tcgetsid - get session ID
#define _XOPEN_SOURCE 500 /* See
The function tcgetsid() returns the session ID of
the current session that has the terminal associated to
controlling terminal. This terminal must be the controlling terminal of
the calling process.
fd refers to the controlling terminal of our session,
the function tcgetsid() will return the session ID of
this session. Otherwise, -1 is returned, and
errno is set
fd is not a valid file descriptor.
The calling process does not have a controlling terminal, or it has
one but it is not described by
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