illumos gate

Here the sigprocmask(2) man page is talking about the 'how' argument (with possible values SIG_BLOCK, SIG_UNBLOCK, SIG_SETMASK), but it is not noted there that we are really talking about 'how':

SYNOPSIS
     #include <signal.h>

     int sigprocmask(int how, const sigset_t *restrict set,
          sigset_t *restrict oset);

DESCRIPTION
     The sigprocmask() function is used to examine and/or  change
     the  caller's  signal  mask. If the value is  SIG_BLOCK, the
     set pointed to by the set argument is added to  the  current
     signal mask. If the value is SIG_UNBLOCK, the set pointed by
     the  set argument is removed from the current  signal  mask.
     If  the  value  is   SIG_SETMASK, the current signal mask is
     replaced by the set pointed to by the  set argument. If  the
     oset  argument  is not  NULL, the previous mask is stored in
     the space pointed to by oset. If the value of the  set argu-
     ment  is  NULL,  the  value   how is not significant and the
     caller's signal mask is unchanged; thus,  the  call  can  be
     used  to inquire about currently blocked signals. If the set
     or oset argument points to an invalid address, the  behavior
     is undefined and errno may be set to EFAULT.