tcgetsid —
get session ID associated with a controlling terminal
Standard C Library (libc, -lc)
The tcgetsid function returns the value of the session
  ID associated with the specified controlling terminal device. The session ID
  is defined as the process group ID of the session leader.
If an error occurs, tcgetsid returns -1 and the global
  variable errno is set to indicate the error, as follows:
  - [EBADF]
- The fd argument is not a valid file descriptor.
- [ENOTTY]
- The calling process does not have a controlling terminal or the underlying
      terminal device represented by fd is not the
      controlling terminal.
Thetcgetsid function conforms to
  X/Open Portability Guide Issue 4, Version 2
  (“XPG4.2”).