getsid - get session ID
#include <unistd.h>
pid_t getsid(pid_t pid);
getsid(0) returns the session ID of the calling process.
getsid(p) returns the session ID of the process with pro
cess ID p.
On error, -1 will be returned. The only error which can
happen is ESRCH, when no process with process ID p was
found.
SVr4, which documents an additional EPERM error condition.
setsid(2)