use Cstring as in GDAL

like JuliaGeo/GDAL.jl#47

without this I received errors like:
```
MethodError: no method matching unsafe_convert(::Type{Ptr{Cstring}}, ::Base.RefValue{Ptr{UInt8}})
```

src/ogr/featurelayer.jl

@@ -166,7 +166,7 @@ function setattributefilter!(layer::FeatureLayer, query::AbstractString)
 end
 
 function clearattributefilter!(layer::FeatureLayer)
-    result = GDAL.setattributefilter(layer.ptr, Ptr{UInt8}(C_NULL))
+    result = GDAL.setattributefilter(layer.ptr, C_NULL)
     @ogrerr result """OGRErr $result: Failed to clear attribute query."""
     layer
 end

src/ogr/fielddefn.jl

@@ -158,8 +158,8 @@ setnullable!(fielddefn::FieldDefn, nullable::Bool) =
 
 "Get default field value"
 function getdefault(fielddefn::FieldDefn)
-    result = @gdal(OGR_Fld_GetDefault::Ptr{UInt8}, fielddefn.ptr::GDALFieldDefn)
-    return result == Ptr{UInt8}(C_NULL) ? "" : unsafe_string(result)
+    result = @gdal(OGR_Fld_GetDefault::Cstring, fielddefn.ptr::GDALFieldDefn)
+    return result == C_NULL ? "" : unsafe_string(result)
 end
 
 """

src/raster/rasterband.jl

@@ -301,7 +301,7 @@ end
 #                   nBXSize::Integer,
 #                   nBYSize::Integer,
 #                   eBDataType::GDALDataType,
-#                   papszOptions::Ptr{Ptr{UInt8}}) =
+#                   papszOptions::Ptr{Cstring}) =
 #     GDALRasterAdviseRead(hRB, nDSXOff, nDSYOff, nDSXSize, nDSYSize, nBXSize,
 #                          nBYSize, eBDataType, papszOptions)::CPLErr
 

src/spatialref.jl

@@ -184,7 +184,7 @@ importURL(url::AbstractString) = importURL!(newspatialref(), url)
 
 "Convert this SRS into WKT format."
 function toWKT(spref::AbstractSpatialRef)
-    wktptr = Ref{Ptr{UInt8}}()
+    wktptr = Ref{Cstring}()
     result = GDAL.exporttowkt(spref.ptr, wktptr)
     @ogrerr result "Failed to convert this SRS into WKT format"
     unsafe_string(wktptr[])
@@ -199,7 +199,7 @@ Convert this SRS into a nicely formatted WKT string for display to a person.
                 stripped off.
 """
 function toWKT(spref::AbstractSpatialRef, simplify::Bool)
-    wktptr = Ref{Ptr{UInt8}}()
+    wktptr = Ref{Cstring}()
     result = GDAL.exporttoprettywkt(spref.ptr, wktptr, simplify)
     @ogrerr result "Failed to convert this SRS into pretty WKT"
     unsafe_string(wktptr[])
@@ -225,15 +225,15 @@ possible. LOCAL_CS coordinate systems are not translatable. An empty string will
 be returned along with OGRERR_NONE.
 """
 function toXML(spref::AbstractSpatialRef)
-    xmlptr = Ref{Ptr{UInt8}}()
+    xmlptr = Ref{Cstring}()
     result = GDAL.exporttoxml(spref.ptr, xmlptr, C_NULL)
     @ogrerr result "Failed to convert this SRS into XML"
     unsafe_string(xmlptr[])
 end
 
 "Export coordinate system in Mapinfo style CoordSys format."
 function toMICoordSys(spref::AbstractSpatialRef)
-    ptr = Ref{Ptr{UInt8}}()
+    ptr = Ref{Cstring}()
     result = GDAL.exporttomicoordsys(spref.ptr, ptr)
     @ogrerr result "Failed to convert this SRS into XML"
     unsafe_string(ptr[])
@@ -305,7 +305,7 @@ function setattrvalue!(spref::AbstractSpatialRef, path::AbstractString,
 end
 
 function setattr!(spref::AbstractSpatialRef, path::AbstractString)
-    result = GDAL.setattrvalue(spref.ptr, path, Ptr{UInt8}(C_NULL))
+    result = GDAL.setattrvalue(spref.ptr, path, C_NULL)
     @ogrerr result "Failed to set attribute $path"
 end
 

src/types.jl

@@ -17,7 +17,7 @@ const GDALStyleManager    = Ptr{GDAL.OGRStyleMgrH}
 const GDALStyleTable      = Ptr{GDAL.OGRStyleTableH}
 const GDALStyleTool       = Ptr{GDAL.OGRStyleToolH}
 
-const StringList          = Ptr{Ptr{UInt8}}
+const StringList          = Ptr{Cstring}
 
 abstract type AbstractGeometry <: GeoInterface.AbstractGeometry end
     # needs to have a `ptr::GDALGeometry` attribute

src/utils.jl

@@ -49,7 +49,7 @@ function unsafe_loadstringlist(pstringlist::Ptr{Cstring})
     (pstringlist == C_NULL) && return stringlist
     i = 1
     item = unsafe_load(pstringlist, i)
-    while Ptr{UInt8}(item) != C_NULL
+    while item != C_NULL
         push!(stringlist, unsafe_string(item))
         i += 1
         item = unsafe_load(pstringlist, i)