Linux man pages : getsid (2)
GETSID(2) Linux Programmer's Manual GETSID(2)
getsid - get session ID
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 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.
EPERM A process with process ID p exists, but it is not in the same
session as the current process, and the implementation considers
this an error.
ESRCH No process with process ID p was found.
SVr4, POSIX 1003.1-2001.
Linux does not return EPERM.
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 inte-
ger n larger than or equal to 500.
Linux 2.5.0 2001-12-17 GETSID(2)