text: NAMEsigset, sighold, sigrelse, sigignore - System V signal API
  SYNOPSIS
  #include <signal.h>
  typedef void (*sighandler_t)(int);
  sighandler_t sigset(int sig, sighandler_t disp);
  int sighold(int sig); int sigrelse(int sig); int sigignore(int sig);
  sigset(), sighold(), sigrelse(), sigignore():
  _XOPEN_SOURCE >= 500
  DESCRIPTIONThese:
  - SIG_DFL
  - Reset the disposition of sig to the default.
  - SIG_IGN
  - Ignore sig.
  - SIG_HOLD
  - Add sig to the process's signal mask, but leave the disposition of sig unchanged..
  RETURN VALUE.
  ERRORSFor sigset() see the ERRORS under sigaction(2) and sigprocmask(2).
  For sighold() and sigrelse() see the ERRORS under sigprocmask(2).
  For sigignore(), see the errors under sigaction(2).
  ATTRIBUTESFor an explanation of the terms used in this section, see attributes(7).
  CONFORMING TOSVr.
  NOTESThese functions appeared in glibc version 2.
  BUGSIn versions of glibc before 2.2, sigset() did not unblock sig if disp was specified as a value other than SIG_HOLD..