getsid - get session ID
On success, a session ID is returned. On error,
(pid_t) -1 will be returned, and
errno is set appropriately.
A process with process ID
pid exists, but it is not in the same session as the calling process, and the implementation considers this an error.
No process with process ID
pid was found.
This system call is available on Linux since version 2.0.
POSIX.1-2001, POSIX.1-2008, SVr4.
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