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Asked: 2026-05-13T08:26:46+00:00

I have an image generator which would benefit from running in threads. I am

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I have an image generator which would benefit from running in threads. I am intending to use POSIX threads, and have written some mock up code based on https://computing.llnl.gov/tutorials/pthreads/#ConVarSignal to test things out.

In the intended program, when the GUI is in use, I want the generated lines to appear from the top to the bottom one by one (the image generation can be very slow).

It should also be noted, the data generated in the threads is not the actual image data. The thread data is read and transformed into RGB data and placed into the actual image buffer. And within the GUI, the way the thread generated data is translated to RGB data can be changed during image generation without stopping image generation.

However, there is no guarantee from the thread scheduler that the threads will run in the order I want, which unfortunately makes the transformations of the thread generated data trickier, implying the undesirable solution of keeping an array to hold a bool value to indicate which lines are done.

How should I deal with this?

Currently I have a watcher thread to report when the image is complete (which really should be for a progress bar but I’ve not got that far yet, it instead uses pthread_cond_wait). And several render threads doing while(next_line());

next_line() does a mutex lock, and gets the value of img_next_line before incrementing it and unlocking the mutex. it then renders the line and does a mutex lock (different to first) to get lines_done checks against height, signals if complete, unlocks and returns 0 if complete or 1 if not.

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1 Answer

  1. Editorial Team

    Given that threads may well be executing in parallel on different cores it’s pretty much inevitable that the results will arrive out of order. I think your appraoch of tracking what’s complete with a set of flags is quite reasonable.

    It’s possible that the overall effect might be nicer if used threads in a different granularity. Say give each thread (say) 20 lines to work on rather than one. Then on completion you’d have bigger blocks available to draw, and maybe drawing stripes would look ok?

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