sigprocmask(2) talks about the 'how' argument
manpage - manual pages
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.