Question

I am writing a ODBC-database class which contains a member function used to fetch a series of attributes and tuples from a given query.

I have a single line of code in the statements below which causes this runtime error to be throw in debug mode:

Unhandled exception at <mem loc> in <prog name>: 0xC0000005: Access violation writing location <mem loc>.

and here is the code where ERROR points out the offending line:

SQLINTEGER length = 0;
vector<vector<string>> data;
this->sReturn = SQLFetch(this->sHandle);

while (this->sReturn == SQL_SUCCESS) {
  vector<string> tuple;

for (int i = 0; i < columnCount; i++) {
  SQLPOINTER value = "";

  switch (info[i].columnType) {
    case 0 : //SQL_UNKNOWN_TYPE
      throw DatabaseAttributeTypeUnknown("The database returned an attribute of an unknown type.");
      break;

    case 1 : //SQL_CHAR
      this->sReturn = SQLGetData(this->sHandle, i + 1, info[i].columnType, value,
        info[i].columnSize*sizeof(SQLCHAR),
ERROR   &length);
        break;

     //Some more cases
  }
}

Any idea on why this error is being thrown? Here is the MSDN documentation on SQLGetData(), which is assigning a value to length.

Thank you for your time.

Answer 1

When a compiler maps executable code to line of code in the sources, they generally cannot differentiate lines in a statement that is split into multiple lines. So if the debugger says that an error happens on a specific line, it can actually be anywhere in the entire statement, so somewhere in:

this->sReturn = SQLGetData(this->sHandle, i + 1, info[i].columnType, value,
    info[i].columnSize*sizeof(SQLCHAR),
    &length);

The only shaky pointer here is value which point to a null static string, so to a one character long array containing a null byte. Also, depending on compiler options, this array can be in a read-only segment of data. While SQLGetData() thinks it points to a location which is at least info[i].columnSize*sizeof(SQLCHAR) bytes in size and where it will write (not read) the data from the SQL column.

I may miss other details, but my first guess is that is what causes the memory access violation.