| GETSID(2) | System Calls Manual | GETSID(2) | 
Standard C Library (libc, -lc)
#include <unistd.h>
pid_t
  
  getsid(pid_t
    pid);
The getsid() function returns the session ID of the
  process specified by pid. If pid
  is 0, the session ID of the calling process is returned. The session ID is
  defined as the process group ID of the process that is the session leader.
If an error occurs, getsid() returns -1 and the global
  variable errno is set to indicate the error, as follows:
  - [ESRCH]
- No process can be found corresponding to that specified by
      pid.
Thegetsid() function conforms to
  X/Open Portability Guide Issue 4, Version 2
  (“XPG4.2”).