Trouble Passing C String in Struct

Question

I'm having trouble getting a char* string from a passed struct. I have the following code:

typedef struct {
    NSInteger id;
    char *title;
} Movie;

...

Movie movie = [self randomMovie];

NSInteger movieID = movie.id;
NSString *movieTitle = [NSString stringWithUTF8String:movie.title];
NSLog(@"movieTitle: %@", movieTitle);

...

- (Movie)randomMovie {
sqlite3_stmt *statement;
NSString *query = @"SELECT id, title FROM movies ORDER BY RANDOM() LIMIT 1;";

Movie movie;

if (sqlite3_prepare_v2(database, [query UTF8String], -1, &statement, nil) == SQLITE_OK) {
    if (sqlite3_step(statement) == SQLITE_ROW) {
        // Get the id and title of the first
        movie.id = sqlite3_column_int(statement, 0);
        movie.title = (char *)sqlite3_column_text(statement, 1);
    }
}

NSLog(@"Random movie %d title: %@", movie.id, [NSString stringWithUTF8String:movie.title]);

sqlite3_finalize(statement);
return movie;
}

This gives me the output:

2013-03-13 10:10:39.438 Fabflix[89156:c07] Random movie 872011 title: Ray
2013-03-13 10:10:39.439 Fabflix[89156:c07] movieTitle: (null)

Does anyone know why the title string isn't being passed correctly from randomMovie? Thanks!

Lily Ballard · Accepted Answer · 2013-03-13 17:20:24Z

2

I'm not terribly familiar with the sqlite3 C API, but are you sure the sqlite3_finalize() call isn't destroying the C string returned from sqlite3_column_text()? You may need to strdup() that thing (and then remember to free() it later).

Update: Yep, from the docs:

The pointers returned are valid until a type conversion occurs as described above, or until sqlite3_step() or sqlite3_reset() or sqlite3_finalize() is called. The memory space used to hold strings and BLOBs is freed automatically.

answered Mar 13, 2013 at 17:20

Lily Ballard

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Comments

user7116 · Accepted Answer · 2013-03-13 17:21:47Z

1

The memory returned by sqlite3_column_text is released as soon as you call sqlite3_finalize. You need to copy the value to your own string first.

typedef struct {
    NSInteger id;
    NSString *title;
} Movie;

/* ... */

movie.title =
    [NSString stringWithUTF8String:(char *)sqlite3_column_text(statement, 1)];

answered Mar 13, 2013 at 17:21

user7116

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