[julia-users] Re: C wrapper ReadStat : read files from Stata, SPSS and SAS.

[Steven G. Johnson]

On Monday, October 12, 2015 at 3:30:18 PM UTC-4, Douglas Bates wrote:
>
> On Monday, October 12, 2015 at 11:24:06 AM UTC-5, Matt wrote:
>>
>> ReadStat is an excellent C library by Evan Millers that allows to read 
>> files from Stata, SPSS and SAS.
>> Evan wrote a wrapper for ReadStat in the Dataread.jl package 
>> https://github.com/WizardMac/DataRead.jl
>> However, the package was not updated with the most recent API of ReadStat.
>> I've been trying to solve this without success (see the WIP pull request 
>> in the repository)
>> The hard part is to write a Julia function (handle_value) that accepts a 
>> C structure with a union field.
>>
>> typedef struct readstat_value_s {
>>     union {
>>         char        char_value;
>>         float       float_value;
>>         double      double_value;
>>         int16_t     i16_value;
>>         int32_t     i32_value;
>>         char       *string_value;
>>     } v;
>>     readstat_types_t        type;
>>     char                    tag;
>>     int                     is_system_missing:1;
>>     int                     is_considered_missing:1;
>> } readstat_value_t;
>>
>

Something like the following should work, because the union types all fit 
into a 64-bit field and will be aligned as such:

immutable readstat_value_t
   union::Int64
   typ::Cint # enum
   tag::Cchar
   bits::UInt8 # bitfield (first two bits are is_system_missing and 
is_considered_missing)
end

function handle_value(p::Ptr{readstat_value_t})
    v = unsafe_load(p)
    if v.tag == READSTAT_TYPE_DOUBLE
        value = unsafe_load(convert(Ptr{Cdouble}, p))
        ...do stuff...
    else if v.tag == READSTAT_TYPE_STRING
        value = bytestring(unsafe_load(convert(Ptr{Cchar}, p)))
        ... do stuff...
    else ...
       ....
    end
end