Arguably, you'd be better off subclassing NSData directly to add the mutation
APIs that you actually need. That clears up any possible confusion about
methods which might affect the length of the data.
On 14 Dec 2012, at 00:13, Robert Monaghan wrote:
> Thanks for the suggestion, Kevin!
>
> I went ahead and created a really crude subclass of NSMutableData. It seems
> to work for my situation.
> Attached is the code, for posterity. (I am sure I won't be the only one
> working around this.)
>
> Any suggestions to make this a bit more "Proper" are welcome.
>
> bob.
>
> //
> // GT_NSMutableData.h
> //
> // Created by Robert Monaghan on 12/13/12.
> // Copyright (c) 2012 Glue Tools LLC. All rights reserved.
> //
>
> #import <Foundation/Foundation.h>
>
> @interface GT_NSMutableData : NSMutableData
> {
> void *_gt_buffer;
> NSUInteger _gt_length;
> BOOL _gt_freeWhenDone;
> }
> @property (readwrite) NSUInteger length;
> @end
>
> //
> // GT_NSMutableData.m
> //
> // Created by Robert Monaghan on 12/13/12.
> // Copyright (c) 2012 Glue Tools LLC. All rights reserved.
> //
>
> #import "GT_NSMutableData.h"
>
> @implementation GT_NSMutableData
> + (id)dataWithBytesNoCopy:(void *)bytes length:(NSUInteger)length
> freeWhenDone:(BOOL)b
> {
> return [[[GT_NSMutableData alloc] initWithBytesNoCopy:bytes
> length:length freeWhenDone:b] autorelease];
> }
>
> - (id)initWithBytesNoCopy:(void *)bytes length:(NSUInteger)length
> freeWhenDone:(BOOL)b
> {
> self = [super init];
> if (self) {
> _gt_buffer = bytes;
> _gt_length = length;
> _gt_freeWhenDone = b;
> }
> return self;
> }
>
> - (void)dealloc
> {
> if (_gt_freeWhenDone)
> free(_gt_buffer);
>
> [super dealloc];
> }
>
> - (void)increaseLengthBy:(NSUInteger)extraLength
> {
> }
>
> - (void)setLength:(NSUInteger)length
> {
> }
>
> - (NSUInteger) length
> {
> return _gt_length;
> }
>
> - (const void *)bytes
> {
> return _gt_buffer;
> }
>
> - (void *)mutableBytes
> {
> return _gt_buffer;
> }
>
> - (void)replaceBytesInRange:(NSRange)range withBytes:(const void *)bytes
> {
> }
>
> - (void)replaceBytesInRange:(NSRange)range withBytes:(const void
> *)replacementBytes length:(NSUInteger)replacementLength
> {
> }
>
> - (void)resetBytesInRange:(NSRange)range
> {
> }
>
> - (void)setData:(NSData *)data
> {
> _gt_buffer = (void *)[data bytes];
> }
>
> @end
>
>
> On Dec 13, 2012, at 3:22 PM, Kevin Perry <kpe...@apple.com> wrote:
>
>> NSMutableData currently ignores (and always has, to my knowledge) the
>> no-copy "hint" and always copies the bytes into an internal buffer in case
>> something tries to change the length of the NSMutableData.
>>
>> It would not be too difficult to make a subclass of NSMutableData that
>> doesn't copy and throws an exception when something tries to change the
>> length. That would violate the Liskov substitution principle though, so at
>> the very least, you want to prevent any code that doesn't understand the
>> fixed-length restriction from getting ahold of one of these objects.
>>
>> [kevin perry];
>>
>> On Dec 13, 2012, at 2:28 PM, Robert Monaghan <b...@gluetools.com> wrote:
>>
>>> Hi Everyone,
>>>
>>> I have just run head long into an issue with NSMutableData and existing
>>> buffers.
>>> I have the following code:
>>>
>>>
>>> UInt8 *sourcebytes = [clImgEngine srcBuffer];
>>> [self setData:[NSMutableData
>>> dataWithBytesNoCopy:sourcebytes length:[clImgEngine inRangeByteCount]
>>> freeWhenDone:NO]];
>>> UInt8 *resultBytes = [[self data] mutableBytes];
>>>
>>> The source is a pinned memory buffer from OpenCL. What I want to do, is to
>>> pass this buffer inside a NSMutableData wrapper and have another object for
>>> some work.
>>> Seems simple enough, except that NSMutableData changes the memory buffer in
>>> the background.
>>>
>>> "sourcebytes" starts with an address such as: 0x000000012361b000
>>> and "resultBytes" returns 0x0000000136fe2000
>>>
>>> Naturally, my work object doesn't see any data from "sourcebytes", as
>>> NSMutableData has moved the buffer.
>>> I thought that freeWhenDone:NO prevented ownership of the buffer..
>>>
>>> Can anyone suggest a way to prevent this from happening? I need the buffer
>>> to stay "pinned".
>>>
>>> Thanks in advance!
>>>
>>> bob.
>>>
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>>
>
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