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. See section Job Control.
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 (see section Process Completion), whether your new handler
applies to those processes or not depends on the particular operating
system.
SIGCHLD.
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.
SIGSTOP signal stops the process. It cannot be handled,
ignored, or blocked.
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 section Special Characters.
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 section Access to the Controlling Terminal.
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; see section 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 (see section 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. In the GNU system, the process
dies with SIGKILL; this avoids the problem of many stopped,
orphaned processes lying around the system.
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