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Grand Central Dispatch (GCD) dispatch source flags

I recently switched from using kqueue to GCD dispatch sources to monitor file changes. This has worked out well and resulted in a much simpler API. I documented my switch here. The only issue I have is that now I cannot access the flags on the event that I was able to in kqueue. For example with kqueue I was able to check if the file was deleted, renamed, or it's attributes were changed with the following:

struct kevent event;

...

if(event.flag & EV_DELETE)
{
    printf("File was deleted\n");
}

Is this API not available with GCD or do I need to set up dispatch sources up for each flag I would like to listen to. Or is it best to use kqueue since it provides greater visibility to the event that has occurred.

like image 213
DHamrick Avatar asked Oct 13 '11 02:10

DHamrick


2 Answers

I found the answer in the Concurrency Programming Guide. I had first looked in the GCD Reference but without luck. The relevant line from the guide was

For a descriptor dispatch source that monitors file system activity, this function returns a constant indicating the type of event that occurred. For a list of constants, see the dispatch_source_vnode_flags_t enumerated type.

Here is an example of how you can use it.

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
int fildes = open("path/to/some/file", O_EVTONLY);
__block dispatch_source_t source = dispatch_source_create(DISPATCH_SOURCE_TYPE_VNODE,fildes, 
                                                  DISPATCH_VNODE_DELETE | DISPATCH_VNODE_WRITE | DISPATCH_VNODE_EXTEND | DISPATCH_VNODE_ATTRIB | DISPATCH_VNODE_LINK | DISPATCH_VNODE_RENAME | DISPATCH_VNODE_REVOKE,
                                                  queue);
dispatch_source_set_event_handler(source, ^
{
    unsigned long flags = dispatch_source_get_mask(source);
    if(flags & DISPATCH_VNODE_DELETE)
        printf("DISPATCH_VNODE_DELETE\n");
    if(flags & DISPATCH_VNODE_WRITE)
        printf("DISPATCH_VNODE_WRITE\n");
    if(flags & DISPATCH_VNODE_EXTEND)
        printf("DISPATCH_VNODE_EXTEND\n");
    if(flags & DISPATCH_VNODE_ATTRIB)
        printf("DISPATCH_VNODE_ATTRIB\n");
    if(flags & DISPATCH_VNODE_LINK)
        printf("DISPATCH_VNODE_LINK\n");
    if(flags & DISPATCH_VNODE_RENAME)
        printf("DISPATCH_VNODE_RENAME\n");
    if(flags & DISPATCH_VNODE_REVOKE)
        printf("DISPATCH_VNODE_REVOKE\n");
});
dispatch_source_set_cancel_handler(source, ^(void) 
{
    close(fildes);
});
dispatch_resume(source);
like image 98
DHamrick Avatar answered Oct 04 '22 20:10

DHamrick


You may change *dispatch_source_get_mask(source)* to *dispatch_source_get_data(source)*, as dispatch_source_get_mask(source) returns all the flags you passed in the creation of the handler instead of the event generated.

like image 24
Miguel Chacón Avatar answered Oct 04 '22 20:10

Miguel Chacón