On Mon, Nov 6, 2017 at 4:25 AM, Chun Zhang <chunzh...@gmail.com> wrote:
>
> Thank you for the reply!
>
> The C libi.PI_init_global_config has the interface of
> PI_init_global_config(int argc, char *argv[]);
> so Swig converts it to PI_init_global_config(arg1 int, arg2 *string)
>
> The code was expecting to a *string. Viper.GetString returns a string as far
> as I can see, is that right? So device is a string, not a *string. (verified
> with reflect.TypeOf). Everything matches except for the crash :(
>
> What really puzzles me is that why when passing the parameter using flag is
> fine, as well as when hard-coding the value. In both cases, device is still
> a string, nothing changed. But whenever trying to read the value from a
> config file using viper or just simple json.decoder, the code complains it
> is a pointer to a pointer. I mean, I literally did a comparison between the
> read-in value and a variable hard-coded as "eth1", it says they are equal.
A Go string contains an internal pointer. Passing a *string to cgo
means that you are passing a pointer to a string that contains a
pointer, so you are passing a pointer to a pointer. That is invalid
according to the cgo pointer passing rules.
A Go string is definitely not a C or C++ string. You can not pass a
pointer to a Go string to C code that expects a char**. Even if cgo
permitted it, it would not work. If SWIG is converting a C char** (or
char*[]) to Go *string, then SWIG is getting it wrong.
Ian
> On Monday, November 6, 2017 at 1:07:46 AM UTC-5, Tamás Gulácsi wrote:
>>
>>
>> 2017. november 6., hétfő 2:11:48 UTC+1 időpontban Chun Zhang a következőt
>> írta:
>>>
>>> Hi, All,
>>>
>>> I am trying to read a configuration file using Viper, the config file is
>>> a very simple json file with one line
>>> {
>>> "device" : "eth1"
>>> }
>>>
>>> I use the following line to read it
>>>
>>> device := viper.GetString("device")
>>>
>>>
>>> then passing this var further into a C library with swig generated api as
>>> arg2
>>>
>>> itc := device //Need to update if there is more devices
>>> libpi.PI_init_global_config(1, &itc)
>>>
>>> func PI_init_global_config(arg1 int, arg2 *string) {
>>>
>>>
>>>
>>> I can retrieve the correct value, and can verify that var device does
>>> have eth1 as the value when printf it. I even did the compare of device ==
>>> "eth1", and the result is true.
>>>
>>> However, when passing this device var to the PI_init_gloable_config API,
>>> I got code crashed with the following error
>>>
>>> panic: runtime error: cgo argument has Go pointer to Go pointer
>>>
>>
>>
>> The value you pass to C must not have a pointer which points to a Go
>> pointer.
>> What are you passing to libpi.PI_init_global_config ?
>> I think "device" is already a *string, so &itc is a **string - try
>> libpi.PI_init_global_config(device).
>>
>> (I assume libpi does some conversion, as the proper way to pass a string
>> ot C (*char) is to convert the string to *char with cs:=C.CString(*device)
>> and later C.free(unsafe.Pointer(cs)) it).
>>>
>>>
>>> If instead of reading this file from configuration json, I hardcode it in
>>> the code as
>>>
>>> device := "eth1"
>>>
>>> then there is no issue at all. If I read this variable using Flag from
>>> command line, like
>>>
>>> flag.StringVar(&device, "Device", "eth1", "NIC to Listen")
>>>
>>>
>>> there was no problem either.
>>>
>>>
>>>
>>> Can anybody please enlighten me what's the difference? Why this error is
>>> triggered? I googled quite a bit, but it does not seem to help. I have go
>>> 1.8.3 installed.
>>>
>>> Thanks,
>>> Chun
>
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