(libc.info.gz) Job Control Signals

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 24.2.5 Job Control Signals
 --------------------------
 
 These signals are used to support job control.  If your system doesn't
 support job control, then these macros are defined but the signals
 themselves can't be raised or handled.
 
    You should generally leave these signals alone unless you really
 understand how job control works.   Job Control.
 
  -- Macro: int SIGCHLD
      This signal is sent to a parent process whenever one of its child
      processes terminates or stops.
 
      The default action for this signal is to ignore it.  If you
      establish a handler for this signal while there are child processes
      that have terminated but not reported their status via 'wait' or
      'waitpid' ( Process Completion), whether your new handler
      applies to those processes or not depends on the particular
      operating system.
 
  -- Macro: int SIGCLD
      This is an obsolete name for 'SIGCHLD'.
 
  -- Macro: int SIGCONT
      You can send a 'SIGCONT' signal to a process to make it continue.
      This signal is special--it always makes the process continue if it
      is stopped, before the signal is delivered.  The default behavior
      is to do nothing else.  You cannot block this signal.  You can set
      a handler, but 'SIGCONT' always makes the process continue
      regardless.
 
      Most programs have no reason to handle 'SIGCONT'; they simply
      resume execution without realizing they were ever stopped.  You can
      use a handler for 'SIGCONT' to make a program do something special
      when it is stopped and continued--for example, to reprint a prompt
      when it is suspended while waiting for input.
 
  -- Macro: int SIGSTOP
      The 'SIGSTOP' signal stops the process.  It cannot be handled,
      ignored, or blocked.
 
  -- Macro: int SIGTSTP
      The 'SIGTSTP' signal is an interactive stop signal.  Unlike
      'SIGSTOP', this signal can be handled and ignored.
 
      Your program should handle this signal if you have a special need
      to leave files or system tables in a secure state when a process is
      stopped.  For example, programs that turn off echoing should handle
      'SIGTSTP' so they can turn echoing back on before stopping.
 
      This signal is generated when the user types the SUSP character
      (normally 'C-z').  For more information about terminal driver
      support, see  Special Characters.
 
  -- Macro: int SIGTTIN
      A process cannot read from the user's terminal while it is running
      as a background job.  When any process in a background job tries to
      read from the terminal, all of the processes in the job are sent a
      'SIGTTIN' signal.  The default action for this signal is to stop
      the process.  For more information about how this interacts with
      the terminal driver, see  Access to the Terminal.
 
  -- Macro: int SIGTTOU
      This is similar to 'SIGTTIN', but is generated when a process in a
      background job attempts to write to the terminal or set its modes.
      Again, the default action is to stop the process.  'SIGTTOU' is
      only generated for an attempt to write to the terminal if the
      'TOSTOP' output mode is set;  Output Modes.
 
    While a process is stopped, no more signals can be delivered to it
 until it is continued, except 'SIGKILL' signals and (obviously)
 'SIGCONT' signals.  The signals are marked as pending, but not delivered
 until the process is continued.  The 'SIGKILL' signal always causes
 termination of the process and can't be blocked, handled or ignored.
 You can ignore 'SIGCONT', but it always causes the process to be
 continued anyway if it is stopped.  Sending a 'SIGCONT' signal to a
 process causes any pending stop signals for that process to be
 discarded.  Likewise, any pending 'SIGCONT' signals for a process are
 discarded when it receives a stop signal.
 
    When a process in an orphaned process group ( Orphaned Process
 Groups) receives a 'SIGTSTP', 'SIGTTIN', or 'SIGTTOU' signal and does
 not handle it, the process does not stop.  Stopping the process would
 probably not be very useful, since there is no shell program that will
 notice it stop and allow the user to continue it.  What happens instead
 depends on the operating system you are using.  Some systems may do
 nothing; others may deliver another signal instead, such as 'SIGKILL' or
 'SIGHUP'.  On GNU/Hurd systems, the process dies with 'SIGKILL'; this
 avoids the problem of many stopped, orphaned processes lying around the
 system.
 
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