问题
I am working on a project which could make good use of POSIX sessions (as described here) but I'm now uncertain whether I'm misunderstanding the concept and/or whether OS X even supports process sessions.
OS X 10.9 has a setsid(2) system call but calling it with arguments 0, 0 seems to do noting. And looking at the output of ps -e -o pid,pgid,sess shows that all processes have a session ID of 0:
PID PGID SESS
1 1 0
11 11 0
12 12 0
13 13 0
14 14 0
15 15 0
16 16 0
17 17 0
[...]
Does OS X support POSIX sessions for processes and, if not, is there another mechanism in place that e.g. a shell can use to send a signal to all processes started from that shell?
Besides looking at active processes on my system, I used the ersatz-setsid command line tool to run a process in a new session, but without success:
$ ./setsid sleep 1000
$ ps -eo pid,pgid,sess,command
PID PGID SESS COMMAND
[...]
10650 10650 0 sleep 1000
[...]
回答1:
In my experience, Mac OSX does support posix sessions. The getsid() system call works. But the ps utility does not (ps -o sess produces 0 for all processes).
回答2:
As asynchronos pointed out, macOS does support the notion of POSIX sessions, but it does not have provisions for querying the session id via ps(1) or top(8); that's why you see 0.
To add some detail:
ps uses sysctl(3) to gather up a list of running processes. That list is returned as an array of (for 64bit machines) struct user64_kinfo_proc (xnu/bsd/sys/sysctl.h). The user-land definition is struct kinfo_proc (/usr/include/sys/sysctl.h).
The kernel function that handles the filling of that array is sysctl_prochandle() (xnu/bsd/kern/kern_sysctl.c).
It, in turn, calls fill_user64_eproc to fill in struct user64_eproc, a member of user64_kinfo_proc.
user64_eproc is the data structure that (I'm guessing) would have a e_psid member denoting the session id of the given process. As such, it has no member. fill_user64_eproc uses the process's struct session to set some flags and record the process's group id into user64_eproc, but it doesn't record the session id because, well, there's nowhere to put it.
Interestingly, the source for ps has a section ifdef'd out that accesses kinfo_proc.ki_sid. This member, of course, also doesn't exist.
So, I'm sure how it's not returned, but not why, unfortunately.
来源：https://stackoverflow.com/questions/35248294/does-os-x-support-posix-sessions