Issue Description:

I am evaluating libdispatch for a cross platform C project. I found a bug ( SR-7316 ) that is also related to dispatch sources. Dispatch sources seem hard to use without blocks, because I can not find a way to pass context to the handlers. This also goes against the api design advice in the documentation. (See https://developer.apple.com/legacy/library/documentation/Darwin/Reference/ManPages/man3/dispatch_api.3.html )

I can think of only 2 ways to circumvent this problem:

1. Using dispatch_get_current_queue and doing a lookup to find the context dependent data.

2. Using some form of dynamic function creation to create a new function depending on the context.

Both seem to have serious problems. The first is deprecated, and the second is hardly a portable solution.

Did I miss anything or is there a pressing reason that dispatch_source_set_*_handler_f do not have a context parameter? I suspect that changing the function signatures is a breaking change that may not be possible, but is there any improvement that can be made to libdispatch to alleviate this problem?