POSIX specification:
https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/sched.h.html
Portability problems fixed by Gnulib:
pid_t
on some platforms:
glibc 2.11, macOS 11.1.
struct sched_param
is not defined on some platforms:
Haiku.
SCHED_FIFO
, SCHED_RR
, SCHED_OTHER
are not defined on
some platforms:
Haiku.
Portability problems not fixed by Gnulib: