On Wed, Nov 5, 2014, at 12:50 AM, Gerriet M. Denkmann wrote:
> I want to monitor a file for changed content.
> Polling is generally not a good idea, so I tried dispatch_source.
>
> But it does not work.
>
> Here the code:
>
> static NSString *const kTestPath = @"/tmp/a.test";
>
> - (void)applicationDidFinishLaunching:(NSNotification *)aNotification
> {
> const char *filename = kTestPath.fileSystemRepresentation;
> int fd = open(filename, O_RDONLY);
> fcntl(fd, F_SETFL, O_NONBLOCK);
> uintptr_t fD = (uintptr_t)fd;
> dispatch_queue_t queue =
> dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
> const uint64_t flags = DISPATCH_VNODE_WRITE | DISPATCH_VNODE_LINK;
> dispatch_source_t my_source =
> dispatch_source_create(DISPATCH_SOURCE_TYPE_VNODE, fD, flags, queue);
> dispatch_source_set_event_handler(my_source, ^{ [self fileDidChange]; }
> );
> dispatch_resume(my_source);
> }
>
> [snip]
>
>
> What am I doing wrong? (Writing directly to kTestPath without rename()
> also does not work).
>
My own test setup, which mimics yours, only works if the file already
exists when the dispatch source is created. This makes sense, since
without O_CREAT, open() returns -1 (an invalid file descriptor).
You need to pass O_RDONLY | O_CREAT to open() to ensure that the file
exists before you start trying to monitor it for changes.
--Kyle Sluder
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