On 02/14/2011 08:14 AM, Oliver Kowalke wrote:
Am 14.02.2011 16:45, schrieb Klaim - Joël Lamotte:
As far as I know, from a previous discussion on this list, async_wait might be the only (cross-platform) way (without having to add another dependency than boost to the project...) to achive the last point, knowing when the child process ends.
Do you mean with async_wait the feature dealing with SIGCHLD and sigwait()? If so it is not cross-platform. As discussed in this thread waiting on signals asynchronously influences the application and other libs - see installing signal handlers. IMHO waiting on signals (SIGCHLD, SIGTERM, SIGSTOP, SIGUSR1, ...) should be implemented in a separate library.
I do not think that a general signal handling library, while potentially useful, is required for Boost.Process to do asynchronous waiting. The reason is that simply letting two independent pieces of code be notified of SIGCHLD is not sufficient, because (in order to be efficient) the SIGCHLD handler needs to then invoke waitpid(-1, ...), which affects the global state of the program. Instead, what is needed is basically a SIGCHLD library, which allows registering a callback to be invoked when a given PID changes state (exited, suspended, or continued). There should be an option to invoke the callback directly in the signal handler, and there should also be a way (possibly built on top of the first invocation method) to invoke a callback asynchronously using ASIO. Ideally, Boost.Process could use this SIGCHLD library by default, but also be able to work with an alternate SIGCHLD multiplexing library.
About the "thread and future + sync. waiting" way of doing it : would it work with any kind of child process end (including any kind of crash)? Or is it part of the limitations you're thinking about?
Yes - waitpid( child_pid, ...) blocking in one separate thread - returning the result in a future. Depending on how many threads are created blocking in waitpid it may reduce the performance (at least at some amount of threads the scheduling overhead may become significant).
In addition to the overhead of one thread per process (which is rather substantial overhead), there is the problem that all other code running in the same process is forced to use this same inefficient waiting mechanism, as opposed to a multiplexing interface as described above. For instance, glib has a similar SIGCHLD handling facility, and while I believe it would be possible to have Boost.Process make use of it, the inefficient waiting scheme would not inter-operate with the process launching facility in glib.