signal(2)                                                         signal(2)




 NAME
      signal, sigset, sighold, sigrelse, sigignore, sigpause -signal
      management

 SYNOPSIS
      #include <signal.h>

      void (*signal(int sig, void (*func)(int)))(int);

      int sighold(int sig);

      int sigignore(int sig);

      int sigpause(int sig);

      int sigrelse(int sig);

      void (*sigset(int sig, void (*disp)(int)))(int);

 DESCRIPTION
      The signal() function chooses one of three ways in which receipt of
      the signal number sig is to be subsequently handled. If the value of
      func is SIG_DFL, default handling for that signal will occur. If the
      value of func is SIG_IGN, the signal will be ignored. Otherwise, func
      must point to a function to be called when that signal occurs. Such a
      function is called a signal handler.

      When a signal occurs, if func points to a function, first the
      equivalent of a:

           signal(sig, SIG_DFL);

      is executed or an implementation-dependent blocking of the signal is
      performed. (If the value of sig is SIGILL, whether the reset to
      SIG_DFL occurs is implementation-dependent.) Next the equivalent of:

           (*func)(sig);

      is executed. The func function may terminate by executing a return
      statement or by calling abort(), exit(), or longjmp().  If func()
      executes a return statement and the value of sig was SIGFPE or any
      other implementation-dependent value corresponding to a computational
      exception, the behaviour is undefined.  Otherwise, the program will
      resume execution at the point it was interrupted.

      If the signal occurs other than as the result of calling abort(),
      kill() or raise(), the behaviour is undefined if the signal handler
      calls any function in the standard library other than one of the
      functions listed on the sigaction() page or refers to any object with
      static storage duration other than by assigning a value to a static
      storage duration variable of type volatile sig_atomic_t.  Furthermore,



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 signal(2)                                                         signal(2)




      if such a call fails, the value of errno is indeterminate.

      At program startup, the equivalent of:

           signal(sig, SIG_IGN);

      is executed for some signals, and the equivalent of:

           signal(sig, SIG_DFL);

      is executed for all other signals (see exec).

      The sigset(), sighold(), sigignore(), sigpause() and segrelse()
      functions provide simplified signal management.

      The sigset() function is used to modify signal dispositions.  The sig
      argument specifies the signal, which may be any signal except SIGKILL
      and SIGSTOP.  The disp argument specifies the signal's disposition,
      which may be SIG_DFL, SIG_IGN or the address of a signal handler. If
      sigset() is used, and disp is the address of a signal handler, the
      system will add sig to the calling process' signal mask before
      executing the signal handler; when the signal handler returns, the
      system will restore the calling process' signal mask to its state
      prior the delivery of the signal. In addition, if sigset() is used,
      and disp is equal to SIG_HOLD, sig will be added to the calling
      process' signal mask and sig's disposition will remain unchanged. If
      sigset() is used, and disp is not equal to SIG_HOLD, sig will be
      removed from the calling process' signal mask.

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

      The sigrelse() function removes sig from the calling process' signal
      mask.

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

      The sigpause() function removes sig from the calling process' signal
      mask and suspends the calling process until a signal is received.

      If the action for the SIGCHLD signal is set to SIG_IGN, child
      processes of the calling processes will not be transformed into zombie
      processes when they terminate. If the calling process subsequently
      waits for its children, and the process has no unwaited for children
      that were transformed into zombie processes, it will block until all
      of its children terminate, and wait(), wait3(), waitid() and waitpid()
      will fail and set errno to ECHILD.

 RETURN VALUE
      If the request can be honoured, signal() returns the value of func()
      for the most recent call to signal() for the specified signal sig.
      Otherwise, SIG_ERR is returned and a positive value is stored in



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 signal(2)                                                         signal(2)




      errno.

      Upon successful completion, sigset() returns SIG_HOLD if the signal
      had been blocked and the signal's previous disposition if it had not
      been blocked. Otherwise, SIG_ERR is returned and errno is set to
      indicate the error.

      For all other functions, upon successful completion, 0 is returned.
      Otherwise, -1 is returned and errno is set to indicate the error.

 ERRORS
      The signal() function will fail if:

           [EINVAL]       The sig argument is not a valid signal number or
                          an attempt is made to catch a signal that cannot
                          be caught or ignore a signal that cannot be
                          ignored.

      The signal() function may fail if:

           [EINVAL]       An attempt was made to set the action to SIG_DFL
                          for a signal that cannot be caught or ignored (or
                          both).

