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I'm looking for the analogous of Cocoa's

-[NSObject performSelectorOnMainThread: withObject: waitUntilDone:]

method.

So basically I have a function that does some work on a separate thread but it must perform some synchronous calls that need to be performed on the main one.

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in cocoa, the message is added to the run loop, which is cleared as part of its iteration.

to simulate this:

  • you'll a want a run loop
  • an abstract message system
  • and a reference counting mechanism (in most cases)
  • a way to add those messages to a run loop for scheduled execution

timers would be a nice addition

to accomplish something similar using pthread interfaces exclusively, start by reading up on conditions pthread_cond_t.

i know of no pthread interface with a 1-1 relationship for what you're trying to accomplish. conditions also operate without run loops, so you may need to bring that to the table, if you do not reuse a run loop implementation. if you use run loops, then you just need a lock to add messages to a thread with a run loop.

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yes, I know how that works in Cocoa. But the question is about thread switching. Run loop works on a single thread, but what if you want to perform some function on another existing thread?
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pthreads are a very low-level abstraction, so there's no easy way to do this with raw pthreads. Typically you'll want to write to a file descriptor to wake up an event loop on the main thread, then pass it a pointer to the function you want to run. You could even write pointer values onto a pipe(), then have the main thread execute them.

To wait synchronously, you can simply have a mutex and condition variable, plus completion flag on these execution request objects. Have the child thread wait on the mutex/condvar/completion flag, then in the main thread (under the mutex) set the flag and signal the cvar. Cleanup of the request structure would be done in the child.

To be more specific, it'd help if you could mention what event loop you have running on your main thread.

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