read data of specific length from a specific position ios

Question

I have the following code for reading a file in length of specific size:

  int chunksize = 1024;
  NSData*  fileData = [[NSFileManager defaultManager] contentsAtPath:URL];
  NSString* fileName = [[message.fileURL lastPathComponent] stringByDeletingPathExtension];
  NSString*  extension = [[message.fileURL pathExtension] lastPathComponent];
  NSFileHandle*  fileHandle = [NSFileHandle fileHandleForReadingAtPath:[self retrieveFilePath:fileName andExtension:extension]];
  file=@"test.png";

    int numberOfChunks =  ceilf(1.0f * [fileData length]/chunksize); //it s about 800

    for (int i=0; i<numberOfChunks; i++)
    {
        NSData *data = [fileHandle readDataOfLength:chunksize];
        ....//some code
    }

// read a chunk of 1024 bytes from position 2048
 NSData *chunkData = [fileHandle readDataOfLength:1024 fromPosition:2048];//I NEED SOMETHING LIKE THIS!!

in order to retrive the total number of chunks in a file and to convert the real number into a integer one i've used that syntax — just ME, Jul 17 '14 at 07:25
I think this will work: `int numberOfChunks = ([fileData length] / chunksize) + (([fileData length] % chunksize) ? 1 : 0);` — trojanfoe, Jul 17 '14 at 07:35

trojanfoe · Accepted Answer · 2014-07-17T07:26:15.603

4

You need to set the file pointer to the offset you want to read from:

[fileHandle seekToFileOffset:2048];

And then read the data:

NSData *data = [fileHandle readDataOfLength:1024];

Be aware that errors are reported in the form of NSExceptions, so you'll want some @try/@catch blocks around most of these calls. In fact the use of exceptions to report errors means I often make my own file-access functions to ease their use; for example:

+ (BOOL)seekFile:(NSFileHandle *)fileHandle
        toOffset:(uint32_t)offset
           error:(NSError **)error
{
    @try {
        [fileHandle seekToFileOffset:offset];
    } @catch(NSException *ex) {
        if (error) {
            *error = [AwzipError error:@"Failed to seek in file"
                                  code:AwzipErrorFileIO
                             exception:ex];
        }
        return NO;
    }

    return YES;
}

edited Jul 17 '14 at 07:26

answered Jul 17 '14 at 07:20

trojanfoe

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a very nice way of explaining this. Thank you very much!! Could I ask you why did you choose to use + in front of the [self seekFile:fileHandler toOffset:2048 error:error] method? You want to make it a public one? – just ME Jul 17 '14 at 07:31
It's because that's a utility class which is used from everywhere in the code. The class holds no state and simply provides utility functions. – trojanfoe Jul 17 '14 at 07:32
That's a class to create custom `NSError` objects, specific to the problem domain. You can just use the base `NSError` methods or create your own error-generating functions. – trojanfoe Jul 17 '14 at 08:00

read data of specific length from a specific position ios

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