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POWhy does Cocoa return an empty string occasionally?
 

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  1. POWhy does Cocoa return an empty string occasionally?
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    <p>I have some code in my application that looks something like this:</p> <pre><code>char *hash = (char*) sqlite3_column_text(get_bookmark, 0); NSString* postHash = [NSString stringWithUTF8String:hash]; </code></pre> <p>This works for me every time; I've never seen it <em>not</em> work. Most of my users do not experience problems (as far as I know). However I find that <code>postHash</code> is an empty string (<code>@""</code>) for some users some of the time.</p> <p>Can anyone explain why?</p> <p>Some more context/speculation:</p> <p>This only seems to happen on jailbroken handsets. Is there anything different about them? I gather that there's usually less memory available. Anything else that could contribute here?</p> <p><code>postHash</code> is used in a table cell and is occasionally seen to be populated correctly so I'm reasonably confident that the database call <em>should</em> work. In fact, if the database also has an empty string it's because of a very similar piece of code so the question remains.</p> <p><code>hash</code> is certainly returning with a non-NULL value. If I force a NULL here, the app crashes. Similarly, <code>postHash</code> is not <code>nil</code> as that would also crash the app (for the same reason).</p> <p>I am thinking that this is possibly memory related. If the method tries to allocate too much memory before <code>-didReceiveMemoryWarning</code> can get called what happens? I know that, at some point, the Springboard ejects the app. But is it possible that Cocoa returns a null string here rather than the expected value? I've heard of a few reports that, as far as I can tell, can only have been caused by an empty string being present where something longer should have been present.</p> <p>Any other speculation, theories or ideas welcome.</p>
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    1. USStephen Darlington
    1. USStephen Darlington
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      1. USRahul Vyas
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    1. CO“But is it possible that Cocoa returns a null string here rather than the expected value?” A “null string” would be either NULL (the C-string) or nil (the NSString). An empty string is altogether different: it's a string with no characters in it, whereas NULL/nil is no string at all.
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      1. USPeter Hosey
    2. COThat's not my understanding of what a "null string" is. When I say "null string" above I mean "empty string", i.e., @"" in Objective-C or "" in C. A nil/NULL value causes the app to crash.
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      1. POWhy does Cocoa return an empty string occasionally?
      1. USStephen Darlington
    3. CO“Null” means 0. The null character is '\0'; i.e., 0. The null pointer is 0. There is no such thing as a “null string” in C, because if it's null, it's not a string; it's just 0.
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      1. USPeter Hosey
 

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