Hi!
On Thu, 22 Nov 2018 11:17:24 +0200, Janne Blomqvist <blomqvist.ja...@gmail.com>
wrote:
> From backtrace.h for backtrace_create_state:
>
> Calling this function allocates resources that can not be freed.
> There is no backtrace_free_state function. The state is used to
> cache information that is expensive to recompute. Programs are
> expected to call this function at most once and to save the return
> value for all later calls to backtrace functions.
>
> So instead of calling backtrace_create_state every time we wish to
> show a backtrace, do it once and store the result in a static
> variable. libbacktrace allows multiple threads to access the state,
> so no need to use TLS.
Hmm, OK, but...
> --- a/libgfortran/runtime/backtrace.c
> +++ b/libgfortran/runtime/backtrace.c
> @@ -146,11 +146,15 @@ full_callback (void *data, uintptr_t pc, const char
> *filename,
> void
> show_backtrace (bool in_signal_handler)
> {
> - struct backtrace_state *lbstate;
> + /* Note that libbacktrace allows the state to be accessed from
> + multiple threads, so we don't need to use a TLS variable for the
> + state here. */
> + static struct backtrace_state *lbstate;
> struct mystate state = { 0, false, in_signal_handler };
> -
> - lbstate = backtrace_create_state (NULL, __gthread_active_p (),
> - error_callback, NULL);
> +
> + if (!lbstate)
> + lbstate = backtrace_create_state (NULL, __gthread_active_p (),
> + error_callback, NULL);
... don't you still have to make sure that only one thread ever executes
"backtrace_create_state", and writes into "lbstate" (via locking, or
atomics, or "pthread_once", or some such)? Or is that situation (more
than one thread entering "show_backtrace" with uninitialized "lbstate")
not possible to happen for other reasons?
Grüße
Thomas
