SQLite and BOOST_SCOPE_EXIT_ALL - c++

Question

Why does the SQLite C interface requires to pass the pointer by the reference when using BOOST_SCOPE_EXIT_ALL, for example?
Suppose that I have the following code:
#include <boost/scope_exit.hpp>
#include <sqlite3.h>
#include <cstdlib>
#include <iostream>
int main()
{
sqlite3* db;
BOOST_SCOPE_EXIT_ALL(db)
{
if (sqlite3_close(db) != SQLITE_OK)
{
std::cerr << sqlite3_errmsg(db) << '\n';
}
};
if (sqlite3_open(":memory:", &db) != SQLITE_OK)
{
std::cerr << sqlite3_errmsg(db) << '\n';
return EXIT_FAILURE;
}
sqlite3_stmt* prepared_stmt = NULL;
BOOST_SCOPE_EXIT_ALL(db, prepared_stmt)
{
if (sqlite3_finalize(prepared_stmt) != SQLITE_OK)
{
std::cerr << sqlite3_errmsg(db) << '\n';
}
};
if (sqlite3_prepare_v2(db, "CREATE TABLE IF NOT EXISTS foo(bar TEXT, baz TEXT)", -1, &prepared_stmt, 0) != SQLITE_OK)
{
std::cerr << sqlite3_errmsg(db) << '\n';
return EXIT_FAILURE;
}
if (sqlite3_step(prepared_stmt) != SQLITE_DONE)
{
std::cerr << sqlite3_errmsg(db) << '\n';
return EXIT_FAILURE;
}
}
It crashes when trying to call sqlite3_close function and doesn't print anything to the stderr. However, if I change
BOOST_SCOPE_EXIT_ALL(db)
to
BOOST_SCOPE_EXIT_ALL(&db)
it states that
unable to close due to unfinalized statements or unfinished backups
when trying to call sqlite3_close function.
If I pass the prepared_stmt by the reference too, it works as expected:
BOOST_SCOPE_EXIT_ALL(db, &prepared_stmt)
Why? I thought that the SQLite library only complains about the addresses that db and prepared_stmt points to.
Thanks in advance.

Answer 1

The first line in main()
sqlite3* db;
is probably why sqlite3_close() is crashing. Your compiler may be reusing a memory location for that declared pointer and may not necessarily be using a location that is null or making it null. Therefore, the call to sqlite3_close() is receiving a bogus pointer.
The docs at https://www.sqlite.org/c3ref/close.html state:
The C parameter to sqlite3_close(C) and sqlite3_close_v2(C) must be
either a NULL pointer or an sqlite3 object pointer obtained from
sqlite3_open(), sqlite3_open16(), or sqlite3_open_v2(), and not
previously closed. Calling sqlite3_close() or sqlite3_close_v2() with
a NULL pointer argument is a harmless no-op.
So, according to the above statement, the following code should remove the crash:
sqlite3* db = nullptr;
Of course, all of this would be moot if you just removed this block of unecessary code entirely:
BOOST_SCOPE_EXIT_ALL(db)
{
if (sqlite3_close(db) != SQLITE_OK)
{
std::cerr << sqlite3_errmsg(db) << '\n';
}
};
There is no reason to close a db that has never been opened.
EDIT: Here is a version of your function as I would try it. I'm really out of time right now so I can't test it, but I think it will work. I removed the boost macros since I'm unfamiliar with them, but you could add those back in. Hopefully this helps!
int main()
{
sqlite3* db = nullptr;
if (sqlite3_open(":memory:", &db) != SQLITE_OK)
goto ERROR_OCCURRED;
sqlite3_stmt* prepared_stmt = nullptr;
if (sqlite3_finalize(prepared_stmt) != SQLITE_OK)
std::cerr << sqlite3_errmsg(db) << '\n';
if (sqlite3_prepare_v2(db, "CREATE TABLE IF NOT EXISTS foo(bar TEXT, baz TEXT)", -1, &prepared_stmt, 0) != SQLITE_OK)
goto ERROR_OCCURRED;
if (sqlite3_step(prepared_stmt) != SQLITE_DONE)
goto ERROR_OCCURRED;
if (sqlite3_close(db) != SQLITE_OK)
{
std::cerr << sqlite3_errmsg(db) << '\n';
sqlite3_close(db);
return EXIT_FAILURE;
}
return 0;
ERROR_OCCURRED:
std::cerr << sqlite3_errmsg(db) << '\n';
sqlite3_close(db);
return EXIT_FAILURE;
}
Unfortunately, I'm out-of-time to check your call to sqlite3_finalize() but I also think that it's probably too early in the function. I don't see a reason to finalize a null pointer.