Question

Parsing a 20MB file that is too large to do in one piece on a 2G iPod touch. I commented out all code except the NSData/NSMutableString part and stil have the leak. I don’t understand….

support method to read chunk from file:

+ (NSData *) dataWithContentsOfFile:(NSString *)path atOffset:(off_t)offset withSize:(size_t)bytes  {
    FILE *file = fopen([path UTF8String], "rb");
    if(file == NULL)
        return nil;

    void *data = malloc(bytes);  // check for NULL!
    fseeko(file, offset, SEEK_SET);
    fread(data, 1, bytes, file);  // check return value, in case read was short!
    fclose(file);

    // NSData takes ownership and will call free(data) when it's released
    return [NSData dataWithBytesNoCopy:data length:bytes];
}

and the barebones code that leaks:

while( cnt<total) {
    NSAutoreleasePool* pool= [[NSAutoreleasePool alloc] init];

    NSData* data= [NSData dataWithContentsOfFile:fullPath atOffset:cnt withSize:MIN(100000,total-cnt)];
    NSString* xmlFragment  = [[NSString alloc] initWithBytes: [data bytes] length:[data length] encoding: NSUTF8StringEncoding];
    cnt+= [xmlFragment length];

    [pool drain];
}

Answer 1

You are doing alloc-init every time you are in the loop but there isn’t a release so there is a leak in your code.

NSString* xmlFragment  = [[NSString alloc] initWithBytes: [data bytes] length:[data length] encoding: NSUTF8StringEncoding];

There is no balancing release for the statement above. If you do,

NSString* xmlFragment  = [[[NSString alloc] initWithBytes: [data bytes] length:[data length] encoding: NSUTF8StringEncoding] autorelease];

It will be deallocated at [pool drain];. However the loop seems pointless. You probably meant to do a bit more like add xmlFragment to a mutable string perhaps?