Sync images from two different cameras

I get images from two different cameras having different frame rates and resolutions. One of the cameras (smart camera) works as a controller. It receives images from another camera and its own image sensor, and then stores a pair of images in the on-board memory. I have a problem here. I have to do stereo matches with every pair of images. Therefore, images should be perfectly synchronized (just a few milliseconds). I point both cameras at the on-screen stopwatch and take pictures to see how far, in time, they are from each other. It turned out to be disappointing that neither they are synchronized, nor the time offset between one pair is constant. I am working on linux and C programming. I tried two posix streams to capture images from separate cameras.The first cam1 () thread signals the other cam2 () thread before starting to capture the image. Thus, both image captures must be started at the same time. But that doesn't work either. I would really appreciate any decision in this regard.