getsid — get
    process session
Standard C Library (libc, -lc)
#include
    <unistd.h>
pid_t
  
  getsid(pid_t
    pid);
The session ID of the process identified by
    pid is returned by
    getsid().
    If pid is zero, getsid()
    returns the session ID of the current process.
Upon successful completion, the getsid()
    system call returns the session ID of the specified process; otherwise, it
    returns a value of -1 and sets errno to indicate an error.
The getsid() system call will succeed
    unless:
  - [ESRCH]
- if there is no process with a process ID equal to
      pid.
Note that an implementation may restrict this system call to
    processes within the same session ID as the calling process.
The getsid() system call appeared in
    FreeBSD 3.0. The getsid()
    system call is derived from its usage in AT&T
    System V UNIX.