Re: [netcdfgroup] NC_STRING and NC_CHAR ... How to read strings?

• To: "netcdfgroup@xxxxxxxxxxxxxxxx" <netcdfgroup@xxxxxxxxxxxxxxxx>, "dmh@xxxxxxxx" <dmh@xxxxxxxx>
• Subject: Re: [netcdfgroup] NC_STRING and NC_CHAR ... How to read strings?
• From: Panov Sergey <sergey.panov@xxxxxxxxxxx>
• Date: Thu, 28 Feb 2019 21:42:59 +0000
• Authentication-results: spf=none (sender IP is ) smtp.mailfrom=sergey.panov@xxxxxxxxxxx;

On Thu, 2019-02-28 at 13:13 -0700, Dennis Heimbigner wrote:
> I think the key is your observation:
> > char buffer[20];
> > inst_var.getVar(buffer);
> > to
> > char * buffer;
> > inst_var.getVar(&buffer);
> If the variable is actually of type string (as opposed to char), then
> you need to pass a buffer into which string pointers (char*) are
> inserted
> so the getVar argument needs to be char** (your second case).
> I assume getVar is overloaded so that in the first case it is
> attempting 
> to read
> a string typed variable using nc_get_vara_text.
> In the second case, you are invoking the char** overloaded getVar, so
> it is properly invoking nc_get_vara_string function.

I get it, but why I am getting a pointer to some string? If I do not
own it, why it is not const? If I own it now, am I responsible for
freeing its memory?

The netcdf-cxx4 library is C++ lib. There should be no place for    
getVar(char**) and getVar(char*) in C++ lib . I expected there 
getVar(string &) and getVar(vector<string> &) calls. 

> =Dennis Heimbigner
>   Unidata
> 
> 
> On 2/27/2019 10:11 PM, Panov Sergey wrote:
> > I guess it was not a segmentation fault, but a error return
> > triggering
> > C++ exception. I was able to get it to trigger a real segmentation
> > fault, but changing
> > 
> > char buffer[20];
> > inst_var.getVar(buffer);
> > 
> > to
> > 
> > char * buffer;
> > inst_var.getVar(&buffer);
> > 
> > gives me a pointer to a correct string. It is just bizarre!!! Does
> > it
> > mean I now can modify that sting and it is OK?
> > 
> > 
> > On Thu, 2019-02-28 at 04:31 +0000, Panov Sergey wrote:
> > > I have a very basic question -- how to read "string" variable in
> > > C or
> > > C++
> > > 
> > > The ncdump tells me the variable type is "string".  When I try to
> > > read
> > > it using
> > > 
> > >   void getVar(char* dataValues) const;
> > > 
> > > function of class NcVar of netcdf-cxx4 I am hitting a
> > > segmentation
> > > fault. I am not sure why it is a segmentation fault, but it comes
> > > from
> > > 
> > > ...
> > >     /* No NC_CHAR conversions, you pervert! */
> > >    if (var->type_info->nc_typeid != *mem_nc_type &&
> > >        (var->type_info->nc_typeid == NC_CHAR || *mem_nc_type ==
> > > NC_CHAR))
> > >      return NC_ECHAR;
> > > ...
> > > 
> > > code in check_for_vara() function in nc4hdf.c .
> > >   The var->type_info->nc_typeid is NC_STRING (12) and this check
> > > should
> > > just fail (the reading of the variable should just fail, but it
> > > "throws
> > > an exception" ... causes a segmentation fault for some reason).
> > > 
> > > The stack is;
> > > check_for_vara nc4hdf.c:477
> > > nc4_get_vara nc4hdf.c:905
> > > nc4_get_vara_tc nc4var.c:1379
> > > NC4_get_vara nc4var.c:1396
> > > NC_get_vara dvarget.c:101
> > > NC_get_var dvarget.c:117
> > > nc_get_var_text dvarget.c:1020
> > > netCDF::NcVar::getVar ncVar.cpp:1340
> > > main cfradial_rd.cpp:47
> > > __libc_start_main 0x00007ffff485a413
> > > _start 0x00000000004021ee
> > > 
> > > The C++ code is calling C function nc_get_var_text() to read this
> > > string which should have been normal.
> > > 
> > > What am I doing wrong and what is the right way of reading
> > > "string"
> > > variables?
> > > 
> > > PS. Does anyone else dislike the official C++ library API or it
> > > is
> > > just
> > > me?
> > >   
> > > _______________________________________________
> > > NOTE: All exchanges posted to Unidata maintained email lists are
> > > recorded in the Unidata inquiry tracking system and made publicly
> > > available through the web.  Users who post to any of the lists we
> > > maintain are reminded to remove any personal information that
> > > they
> > > do not want to be made public.
> > > 
> > > 
> > > netcdfgroup mailing list
> > > netcdfgroup@xxxxxxxxxxxxxxxx
> > > For list information or to unsubscribe,  visit:
> > > http://www.unidata.ucar.edu/mailing_lists/
> > _______________________________________________
> > NOTE: All exchanges posted to Unidata maintained email lists are
> > recorded in the Unidata inquiry tracking system and made publicly
> > available through the web.  Users who post to any of the lists we
> > maintain are reminded to remove any personal information that they
> > do not want to be made public.
> > 
> > 
> > netcdfgroup mailing list
> > netcdfgroup@xxxxxxxxxxxxxxxx
> > For list information or to unsubscribe,  visit: 
> > http://www.unidata.ucar.edu/mailing_lists/

• Follow-Ups:
  • Re: [netcdfgroup] NC_STRING and NC_CHAR ... How to read strings?
    • From: Dennis Heimbigner

• References:
  • [netcdfgroup] NC_STRING and NC_CHAR ... How to read strings?
    • From: Panov Sergey
  • Re: [netcdfgroup] NC_STRING and NC_CHAR ... How to read strings?
    • From: Panov Sergey
  • Re: [netcdfgroup] NC_STRING and NC_CHAR ... How to read strings?
    • From: Dennis Heimbigner