intended to translate from one CRS to another in C language

GDAL_C_COORD_TRANSF.h

   //  #include <stdio.h>

   #include <C:\OSGeo4W64\include\ogr_srs_api.h>

/*
   2019 Kaue de Moraes Vestena

   Intended for integration in C code
*/



struct lat_lgt
{
   // struct to store the result for the version 1
   double lat;
   double lgt;
};




struct lat_lgt transform_lat_lgt_v1(double E,double N,int dest_EPSG)
{
   /*
      Transform a pair of projected E,N coordinates from a Coordinate Reference System to WGS84 Longitude,Latitude

      arguments:
      E : input X coordinate
      Y : input Y coordinate
      dest_EPSG : EPSG code for the source CRS
      
      for more infos about EPSG codes, visit: http://epsg.io/ and the wikipedia article

   */

   // basic declarations
   struct lat_lgt result;

   double lat,lgt,h;

   // GDAL declarations

   OGRSpatialReferenceH DEST_CRS,WGS84_T;

   DEST_CRS = OSRNewSpatialReference(NULL);
   WGS84_T = OSRNewSpatialReference(NULL);


   // creating the spatial references   
   OSRImportFromEPSG(DEST_CRS,dest_EPSG);
   OSRImportFromEPSG(WGS84_T,4326);

   // the transformer object
   OGRCoordinateTransformationH the_transf = OCTNewCoordinateTransformation(WGS84_T,DEST_CRS);

   // doing the transformation
   OCTTransform(the_transf, 1, &lgt, &lat, &h);



   result.lat = lat;
   result.lgt = lgt;

   return result;


   /*
      SAMPLE RUN:

      double Es = 670862.352;
      double Ns = 7185074.028;
      int UTM22S_SIRGAS = 31982;


      lat_lgt transformed = transform_lat_lgt_v1(Es,Ns,UTM22S_SIRGAS)


      *************


      transformed.lat : -25.44151
      transformed.lgt : -49.30082
   */
}


char *transform_lat_lgt_v2(char *E,char *N,char *dest_EPSG)
{
   /*
      Transform a pair of projected E,N coordinates from a Coordinate Reference System to WGS84 Longitude,Latitude

      second version that calls GDAL CLI tool, so we don't need to import any libraries

      arguments:
      E : input X coordinate AS C-STR
      Y : input Y coordinate AS C-STR
      dest_EPSG : EPSG code for the source CRS AS C-STR
      
      for more infos about EPSG codes, visit: http://epsg.io/ and the wikipedia article

   */


   // printf("%s %s %s",E,N,dest_EPSG);

   // predeclarations
  char buffer [500];

   char *outstr = malloc(100);

  int cx;

   // formatting the string containing the command
  cx = snprintf(buffer,499,"echo %s %s | gdaltransform -s_srs EPSG:%s -t_srs EPSG:4326 -output_xy",E,N,dest_EPSG);

   // popen command to store output of command line tool
   FILE *fp;

   fp = popen(buffer,"r");


   //passing from popen stream to string
   while (fgets(outstr, 500, fp) != NULL) 
   {
   //  nothing to do inside
   }

   return outstr;
   


   /*
      sample run:

   char strE[] = "670862.352" ;
   char strN[] = "7185074.028";
   char strEPSG[] = "31982";

   char *out;

   out = transform_lat_lgt_v2(strE,strN,strEPSG);

   printf("%s", out);


   ******************
   should print:
   -49.3008152048423 -25.4415134529186

   */
}