      The sigset(), sighold(), sigrelse(), sigignore(), and sigpause()
      functions will fail if:

           [EINVAL]       The sig argument is an illegal signal number.

      The sigset(), and sigignore() functions will fail if:

           [EINVAL]       An attempt is made to catch a signal that cannot
                          be caught, or to ignore a signal that cannot be
                          ignored.

 APPLICATION USAGE
      The sigaction() function provides a more comprehensive and reliable
      mechanism for controlling signals; new applications should use
      sigaction() rather than signal().

      The sighold() function, in conjunction with sigrelse() or sigpause(),
      may be used to establish critical regions of code that require the
      delivery of a signal to be temporarily deferred.

      The sigsuspend() function should be used in preference to sigpause()
      for broader portability.

 SEE ALSO
      exec, pause(), sigaction(), waitid(), <signal.h>.





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 signal(2)                                                         signal(2)




 CHANGE HISTORY
      First released in Issue 1.

      Derived from Issue 1 of the SVID.

 Issue 4
      The following changes are incorporated for alignment with the ISO C
      standard:

           -         The function is no longer marked as an extension.

           -         The argument int is added to the definition of func()
                     in the SYNOPSIS section.

           -         In Issue 3, this interface cross-referred to
                     sigaction().  This issue provides a complete
                     description of the function as defined in ISO C
                     standard.

      Another change is incorporated as follows:

           -         The APPLICATION USAGE section is added.

 Issue 4, Version 2
      The following changes are incorporated for X/OPEN UNIX conformance:

           -         The sighold(), sigignore(), sigpause(), sigrelse(), and
                     sigset() functions are added to the SYNOPSIS.

           -         The DESCRIPTION is updated to describe semantics of the
                     above interfaces.

           -         Additional text is added to the RETURN VALUE section to
                     describe possible returns from the sigset() function
                     specifically, and all of the above functions in
                     general.

           -         The ERRORS section is restructured to describe possible
                     error returns from each of the above functions
                     individually.

           -         The APPLICATION USAGE section is updated to describe
                     certain programming considerations associated with the
                     X/OPEN UNIX functions.










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 signal(2)                                                         signal(2)




                                 HP-UX EXTENSIONS



 SYNOPSIS
      void (*signal(int sig, void (*action)(int)))(int);

      void (*sigset(int sig, void (*func)(int)))(int);

 DESCRIPTION
      The system defines a set of signals that can be delivered to a
      process. The set of signals is defined in signal(5), along with the
      meaning and side effects of each signal.  An alternate mechanism for
      handling these signals is defined here. The facilities described here
      should not be used in conjunction with the other facilities described
      under signal(2), sigvector(2), sigblock(2), sigsetmask(2),
      sigpause(2), and sigspace(2).

      Acceptable values for sig are defined in <signal.h>.

           SIG_DFL     Execute the default action, which varies depending on
                       the signal.  The default action for most signals is
                       to terminate the process (see signal(5)).

                       A pending signal is discarded (whether or not it is
                       blocked) if action is set to SIG_DFL but the default
                       action of the pending signal is to ignore the signal
                       (as in the case of SIGCLD).

           SIG_IGN     Ignore the signal.
                       When signal() is called with action set to SIG_IGN
                       and an instance of the signal sig is pending, the
                       pending signal is discarded, whether or not it is
                       blocked.

                       SIGKILL and SIGSTOP signals cannot be ignored.

           address     Catch the signal.
                       Upon receipt of signal sig, reset the value of action
                       for the caught signal to SIG_DFL (except signals
                       marked with "not reset when caught"; see signal(5)),
                       call the signal-catching function to which address
                       points, and resume executing the receiving process at
                       the point where it was interrupted.

                       The signal-catching function is called with the
                       following three parameters:

                            sig     The signal number.





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 signal(2)                                                         signal(2)




                            code    A word of information usually provided
                                    by the hardware.

                            scp     A pointer to the machine-dependent
                                    structure sigcontext defined in
                                    <signal.h>.

                       The pointer scp is valid only during the context of
                       the signal-catching function.  The structure pointer
                       scp is always defined.

                       The code word is always zero for all signals except
                       SIGILL and SIGFPE.  For SIGILL, code has the
                       following values:

                            8       illegal instruction trap;
                            9       break instruction trap;
                            10      privileged operation trap;
                            11      privileged register trap.

