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
|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
getsid(0) returns the session ID of the calling process. getsid()
returns the session ID of the process with process ID pid. If pid is
0, getsid() returns the session ID of the calling process.
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 pid 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 pid was found.
This system call is available on Linux since version 2.0.
POSIX.1-2001, POSIX.1-2008, SVr4.
Linux does not return EPERM.
See credentials(7) for a description of sessions and session IDs.
getpgid(2), setsid(2), credentials(7)
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Linux 2017-09-15 GETSID(2)