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    <p>Use <code>@synchronized(logBuffer)</code>:</p> <pre><code>- (void) addToLog:(NSString*)str { @synchronized(logBuffer) { [logBuffer addObject:s]; } if ([logBuffer count] &gt;= kBufferSize) { [self writeLogOnFile]; } } - (void) writeLogOnFile { @synchronized(logBuffer) { NSString *multiline = [logBuffer componentsJoinedByString:@"\r\n"]; multiline = [NSString stringWithFormat:@"%@\n", multiline]; NSData *data = [multiline dataUsingEncoding:NSUTF8StringEncoding]; NSFileHandle *outputFileHandle = [NSFileHandle fileHandleForWritingAtPath:logFilePath]; [outputFileHandle seekToEndOfFile]; [outputFileHandle writeData:data]; [outputFileHandle closeFile]; [logBuffer removeAllObjects]; // This is where the exception is thrown } } </code></pre> <p>Edit: Since I'm using <code>@synchronized</code>, we can get rid of the buffer copy and just synch.</p> <p>In continuation, considering the comments and edited question:</p> <p>If you only call <code>writeLogOnFile</code> from <code>addToLog</code>, then I would do one of two things:</p> <ol> <li><p>Merge the <code>writeLogOnFile</code> code into <code>addToLog</code>, since it's 1-to-1 anyway. This ensures that nothing will ever directly call <code>writeLogOnFile</code>. In this case, wrap <code>addToLog</code> completely within <code>@synchronized(logBuffer) {}</code></p></li> <li><p>If you want to keep <code>writeLogOnFile</code> separate for whatever reason, then make this method private to the class. In this case, you can get rid of <code>@synchronized(logBuffer)</code> within <code>writeLogOnFile</code> since in theory you know what you are doing within the class, but you should also wrap <code>addToLog</code> completely within <code>@synchronized(logBuffer) {}</code></p></li> </ol> <p>As you can see, in both cases you should absolutely make <code>addToLog</code> completely single-threaded through <code>@synchronized</code> (or keep the original answer). It's very simple, keeps your code clean, and gets rid of all the threading issues that your edited question is trying to work around. The <code>@synchronized</code> pattern was created specifically to avoid writing all the wrapper code that you wrote to solve your problem, namely forcing everything through the main thread (or a specific thread).</p> <p>For completeness, here's the complete code that I would write:</p> <pre><code>- (void) addToLog:(NSString*)str { @synchronized(logBuffer) { [logBuffer addObject:s]; if ([logBuffer count] &gt;= kBufferSize) { // write log to file NSString *multiline = [logBuffer componentsJoinedByString:@"\r\n"]; multiline = [NSString stringWithFormat:@"%@\n", multiline]; NSData *data = [multiline dataUsingEncoding:NSUTF8StringEncoding]; NSFileHandle *outputFileHandle = [NSFileHandle fileHandleForWritingAtPath:logFilePath]; [outputFileHandle seekToEndOfFile]; [outputFileHandle writeData:data]; [outputFileHandle closeFile]; [logBuffer removeAllObjects]; } } } </code></pre>
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    1. COI would instead do `@synchronized(logBuffer) { NSArray *bufferCopy = ...; [logBuffer removeAllObjects]; }` at the start of `writeLogOnFile`. This guards against the possibility that a log item will be added between duplicating the array and removing the objects--any added objects would not be written to the file but would just be removed when `logBuffer` is cleared.
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    2. COActually, the second @syncronized is unnecessary if the only way to enter write... is through the (synchronized) addToLog. But this isn't the way I'd do it, I don't think.
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    3. COMy answer is really a sample of what can be done. I assume that @dimitrios is probably calling `[logBuffer addObject:s]` from many places, and similarly with `[self writeLogOnFile]`. Using `@synchronized` allows one to be very flexible with how and when to call this.
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