Bob Rossi <bob@xxxxxxxxxx> writes: > On Sat, Jan 15, 2011 at 08:09:16PM -0800, Ian Lance Taylor wrote: > >> What you describe is not safe in all known configurations. There are >> configurations where the unwind library invoked to throw the exception >> will not be able to find the unwind information defined by the shared >> library, which means that it will not be able to reliably unwind through >> the functions in the shared library, and, in particular, will not be >> able to run destructors for local variables defined in functions in the >> shared library. > > I'm assumming that you understood that the shared libraries are pure C, > with no C++ in them at all. (There won't be any destructors for local > variables defined in the shared library since it's pure C.) Only the > main executable with throw and catch exceptions. There is a gcc extension that permits destructors for local variables in C code, and the GNU/Linux pthread library uses it when available. Look at pthread_cleanup_push in <pthread.h>. Anyhow, whether you rely on that or not, the unwind library still needs to find the unwind info just to unwind the stack reliably on x86_64. >> However, what you describe is safe on current versions of GNU/Linux. It >> is safe if 1) your linker supports the --eh-frame-hdr option (binutils >> 2.12 ; 2) your glibc provides the dl_iterate_phdr option; 3) your gcc >> was configured to be aware of both these facts. These facts have been >> true for at least five years on GNU/Linux systems. >> >> What is the range of configurations you have to consider? > > Well, essentially any configuration that someone asks to run the > software on. > > Regarding points 1, 2, and 3. Do those have to be true at link time > (when i'm building the executable on the machines I control) > or at load time (when the user goes to run the executable on the > machines they control)? Unless you are linking statically, the dl_iterate_phdr function has to be available at runtime, in libc.so.6. Every other aspect is only required at link time, on a machine you control. You are probably safe enough on GNU/Linux in practice, the question you'll have to consider is whether you have to run on non-GNU/Linux systems. Ian
