How can I reuse an NSData to read multiple large files?

I need to read several dozen files and do some trivial processing with their contents. Each file individually won't cause problems, but having all the data loaded at once wi开发者_高级运维ll quickly exhaust my memory.

I started with:

for (NSString *filename in filenames)
    do_something([NSData dataWithContentsOfFile:filename]);

Then of course, I remembered that Objective-C on the iPhone is not really garbage collected, and those would all stick around until the end of the frame anyway. Okay:

for (NSString *filename in filenames) {
    NSData *d = [[NSData alloc] initWithContentsOfFile:filename];
    do_something(d);
    [d release];
}

This nominally only uses as much memory as the largest file, but that's only assuming the allocator is playing friendly at the moment - it could also thrash and fragment everything.

Is there some way I can make an NSMutableData, and keep reusing that Data's buffer, growing it as necessary? I need it as an NSData for other third-party APIs. The best idea I have at the moment is mallocing/reallocing a char* buffer as I go, reading using e.g. stdio, and constructing NSDatas with freeWhenDone:NO backed by that; that way I only thrash/retain a small amount per file.

What you are doing is the second example is fine. Even if you reused an NSMutableData object for its capacity another NSData object would need to be created with the file contents. If you are running into memory issues consider modifying do_something() to work with NSInputStreams.

You could use -[NSData initWithContentsOfMappedFile:] with your second example to keep the memory usage as low as possible.

From the documentation:

A mapped file uses virtual memory techniques to avoid copying pages of the file into memory until they are actually needed.