x` exists as long as the result of this
function will be used. Accordingly, the argument `x` to this function should never be an
expression, only a variable name or field reference. For example, `x=a.b.c` is acceptable,
but `x=[a,b,c]` is not.

The `unsafe` prefix on this function indicates that using the result of this function after
the `x` argument to this function is no longer accessible to the program may cause undefined
behavior, including program corruption or segfaults, at any later time.

See also [`cconvert`](@ref)
"""
function unsafe_convert end

unsafe_convert(::Type{Ptr{UInt8}}, x::Symbol) = ccall(:jl_symbol_name, Ptr{UInt8}, (Any,), x)
unsafe_convert(::Type{Ptr{Int8}}, x::Symbol) = ccall(:jl_symbol_name, Ptr{Int8}, (Any,), x)
unsafe_convert(::Type{Ptr{UInt8}}, s::String) = convert(Ptr{UInt8}, pointer_from_objref(s)+sizeof(Int))
unsafe_convert(::Type{Ptr{Int8}}, s::String) = convert(Ptr{Int8}, pointer_from_objref(s)+sizeof(Int))
# convert strings to String etc. to pass as pointers
cconvert(::Type{Ptr{UInt8}}, s::AbstractString) = String(s)
cconvert(::Type{Ptr{Int8}}, s::AbstractString) = String(s)

unsafe_convert(::Type{Ptr{T}}, a::Array{T}) where {T} = ccall(:jl_array_ptr, Ptr{T}, (Any,), a)
unsafe_convert(::Type{Ptr{S}}, a::AbstractArray{T}) where {S,T} = convert(Ptr{S}, unsafe_convert(Ptr{T}, a))
unsafe_convert(::Type{Ptr{T}}, a::AbstractArray{T}) where {T} = error("conversion to pointer not defined for $(typeof(a))")

# unsafe pointer to array conversions
"""
    unsafe_wrap(Array, pointer::Ptr{T}, dims, own=false)

Wrap a Julia `Array` object around the data at the address given by `pointer`,
without making a copy.  The pointer element type `T` determines the array
element type. `dims` is either an integer (for a 1d array) or a tuple of the array dimensions.
`own` optionally specifies whether Julia should take ownership of the memory,
calling `free` on the pointer when the array is no longer referenced.

This function is labelled "unsafe" because it will crash if `pointer` is not
a valid memory address to data of the requested length.
"""
function unsafe_wrap(::Union{Type{Array},Type{Array{T}},Type{Array{T,N}}},
                     p::Ptr{T}, dims::NTuple{N,Int}, own::Bool=false) where {T,N}
    ccall(:jl_ptr_to_array, Array{T,N}, (Any, Ptr{Cvoid}, Any, Int32),
          Array{T,N}, p, dims, own)
end
function unsafe_wrap(::Union{Type{Array},Type{Array{T}},Type{Array{T,1}}},
                     p::Ptr{T}, d::Integer, own::Bool=false) where {T}
    ccall(:jl_ptr_to_array_1d, Array{T,1},
          (Any, Ptr{Cvoid}, Csize_t, Cint), Array{T,1}, p, d, own)
end
unsafe_wrap(Atype::Type, p::Ptr, dims::NTuple{N,<:Integer}, own::Bool=false) where {N} =
    unsafe_wrap(Atype, p, convert(Tuple{Vararg{Int}}, dims), own)

"""
    unsafe_load(p::Ptr{T}, i::Integer=1)

Load a value of type `T` from the address of the `i`th element (1-indexed) starting at `p`.
This is equivalent to the C expression `p[i-1]`.

The `unsafe` prefix on this function indicates that no validation is performed on the
pointer `p` to ensure that it is valid. Incorrect usage may segfault your program or return
garbage answers, in the same manner as C.
"""
unsafe_load(p::Ptr, i::Integer=1) = pointerref(p, Int(i), 1)

"""
    unsafe_store!(p::Ptr{T}, x, i::Integer=1)

Store a value of type `T` to the address of the `i`th element (1-indexed) starting at `p`.
This is equivalent to the C expression `p[i-1] = x`.

The `unsafe` prefix on this function indicates that no validation is performed on the
pointer `p` to ensure that it is valid. Incorrect usage may corrupt or segfault your
program, in the same manner as C.
"""
unsafe_store!(p::Ptr{Any}, @nospecialize(x), i::Integer=1) = pointerset(p, x, Int(i), 1)
unsafe_store!(p::Ptr{T}, x, i::Integer=1) where {T} = pointerset(p, convert(T,x), Int(i), 1)

# convert a raw Ptr to an object reference, and vice-versa
"""
    unsafe_pointer_to_objref(p::Ptr)

Convert a `Ptr` to an object reference. Assumes the pointer refers to a valid heap-allocated
Julia object. If this is not the case, undefined behavior results, hence this function is
considered "unsafe" and should be used with care.
"""
unsafe_pointer_to_objref(x::Ptr) = ccall(:jl_value_ptr, Any, (Ptr{Cvoid},), x)

"""
    pointer_from_objref(x)

Get the memory address of a Julia object as a `Ptr`. The existence of the resulting `Ptr`
will not protect the object from garbage collection, so you must ensure that the object
remains referenced for the whole time that the `Ptr` will be used.
"""
pointer_from_objref(@nospecialize(x)) = ccall(:jl_value_ptr, Ptr{Cvoid}, (Any,), x)
data_pointer_from_objref(@nospecialize(x)) = pointer_from_objref(x)::Ptr{Cvoid}

eltype(::Type{Ptr{T}}) where {T} = T

## limited pointer arithmetic & comparison ##

==(x::Ptr, y::Ptr) = UInt(x) == UInt(y)
isless(x::Ptr, y::Ptr) = isless(UInt(x), UInt(y))
-(x::Ptr, y::Ptr) = UInt(x) - UInt(y)

+(x::Ptr, y::Integer) = oftype(x, Intrinsics.add_ptr(UInt(x), (y % UInt) % UInt))
-(x::Ptr, y::Integer) = oftype(x, Intrinsics.sub_ptr(UInt(x), (y % UInt) % UInt))
+(x::Integer, y::Ptr) = y + x

"""
Temporarily protects an object from being garbage collected, even
if it would otherwise be unreferenced.

The last argument is the expression to preserve objects during.
The previous arguments are the objects to preserve.
"""
macro gc_preserve(args...)
    syms = args[1:end-1]
    for x in syms
        isa(x, Symbol) || error("Preserved variable must be a symbol")
    end
    s, r = gensym(), gensym()
    esc(quote
        $s = $(Expr(:gc_preserve_begin, syms...))
        $r = $(args[end])
        $(Expr(:gc_preserve_end, s))
        $r
    end)
end