print context error of xmlValidCtxt into external .txt file using file pointer

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C++

FILE *fp =fopen("Engine//Validation.txt","a");
      //   char *buffer;
        // string resultString;
   xmlValidCtxt context;
      context.userData = (void *) stderr;
    context.error = (xmlValidityErrorFunc)fprintf;
      context.warning  = (xmlValidityWarningFunc)fprintf;
     // sprintf(buffer,"ABC");
     //  resultString.append(buffer);
      xmlDoc	*doc = xmlReadFile(fileName.c_str(), NULL, 0);

      xmlDtd  *dtd = xmlParseDTD((const xmlChar *)dtdFileName.c_str(), (const xmlChar *)dtdFileName.c_str());


      int retval = xmlValidateDtd(&context,doc,dtd);


    char convertedString[10];
   itoa(retval,convertedString,10);

   result.append("\nValidation of the file: " + fileName + " with dtd: " + dtdFileName + "\n" + "Result: " + convertedString +"\n");

   fprintf(fp,result.c_str());
  
   fclose(fp);

I have written this code for validate xml with dtd,
I want print error and worning into external .txt file using file pointer how to print it? Help me...