Wayland (& gstreamer) uses fd-passing to implement zero copy of graphics-textures/video frames across process. this is typically used in conjunction with buffers allocated by a mechanism like dmabuf which allocates a memory area and provides a fd associated with it to userland. this (translated) fd can be used by the other process to map the same region of memory.

I don't follow. How can file descriptors be passed through a shared memory to another process and remain valid in its context?

Assuming it's not Windows, where it is possible to explicitly clone a handle for a specific process.

The linked code example doesn't seem to do fd passing, but I guess the use case of fd passing in shared memory context would be the other way around: passing fd's that point to shared memory. The shared memory could then be mmap'ed using the passed fd as the handle.