Silence "unused result" warning in bluesim probe module

[danderson]

In modern glibc, asprintf's return value is annotated as "must use", because it can fail with -1 in OOM conditions.

This is not a correctness issue on modern OSes, since the surrounding code will at worst lead to a null pointer deref and crash in this condition. Further, most modern OSes allocate virtual memory optimistically, and crash later on failure to fault in rather than returning -1 to an allocation call. So, this change's main purpose is to declutter bluesim compilation logs.

[danderson]

Sample of the log clutter this removes:

In file included from /nix/store/ksabz0s564dzcsiaj04gmkb46iq7s0kn-bluespec-2024.07/lib/Bluesim/bluesim_primitives.h:12,
                 from out/bluesim/test/DMAChannel/mkTB.cxx:7:
/nix/store/ksabz0s564dzcsiaj04gmkb46iq7s0kn-bluespec-2024.07/lib/Bluesim/bs_prim_mod_probe.h: In member function ‘unsigned int MOD_Probe<T>::dump_VCD_defs(unsigned int) [with T = unsigned char]’:
/nix/store/ksabz0s564dzcsiaj04gmkb46iq7s0kn-bluespec-2024.07/lib/Bluesim/bs_prim_mod_probe.h:45:13: warning: ignoring return value of ‘int asprintf(char**, const char*, ...)’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
   45 |     asprintf(&buf, "%s$PROBE", inst_name);
      |     ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Github actually makes it slightly nicer by declining to linewrap it. With bonus linewraps it ends up much less readable, and mixed in with more interesting elaboration warnings that I'd like to see better :)

[quark17]

Sounds good, thanks! Can I suggest, though, not using assert, and instead just do something like this:

if (sz < 0)
  perror("dump_VCD_defs: asprintf");

I didn't see any place else that uses assert in the Bluesim code, and we don't need to ability to turn off the assertion, I wouldn't think, so might as well inline the abort call. It'd be best to match whatever convention Bluesim uses for aborting, but I didn't see many such places (only a few uses of perror and nothing else). I assume that asprintf would set errno if it failed, so I assume perror would be OK to use here.

[danderson]

Works for me! I wasn't sure how to report an error from here, since as you say there's no obvious precedent. And, I was reluctant to start doing deeper surgery to plumb error results around for a condition that really isn't ever going to happen :)

Updated the implementation as you suggested, make install-src and make check-suite are happy here.