![]() ![]() ![]() All to clean up what is, conceptually at least, an internal data structure. But that would mean keeping the map pointer around and then making a separate call. So, what does the code need to look like in order to make this call? Ideally the cleanup should happen automatically. What needs to happen is a call to munmap(2) when the NSData deallocates. I just don't want NSData to do it, because it doesn't know the bytes came from a mapping and won't clean them up properly. Why create it this way? Because (a) I don't want to copy the bytes, since that would negate the advantage of memory mapping, and (b) I don't want NSData to try and clean up those bytes when it gets deallocated.īut I do need to clean up those bytes. I can convert the pointer into an NSData using one of NSData's longer convenience initializers. ![]() At this point, mappedFile points to a bunch of bytes which is more or less indistinguishable from what you'd get if you had actually read the file into memory. Once the mapping exists, the code disposes of the file descriptor via close(2). That, combined with the file's size, is enough to create the memory mapping. To call mmap(2) this code first gets a file descriptor for the file via open(2). If (mappedFile = MAP_FAILED) failed, errno=%d, %s", errno, strerror(errno)) MappedFile = mmap(0,, PROT_READ, MAP_FILE|MAP_PRIVATE, fd, 0) NSDictionary *fileAttributes = attributesOfItemAtPath:path error:nil] Given an NSString *path pointing to a file, you can create an memory mapped file like this: NSData isn’t actually mapping files itself, instead it’s using the Unix mmap(2) call. To create memory mapped files NSData is making use of iOS’s excellent Unix core. In short, it’s exactly what you need when your data file is too big to load, and if NSData won’t do it, I’ll just have to force it. When you access bytes in the memory map, data blocks are selectively read from the file as needed, and disposed of when they aren’t. It’s as if the file was already loaded into memory but had since been swapped back out. When you create a memory mapped file, the operating system gives you a memory pointer that you can use to access the file’s data. ![]() But the full power of virtual memory is at your disposal. It’s a way of using virtual memory to your advantage when you have a really big file and you don’t want to spend the RAM on it.Ĭontrary to common misconception, iOS does have virtual memory. Memory mapping is a cool Unix trick that lets you load a file into memory without, as it were, actually loading it into memory. So what could he do? The wheels of my mind started turning.īut first, a brief aside about memory mapping. Mapping wasn’t working despite using NSDataReadingMapped Always. Instruments was showing that the app was taking the full memory hit when the NSData was created. So, fine, whatever, it’s a different call, so what? Well, it wasn’t working. Starting with iOS 5 though, this method has been deprecated. Given NSString *path pointing to a file, you could create an NSData with almost no memory hit regardless of file size by creating it as: Until recently the obvious thing would have been to tell NSData to create a memory-mapped instance. It would load just fine, into an NSData, but before he could finish with it the app would run short of memory and die. He had a ginormous data file he needed to load up and process, and that memory hit was more than the app could bear. Here chromium url: /viewvc/chrome/trunk/src/content/browser/speechĬhrome records audio chunks, accepts only a FLAC file, an open source audio codec (free lossless audio codec) file with a sample rate of 16000.A couple of weeks ago Matt Long was having a problem with an app running out of memory. If you want to add in your “ test” project, (test project because it’s not a public API), you need to read Chrome Browser source code. Do you want use google speech api to recognize text from a dictate? ![]()
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