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The
function returns the value of the process group ID of the foreground process
group associated with the terminal device. If there is no foreground process
group, returns an invalid process ID.
If an error occurs, returns
-1 and the global variable is set to indicate the error, as follows:
The argument is not a valid file descriptor. The calling process does
not have a controlling terminal or the underlying terminal device represented
by is not the controlling terminal.
The function
is expected to be compliant with the specification.
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