Either I'm confused or the gc is

Steven Schveighoffer schveiguy at gmail.com
Sun Nov 15 18:43:00 UTC 2020

On 11/15/20 10:05 AM, H. S. Teoh wrote:
> On Sun, Nov 15, 2020 at 01:43:14PM +0000, donallen via Digitalmars-d wrote:
>> With the help of an excellent suggestion by Steve Schveighoffer, I
>> finally identified this problem yesterday. It was my error, not D's.
> [...]
>> void bind_text(sqlite3_stmt* stmt, int index, const char[] the_text)
>> {
>>      if (sqlite3_bind_text(stmt, index, toStringz(the_text), -1, null)
>> != sqlite_ok)
>>      {
>>          stderr.writeln("bind_text failed");
>>          auto db = sqlite3_db_handle(stmt);
>>          stderr.writeln("Error: %s", fromStringz(sqlite3_errmsg(db)));
>>          exit(EXIT_FAILURE);
>>      }
>> }
> [...]
>> The culprit is the call to toStringz. The documentation for that
>> function give some good advice that I failed to follow:
>> "Important Note: When passing a char* to a C function, and the C
>> function keeps it around for any reason, make sure that you keep a
>> reference to it in your D code. Otherwise, it may become invalid
>> during a garbage collection cycle and cause a nasty bug when the C
>> code tries to use it."
> Ouch.  I'm dismayed that I missed this when I last looked at this code
> (which I believe you did post here). ;-)  Passing a GC-allocated string
> to C code without retaining a reference somewhere on the D side is a
> known gotcha when interfacing with C.  In fact, I'm almost certain I ran
> into this very problem with toStringz myself in the past, yet I still
> failed to notice it when I looked at your code.

I had the same reaction! I also did not see the toStringz as a problem 
when I reviewed.

I will note, also, that toStringz has this quirk where in certain cases 
it just returns the pointer of the array, which is why I think the .idup 
fix worked (that was puzzling to me and I think pulled me away from it 
being a problem with storing a pointer in C-land only).


> In any case, I'm glad that this means you'll be around here more. ;-)



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