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faq.md

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FAQ questions

Why use a new ABI? Why not an attribute?

  • Unlike ABIs, there is no syntax for assigning attributes to function pointers. The ABI must be part of the type system so that callers of function pointers know whether or not the function may unwind.

What should I do if I want to have C++ functions that unwind into Rust? [DRAFT; TODO]

  • mark declarations in Rust extern "C unwind"
  • Do not use catch_unwind in calling code
  • compile w/ panic = unwind
  • Check compiler, linker, & platform documentation (TODO: expand on this)
  • Re-test for every compiler update
  • (TODO: other caveats?)

How does cross-language unwinding differ from cross-language setjmp/longjmp?

  • setjmp/longjmp across Rust frames is currently intended to have well defined behavior as long as those frames do not contain destructors, although we don't have any documentation to that effect. See rust-lang/unsafe-code-guidelines#210 for more details.
  • When crossing frames that do contain destructors, the behavior of longjmp is Undefined Behavior; conversely, a primary goal of defining cross-language unwinding behavior is to support crossing frames with destructors.
  • Rust does not have a concept of Copy for stack-frames, which would permit the compiler to check that longjmp may safely traverse those frames. Such a language feature may be added in the future, but although it would be useful for longjmp, it would not be useful for unwinding.
  • It should never be assumed that drop will be called for objects in intermediate frames traversed by a longjmp, but this may occur on certain platforms. Rust provides no guarantee either way (which is why this is considered Undefined Behavior). Cross-language unwind, however, will be defined such that Drop objects whose frames are unwound are guaranteed droped.
  • Unwinding across Rust frames when panic = abort is currently undefined behavior, but we plan to define the behavior to cause the application to abort. The behavior of setjmp/longjmp, however, is independent of the panic runtime.
  • unwinding involves the use of a personality function, which raises additional cross-language compatibility concerns; setjmp/longjmp does not.

What new constraints does this ABI place on Rust's unwinding mechanism?

  • Ideally: none. However, if Rust's default unwinding mechanism changes, a translation layer will be required to maintain the C unwind ABI.
  • One more concern is how panic = abort will be handled; please refer to the roadmap for details.