popen with argv
system with argv
spawnvp with argv
select nfds readfds writefds exceptfds timeout => [b c d e]
pselect nfds readfds writefds exceptfds timeout sigmask => [b c d]
nfds = (if (nil? allTheFds) 0 $else (extremum max (allTheFds))) + 1 fdSetSize
pathconf _PC_NAME_MAX
nanosleep
clock_nanosleep
getgroups : try group sizes from 1 up, in 2 complements, calling getgroups, checking the result and errno, if errno = EINVAL, recurse.
max is ngroups_max = sysconf(_SC_NGROUPS_MAX) + 1;

make it whitelist-based.
factor out non-POSIX parts into OS module.
fcntl
select
poll
pselect
epoll
dup3 is linux-specific.

map (\e let value = (head (tail e)) in 
        let index = (head e) in 
        (if (index = 4) (value + 1) 
         (if (index = 5) (value + 1900) 
          value))) 
    (enumerate $ run (POSIX.time [] ;\t POSIX.localtime t))

types:
ANSI   <setjmp.h>       typedef   jmp_buf
ANSI   <signal.h>       typedef   sig_atomic_t
ANSI   <stdarg.h>       typedef   va_list
ANSI   <stddef.h>       typedef   ptrdiff_t
ANSI   <stddef.h>       typedef   size_t
ANSI   <stddef.h>       typedef   wchar_t
ANSI   <stdio.h>        typedef   FILE
ANSI   <stdio.h>        typedef   fpos_t
ANSI   <stdio.h>        typedef   size_t
ANSI   <stdlib.h>       typedef   div_t
ANSI   <stdlib.h>       typedef   ldiv_t
ANSI   <stdlib.h>       typedef   size_t
ANSI   <stdlib.h>       typedef   wchar_t
ANSI   <string.h>       typedef   size_t
ANSI   <time.h>         typedef   clock_t
ANSI   <time.h>         typedef   size_t
ANSI   <time.h>         typedef   time_t
POSIX  <dirent.h>       typedef   DIR
POSIX  <setjmp.h>       typedef   sigjmp_buf
POSIX  <sys/times.h>    typedef   clock_t
POSIX  <sys/types.h>    typedef   dev_t
POSIX  <sys/types.h>    typedef   gid_t
POSIX  <sys/types.h>    typedef   ino_t
POSIX  <sys/types.h>    typedef   mode_t
POSIX  <sys/types.h>    typedef   nlink_t
POSIX  <sys/types.h>    typedef   off_t
POSIX  <sys/types.h>    typedef   pid_t
POSIX  <sys/types.h>    typedef   size_t
POSIX  <sys/types.h>    typedef   ssize_t
POSIX  <sys/types.h>    typedef   uid_t
POSIX  <termios.h>      typedef   cc_t
POSIX  <termios.h>      typedef   speed_t
POSIX  <termios.h>      typedef   tcflag_t
POSIX  <unistd.h>       typedef   size_t
POSIX  <unistd.h>       typedef   ssize_t

If you're going to provide any of these, then you should provide at least the
ANSI C types ptrdiff_t, size_t, wchar_t, FILE, fpos_t, clock_t, and time_t.
You should strongly consider providing the other ANSI C types
jmp_buf, sig_atomic_t, div_t, and ldiv_t, not because they
would be particularly useful, but simply because the additional
implementation effort required would be quite trivial.
va_list is probably the most problematic, since it is often dependent
on the particular C compiler.

nicer wrapper for posixSpawn!