NSData的行为initWithBytesNoCopy：length：freeWhenDone： [英] Behavior of NSData initWithBytesNoCopy:length:freeWhenDone:

问题描述

我想要一个固定长度的可变内容共享数据缓冲区，这是如何创建它：

  void * buffer = malloc（length）; 
 //初始化缓冲区内容
 NSData * sharedData = [[NSData alloc] initWithBytesNoCopy：buffer length：length freeWhenDone：YES] 
  

在我创建了一个NSData之后，如果我修改 buffer 会发生什么？ NSData会反映我对 buffer ？

所做的更改

我可以保证 sharedData 当我要修改 buffer 时，不会得到dealloc。

想要使用它：

---

  void * my_alloc（CFIndex allocSize，CFOptionFlags hint， void * info）{return NULL;} 
 void my_dealloc（void * ptr，void * info）{
 mach_vm_deallocate（mach_task_self（），（mach_vm_address_t）ptr，（size_t）info）; 
} 
  

---

  size_t length = //一些数字
 mach_vm_address_t buffer; 
 mach_vm_allocate（mach_task_self（），& buffer，length，VM_FLAGS_ANYWHERE）; 
 //做某些缓冲，例如使用mach RPC传递给其他进程，并期望其他进程将修改内容
 CFAllocatorContext context = {0，（void *）length，NULL，NULL， my_alloc，NULL，my_dealloc，NULL}; 
 CFAllocatorRef allocator = CFAllocatorCreate（NULL，& context）; 
 CFDataCreateWithBytesNoCopy（NULL，（const UInt8 *）buffer，length，allocator）; 
   initWithBytesNoCopy：  code>将有效地在现有缓冲区周围创建一个 NSData 包装;所以是的，通过 [sharedData bytes] 访问的任何内容都会看到您所做的更新。
 
 
 
当然，没有链接从 NSData 实例创建的其他对象，因此例如 [NSImage initWithData：sharedData] 为 NSImage 实例创建副本，这将不会反映任何更改。
 
 
 
此外， $ c> freeWhenDone：YES   NSData 会在删除最后一个引用时破坏缓冲区，所以请留意=）
 
 
 
 
 
 
因此，由于 NSData 实际上是< c $ c> malloc（） allocation，yes它将反映对该内存的更改（由任何进程）;但是它会调用 free（），它是一个坏主意，使用它包装另一种方式创建的缓冲区（ mach_vm_allocate ）with  freeWhenDone：YES 。
 
 
 
如果你真的不需要使用自定义分配器（为什么？），我想你会更好的：
  NSMutableData * sharedData = [NSMutableData dataWithCapacity：length] ; 
 //`dataWithLength：`将首先完全分配和清零缓冲区，如果你喜欢
 void * buffer = [sharedData mutableBytes]; 
 //对'buffer`，mach RPC等做某事
 // 3：利润。 
  
 
I want to have a fix length mutable content shared data buffer and that is how do I create it:
void *buffer = malloc(length);
// initialize buffer content
NSData *sharedData = [[NSData alloc] initWithBytesNoCopy:buffer length:length freeWhenDone:YES]
What happen if I modify buffer after I created a NSData from it?  Will NSData reflect the change I did to buffer?

I can guaranty that sharedData will not get dealloc when I want to modify buffer.

This is how I actually want to use it:



void *my_alloc(CFIndex allocSize, CFOptionFlags hint, void *info) {return NULL;}
void my_dealloc(void *ptr, void *info) {
    mach_vm_deallocate(mach_task_self(), (mach_vm_address_t)ptr, (size_t)info);
}




size_t length = //some number
mach_vm_address_t buffer;
mach_vm_allocate(mach_task_self(), &buffer, length, VM_FLAGS_ANYWHERE);
// do something to buffer, for example pass to other process using mach RPC and expect other process will modify the content
CFAllocatorContext context = {0, (void *)length, NULL, NULL, NULL, my_alloc, NULL, my_dealloc, NULL};
CFAllocatorRef allocator = CFAllocatorCreate(NULL, &context);
CFDataCreateWithBytesNoCopy(NULL, (const UInt8 *)buffer, length, allocator);

解决方案
The initWithBytesNoCopy: will effectively create an NSData wrapper around the existing buffer; so yes, things accessing through [sharedData bytes] will see any updates you make.

Of course, it does nothing to link other objects which are created from the NSData instance, so for instance an [NSImage initWithData:sharedData] may make a copy for the NSImage instance, which won't reflect any changes.

Also, with freeWhenDone:YES the NSData will destroy the buffer when the last reference is removed, so look out for that =)



So, given that the NSData is effectively a thin wrapper around a malloc() allocation, yes it will reflect changes made to that memory (by any process); but as it will call free() on it, it's a bad idea to use it to wrap a buffer created another way (mach_vm_allocate) with freeWhenDone:YES.

If you don't really-really need to use a custom allocator (why?), I think you'd be better off with:
NSMutableData* sharedData = [NSMutableData dataWithCapacity:length];
// `dataWithLength:` will fully allocate and zero the buffer first, if you prefer
void* buffer = [sharedData mutableBytes];
// do something to `buffer`, mach RPC, etc.
// 3: profit.


                        
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