I tried testlwp-static, and it too segfaults, after about 5 context
switches.

  If the information is useful, it is probably better to use sigaction
  with the SA_SIGINFO flag set, which is defined by POSIX.

I certainly agree that it's better to stick to standards.

uc_stack.ss_sp was nonnull, but pointed to 0.

Can anyone confirm if head lwp works on any version of NetBSD?