Get session id
#define _XOPEN_SOURCE 500 /* See feature_test_macros(7) */
pid_t tcgetsid(int fd);
The function tcgetsid() returns the session ID of the current session that has the terminal associated to fd as controlling terminal. This terminal must be the controlling terminal of the calling process.
When 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 appropriately.
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 fd.
tcgetsid() is provided in glibc since version 2.1.
The tcgetsid() function is thread-safe.
This function is implemented via the TIOCGSID ioctl(2), present since Linux 2.1.71.
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