GETSID(2) Linux Programmer's Manual GETSID(2)
getsid - get session ID
pid_t getsid(pid_t pid);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
_XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
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 success, a session ID is returned. 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 calling process, and the implementation considers
this an error.
ESRCH No process with process ID p was found.
This system call is available on Linux since version 2.0.
Linux does not return EPERM.
getpgid(2), setsid(2), credentials(7)
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Linux 2010-09-26 GETSID(2)