getsid(0) returns the session ID of the calling process.
getsid(p) returns the session ID of the process with process ID
p. (The session ID of a process is the process group ID of the
session leader.)
On error, (pid_t) -1 will be returned, and
errno is set appropriately.
Linux has this system call since Linux 1.3.44.
There is libc support since libc 5.2.19.
To get the prototype under glibc, define both _XOPEN_SOURCE and
_XOPEN_SOURCE_EXTENDED, or use "#define _XOPEN_SOURCE n"
for some integer n larger than or equal to 500.