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       sigset, sighold, sigrelse, sigignore - System V signal API


       #define _XOPEN_SOURCE 500
       #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);


       These  functions are provided in glibc as a compatibility interface for
       programs that make use of the historical System V signal API.  This API
       is   obsolete:  new  applications  should  use  the  POSIX  signal  API
       (sigaction(2), sigprocmask(2), etc.)

       The sigset() function modifies the disposition of the signal sig.   The
       disp  argument  can be the address of a signal handler function, or one
       of the following constants:

              Reset the disposition of sig to the default.

              Ignore sig.

              Add sig to the process’s signal mask, but leave the  disposition
              of sig unchanged.

       If disp specifies the address of a signal handler, then sig is added to
       the process’s signal mask during execution of the handler.

       If disp was specified as a value  other  than  SIG_HOLD,  then  sig  is
       removed from the process’s signal mask.

       The dispositions for SIGKILL and SIGSTOP cannot be changed.

       The sighold() function adds sig to the calling process’s signal mask.

       The  sigrelse()  function removes sig from the calling process’s signal

       The sigignore() function sets the disposition of sig to SIG_IGN.


       On success, sigset() returns SIG_HOLD if sig  was  blocked  before  the
       call, or the signal’s previous disposition if it was not blocked before
       the call.  On error, sigset() returns -1, with errno  set  to  indicate
       the error.  (But see BUGS below.)

       The  sighold(),  sigrelse(),  and  sigignore()  functions  return  0 on
       success; on error, these functions return -1 and set errno to  indicate
       the error.


       For 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).


       SVr4,  POSIX.1-2001.   These functions are obsolete: do not use them in
       new programs.  POSIX.1-2008 marks sighold(),  sigignore(),  sigpause(),
       sigrelse(),   and   sigset()  as  obsolete,  recommending  the  use  of
       sigaction(2),  sigprocmask(2),  pthread_sigmask(3),  and  sigsuspend(2)


       These functions appeared in glibc version 2.1.

       The  sighandler_t type is a GNU extension; it is only used on this page
       to make the sigset() prototype more easily readable.

       The sigset() function provides reliable signal handling  semantics  (as
       when calling sigaction(2) with sa_mask equal to 0).

       On  System  V,  the signal() function provides unreliable semantics (as
       when  calling  sigaction(2)  with  sa_mask  equal  to  SA_RESETHAND   |
       SA_NODEFER).    On   BSD,   signal()   provides   reliable   semantics.
       POSIX.1-2001  leaves  these  aspects  of  signal()  unspecified.    See
       signal(2) for further details.

       In  order  to  wait  for  a  signal,  BSD  and System V both provided a
       function named sigpause(3), but this function has a different  argument
       on the two systems.  See sigpause(3) for details.


       In  versions  of glibc before 2.2, sigset() did not unblock sig if disp
       was specified as a value other than SIG_HOLD.

       In versions of glibc before 2.5, sigset() does not correctly return the
       previous  disposition  of  the  signal in two cases.  First, if disp is
       specified as  SIG_HOLD,  then  a  successful  sigset()  always  returns
       SIG_HOLD.   Instead,  it  should return the previous disposition of the
       signal (unless the signal was blocked, in which case SIG_HOLD should be
       returned).  Second, if the signal is currently blocked, then the return
       value of a  successful  sigset()  should  be  SIG_HOLD.   Instead,  the
       previous  disposition  of  the signal is returned.  These problems have
       been fixed since glibc 2.5.


       kill(2), pause(2), sigaction(2), signal(2),  sigprocmask(2),  raise(3),
       sigpause(3), sigvec(3), feature_test_macros(7), signal(7)


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