A more efficient way to Retrieve records into a JSON array in MySQL? C - mysql

Question

I am using MySQL in my C application. I have created a function
GetInDB(char *query, cJSON **json){...
that will allocate the json as a json array, query the db, and store the records in the array. The problem is that it seems difficult to quickly get the field name of each column in a table. Since I don't know the string length of each name in advance, I have to iterate through the results and dynamically allocate (!) a string that is large enough to hold each one. Then, I have to iterate through each record. Is there a better way?
int GetInDB(const char *query, cJSON **json)
{
if(*json != NULL){
cJSON_Delete(*json);
}
*json = cJSON_CreateArray();
if (mysql_query(mysqldb, query))
{
fprintf(stderr, "%s\n", mysql_error(mysqldb));
return -1;
}
MYSQL_RES *result = mysql_store_result(mysqldb);
if (result == NULL)
{
fprintf(stderr, "%s\n", mysql_error(mysqldb));
return -1;
}
int num_fields = mysql_num_fields(result);
int i;
MYSQL_ROW row;
MYSQL_FIELD *field;
char *Columns[num_fields];
int which = 0;
while((field = mysql_fetch_field(result)))
{
Columns[which] = (char*)malloc(strlen(field->name)+1);
strcpy(Columns[which], field->name);
which++;
}
while ((row = mysql_fetch_row(result)))
{
cJSON *obj = cJSON_CreateObject();
for(i = 0; i < num_fields; i++)
{
cJSON_AddStringToObject(obj, Columns[i], row[i] ? row[i] : "NULL");
}
cJSON_AddItemToArray(*json,obj);
}
mysql_free_result(result);
for(i=0; i<num_fields; i++){
free(Columns[i]);
}
return 0;
}