Why not export and use the canonical way of sigemptyset/sigfillset/sigaddset/sigdelset/sigismember instead of pushing for more potential non-conformant code? For glibc sigfillset will correctly fill all the signal set structure while removing the internal used signals. This is par what POSIX specifies [1] where it states that: 

'either sigemptyset() or sigfillset() must be called prior to any other use of the signal set'

And more importantly:

'For example, blocking or ignoring an implementation-defined signal may have undesirable side-effects, whereas the default action for that signal is harmless. In such a case, it would be preferable for such a signal to be excluded from the signal set returned by sigfillset().'

Also keep that since is an implementation detail, different libcs can use different internal signals. UCLIBC, for instance, uses the same 2 signals as GLIBC, however MUSL allocates signal 32, 33, and 34 for internal usage (and excludes in sigfillset and warns with EINVAL on sigaddset). 

[1] http://pubs.opengroup.org/onlinepubs/9699919799/