getsid(2)

NAME
     getsid - get session ID

SYNOPSIS
     #include <sys/types.h>
     pid_t getsid(pid_t pid);

DESCRIPTION
     The function getsid returns the session ID of the process whose process
     ID is equal to pid.  If pid is equal to (pid_t)0, getsid returns the
     session ID of the calling process.

RETURN VALUE
     Upon successful completion, the function getsid returns the session ID of
     the specified process; otherwise, it returns a value of (pid_t)-1 and
     sets errno to indicate an error.

ERRORS
     Under the following conditions, the function getsid fails and sets errno
     to:

     EPERM     if the process  whose process ID is equal to pid is not in the
               same session as the calling process, and the implementation
               does not allow access to the session ID of that process from
               the calling process.

     ESRCH     if there is no process with a process ID equal to pid.

SEE ALSO
     exec(2), fork(2), getpid(2), setpgid(2), setsid(2)