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(libc.info)Job Control is Optional

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Job Control is Optional
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   Not all operating systems support job control.  The GNU system does
support job control, but if you are using the GNU library on some other
system, that system may not support job control itself.

   You can use the `_POSIX_JOB_CONTROL' macro to test at compile-time
whether the system supports job control.  Note: System Options.

   If job control is not supported, then there can be only one process
group per session, which behaves as if it were always in the foreground.
The functions for creating additional process groups simply fail with
the error code `ENOSYS'.

   The macros naming the various job control signals (Note: Job Control
Signals) are defined even if job control is not supported.  However,
the system never generates these signals, and attempts to send a job
control signal or examine or specify their actions report errors or do
nothing.