                       For SIGFPE, code has the following values:

                            12      overflow trap;
                            13      conditional trap;
                            14      assist exception trap;
                            22      assist emulation trap.

                       As defined by the IEEE POSIX Standard, HP-UX does not
                       raise an exception on floating-point divide by zero.
                       The result of floating-point divide by zero is
                       infinity which can be checked by isinf(3M).

                       The signals SIGKILL and SIGSTOP cannot be caught.

           sigset() allows the calling process to choose one of four ways to
           handle the receipt of a specific signal.  sig specifies the
           signal and func specifies the choice.

           sig can be any one of the signals described under signal(5)
           except SIGKILL or SIGSTOP.

           func is assigned one of four values: SIG_DFL, SIG_IGN, SIG_HOLD,
           or a function address. The actions prescribed by SIG_DFL and
           SIG_IGN are described under signal(5).  The action prescribed by
           SIG_HOLD and function address are described below:

                SIG_HOLD                 Hold signal. The signal sig is held
                                         upon receipt. Any pending signal of
                                         this signal type remains held. Only
                                         one signal of each type is held.
                                         Note: the signals SIGKILL, SIGCONT,



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 signal(2)                                                         signal(2)




                                         and SIGSTOP cannot be held.

                function address
                     Catch signal.  func must be a pointer to a function,
                     the signal- catching handler, that is called when
                     signal sig occurs.  sigset() specifies that the process
                     calls this function upon receipt of signal sig.  Any
                     pending signal of this type is released. This handler
                     address is retained across calls to the other signal
                     management functions listed here. Upon receipt of
                     signal sig, the receiving process executes the
                     signal-catching function pointed to by
                     funcasdescribedunder signal(5) with the following
                     differences:

                Before calling the signal-catching handler, the system
                signal action of sig is set to SIG_HOLD.  During a normal
                return from the signal-catching handler, the system signal
                action is restored to func and any held signal of this type
                is released. If a non-local goto (longjmp(3C)) is taken,
                sigrelse() must be called to restore the system signal
                action to func and release any held signal of this type.

           sighold() holds the signal sig.  sigrelse() restores the system
           signal action of sig to that specified previously by sigset().
           sighold() and sigrelse() are used to establish critical regions
           of code. sighold() is analogous to raising the priority level and
           deferring or holding a signal until the priority is lowered by
           sigrelse().

           sigignore() sets the action for signal sig to SIG_IGN (see
           signal(5)).

           sigpause() suspends the calling process until it receives an
           unblocked signal.  If the signal sig is held, it is released
           before the process pauses. sigpause() is useful for testing
           variables that are changed when a signal occurs. For example,
           sighold() should be used to block the signal first, then test the
           variables.  If they have not changed, call sigpause() to wait for
           the signal.

           These functions can be linked into a program by giving the -lV3
           option to the ld command (see ld(1)).

 ERRORS
      sigset() fails and the system signal action for sig is not changed if
      any of the following occur:

           [EFAULT]                 The func argument points to memory that
                                    is not a valid part of the process
                                    address space. Reliable detection of



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 signal(2)                                                         signal(2)




                                    this error is implementation-dependent.

      sigset(), sighold(), sigrelse(), sigignore(), and sigpause() fail and
      the system signal action for sig is not changed if any of the
      following occur:

           [EINVAL]                 An attempt is made to ignore, hold, or
                                    supply a handler for a signal that
                                    cannot be ignored, held, or caught; see
                                    signal(5).

      sigpause returns when the following occurs:

           [EINTR]                  A signal was caught.

 EXAMPLES
      The following call to signal() sets up a signal-catching function for
      the SIGINT signal:

           void myhandler();

           (void) signal(SIGINT, myhandler);

 WARNINGS
      signal() should not be used in conjunction with the facilities
      described under bsdproc(2), sigaction(2), sigset(2V), or sigvector(2).

      signal() does not detect an invalid value for action, and if it does
      not equal SIG_DFL or SIG_IGN, or point to a valid function address,
      subsequent receipt of the signal sig causes undefined results.

 AUTHOR
      signal() was developed by HP, AT&T, and the University of California,
      Berkeley.

 SEE ALSO
      kill(1), init(1M), exit(2), kill(2), lseek(2), pause(2), sigaction(2),
      sigvector(2), wait(2), abort(3C), setjmp(3C), signal(5).

 STANDARDS CONFORMANCE
      signal(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, ANSI C













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