sigprocmask
===========

POSIX specification:
 https://pubs.opengroup.org/onlinepubs/9699919799/functions/sigprocmask.html

Gnulib module: sigprocmask

Portability problems fixed by Gnulib:

  * This function is missing on some platforms: mingw, MSVC 14.

Portability problems not fixed by Gnulib:

  * In case of failure, the return value is wrong on some platforms: NetBSD 9.0 when libpthread is in use.

Note: Although sigprocmask officially has undefined behaviour in multi-threaded programs, in practice it is essentially equivalent to pthread_sigmask, with only a difference regarding the error return convention.  It's simpler to use sigprocmask, since it does not require linking with -lpthread on some platforms: glibc, NetBSD, OpenBSD, AIX, IRIX.