write nsdata in a file at a specific position

Question

I have the following code for writing data in a file:

NSData *chunk=...; //some data
 NSArray *docDirectories = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *docDirectory = [docDirectories objectAtIndex:0];
    NSString *fileName   = [docDirectory stringByAppendingPathComponent:@"TestFile.txt"];
 [chunk writeToFile:fileName atomically:NO];

If I know the size of the file (let;s say 10*chunk) and if i also receive the position of each chunk in the total length of the file, how can I add write data to file at that specific position?

Answer 1

To solve your questions, your best bet is to use NSOutputStream, it makes operations like these easier to handle.

That being said, you would append to the end of the file like this:

NSOutputStream *stream = [[NSOutputStream alloc] initToFileAtPath:filePath append:YES];
[stream open];
NSData *chunk = ...; // some data
[stream write:(uint8_t *)[chunk bytes] maxLength:[chunk length]];
[stream close];
// remember to always handle memory (if not using ARC) //

To insert a chunk of data in the middle of a file is somewhat more involved:

NSData *chunk = ...; // some data
NSString *filePath = ... ; // get the file //
NSUInteger insertionPoint = ...; // get the insertion point //
// make sure the file exists, if it does, do the following //
NSData *oldData = [NSData dataWithContentsOfFile:filePath];
// error checking would be nice... if (oldData) ... blah //
NSOutputStream *stream = [[NSOutputStream alloc] initToFileAtPath:filePath append:NO];
[stream open];
[stream write:(uint8_t *)[oldData bytes] maxLength:insertionPoint]; // write the old data up to the insertion point //
[stream write:(uint8_t *)[chunk bytes] maxLength:[chunk length]]; // write the new data //
[stream write:(uint8_t *)&[oldData bytes][insertionPoint] maxLength:[oldData length] - insertionPoint]; // write the rest of old data at the end of the file //
[stream close];
// remember to always handle memory (if not using ARC) //

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