m4_dnl hm_rv_succfail returns 0 (ok) or -1 (see errno)
m4_dnl hm_rv_len(<max>) returns length read/written, must be <=max, -1 => errno
m4_dnl hm_rv_fcntl syscall is fcntl, do special processing
m4_dnl hm_rv_succfail returns 0 (ok) or -1 (see errno)
m4_dnl hm_rv_len(<max>) returns length read/written, must be <=max, -1 => errno
m4_dnl hm_rv_fcntl syscall is fcntl, do special processing