setsid — create session and set process group ID

       The  setsid()  system  call  creates a new session.  The calling process is the session leader of the new
       session, is the process group leader of a new process group and has no controlling terminal.  The calling
       process is the only process in either the session or the process group.

       The setsid() system call will fail if:

       [EPERM]            The  calling  process  is already a process group leader, or the process group ID of a
                          process other than the calling process matches the process ID of the calling process.

       Standard C Library (libc, -lc)

       setsid — create session and set process group ID

       Upon successful completion, the setsid() system call returns the value of the process group ID of the new
       process group, which is the same as the process ID of the calling process.  If an error occurs,  setsid()
       returns -1 and the global variable errno is set to indicate the error.

setpgid(2), tcgetpgrp(3), tcsetpgrp(3)

       The setsid()  system  call  is  expected  to  be  compliant  with  the  ISO/IEC  9945-1:1990  (“POSIX.1”)
       specification.

Debian                                            June 4, 1993                                         SETSID(2)

#include<unistd.h>pid_tsetsid(void);