make Cstring more usable as a return value (see JuliaLang#10799)

base/c.jl

@@ -60,6 +60,11 @@ end
 
 convert{T<:Union{Int8,UInt8}}(::Type{Cstring}, p::Ptr{T}) = box(Cstring, unbox(Ptr{T}, p))
 convert(::Type{Cwstring}, p::Ptr{Cwchar_t}) = box(Cwstring, unbox(Ptr{Cwchar_t}, p))
+convert{T<:Union{Int8,UInt8}}(::Type{Ptr{T}}, p::Cstring) = box(Ptr{T}, unbox(Cstring, p))
+convert(::Type{Ptr{Cwchar_t}}, p::Cwstring) = box(Ptr{Cwchar_t}, unbox(Cwstring, p))
+
+# here, not in pointer.jl, to avoid bootstrapping problems in coreimg.jl
+pointer_to_string(p::Cstring, own::Bool=false) = pointer_to_string(convert(Ptr{UInt8}, p), own)
 
 # convert strings to ByteString etc. to pass as pointers
 cconvert(::Type{Cstring}, s::AbstractString) = bytestring(s)

base/libc.jl

@@ -194,5 +194,7 @@ free(p::Ptr) = ccall(:free, Void, (Ptr{Void},), p)
 malloc(size::Integer) = ccall(:malloc, Ptr{Void}, (Csize_t,), size)
 realloc(p::Ptr, size::Integer) = ccall(:realloc, Ptr{Void}, (Ptr{Void}, Csize_t), p, size)
 calloc(num::Integer, size::Integer) = ccall(:calloc, Ptr{Void}, (Csize_t, Csize_t), num, size)
+free(p::Cstring) = free(convert(Ptr{UInt8}, p))
+free(p::Cwstring) = free(convert(Ptr{Cwchar_t}, p))
 
 end # module

base/strings/basic.jl

@@ -17,7 +17,7 @@ function bytestring(p::Union{Ptr{UInt8},Ptr{Int8}})
     p == C_NULL ? throw(ArgumentError("cannot convert NULL to string")) :
     ccall(:jl_cstr_to_string, Any, (Ptr{UInt8},), p)::ByteString
 end
-bytestring(s::Cstring) = bytestring(box(Ptr{Cchar}, unbox(Cstring,s)))
+bytestring(s::Cstring) = bytestring(convert(Ptr{UInt8}, s))
 
 function bytestring(p::Union{Ptr{UInt8},Ptr{Int8}},len::Integer)
     p == C_NULL ? throw(ArgumentError("cannot convert NULL to string")) :

base/unicode/utf32.jl

@@ -256,7 +256,7 @@ elseif sizeof(Cwchar_t) == 4
     const WString = UTF32String
     const wstring = utf32
 end
-wstring(s::Cwstring) = wstring(box(Ptr{Cwchar_t}, unbox(Cwstring,s)))
+wstring(s::Cwstring) = wstring(convert(Ptr{Cwchar_t}, s))
 
 # Cwstring is defined in c.jl, but conversion needs to be defined here
 # to have WString

test/strings/basic.jl

@@ -243,6 +243,16 @@ let s = "ba\0d"
     @test_throws ArgumentError Base.unsafe_convert(Cwstring, wstring(s))
 end
 
+cstrdup(s) = @windows? ccall(:_strdup, Cstring, (Cstring,), s) : ccall(:strdup, Cstring, (Cstring,), s)
+let p = cstrdup("hello")
+    @test bytestring(p) == "hello" == pointer_to_string(cstrdup(p), true)
+    Libc.free(p)
+end
+let p = @windows? ccall(:_wcsdup, Cwstring, (Cwstring,), "tést") : ccall(:wcsdup, Cwstring, (Cwstring,), "tést")
+    @test wstring(p) == "tést"
+    Libc.free(p)
+end
+
 # issue # 11389: Vector{UInt32} was copied with UTF32String, unlike Vector{Char}
 a = UInt32[48,0]
 b = UTF32String(a)