sqlite3 Program received signal SIGSEGV, Segmentation fault in sqlite3_get_table () - c++

Question

I have pasted my code below. It gives seg fault at sqlite3_get_table (). I want to get the number of row in table and return that. If number of row is greater than 0, the program needs to read all rows.
int countRowsInTable()
{
sqlite3 *db;
char *zErrMsg = 0;
int rc;
char const *sql;
char ***pazResult;
int *pnRow; /* Number of result rows written here */
int *pnColumn; /* Number of result columns written here */
rc = sqlite3_open("DeviceDetails.db", &db);
if( rc )
{
fprintf(stderr, "Can't open database: %s\n", sqlite3_errmsg(db));
return(0);
}
else
{
fprintf(stderr, "Opened database successfully\n");
}
/* Create SQL statement */
sql = "select * from Device_Details";
/* Execute SQL statement */
// rc = sqlite3_exec(db, sql, 0/*callback*/, 0/*(void*)data*/, &zErrMsg);
rc = sqlite3_get_table(db, sql, pazResult, pnRow, pnColumn, &zErrMsg);
void sqlite3_free_table(char **result);
if( rc != SQLITE_OK )
{
fprintf(stderr, "SQL error: %s\n", zErrMsg);
sqlite3_free(zErrMsg);
}
else
{
fprintf(stdout, "Operation done successfully\n");
cout<<"rows="<<pnRow<<endl;
sleep(10);
}
sqlite3_close(db);
cout<<"num rows =" << *pnRow <<endl;
}
after reading your suggestion I have changed my code. Now I use select count (*), but where (which) variable will hold the number of rows. Do, I need to implement the callback function in sqlite3_exec. Also I need to get data if row count is greater than zero.If I get the data in call back function, how to pass it to the function calling countRows in table? may be I can keep a global variable. As soon as call back is executed , it will signal the calling function through condition variable. So I have to maintain countRowsInTable function in a new thread. I there any simpler approach?
int countRowsInTable()
{
sqlite3 *db;
char *zErrMsg = new char[64];
int rc;
char const *sql;
char ***pazResult;
int *pnRow = new int; /* Number of result rows written here */
int *pnColumn = new int; /* Number of result columns written here */
//*pnRow = 0;
**pazResult = new char[20480];
rc = sqlite3_open("DeviceDetails.db", &db);
if( rc )
{
fprintf(stderr, "Can't open database: %s\n", sqlite3_errmsg(db));
return(0);
}
else
{
fprintf(stderr, "Opened database successfully\n");
}
/* Create SQL statement */
sql = "select count(*) from Device_Details";
/* Execute SQL statement */
rc = sqlite3_exec(db, sql, 0/*callback*/, 0/*(void*)data*/, &zErrMsg);
//rc = sqlite3_get_table(db, sql, pazResult, pnRow, pnColumn, &zErrMsg);
//void sqlite3_free_table(char **result);
if( rc != SQLITE_OK )
{
fprintf(stderr, "SQL error: %s\n", zErrMsg);
sqlite3_free(zErrMsg);
}
else
{
fprintf(stdout, "Operation done successfully\n");
}
}

Answer 1

Your are not calling the function with the right parameters. You are passing uninitialized pointers, whereas you must create the variables and pass pointers on them so SQLite can write the values.
const char *sql = "select * from Device_Details";
char **result;
int nrow;
int ncolumn;
char *errmsg;
int rc;
rc = sqlite3_get_table(db, sql, &result, &nrow, &ncolumn, &errmsg);
However, you should be aware the SQLite manual discourage the use of the function: "This is a legacy interface that is preserved for backwards compatibility. Use of this interface is not recommended."
Also, you should remove void sqlite3_free_table(char **result) that you probably pasted by mistake.