1Each mount of the devpts filesystem is now distinct such that ptys 2and their indicies allocated in one mount are independent from ptys 3and their indicies in all other mounts. 4 5All mounts of the devpts filesystem now create a /dev/pts/ptmx node 6with permissions 0000. 7 8To retain backwards compatibility the a ptmx device node (aka any node 9created with "mknod name c 5 2") when opened will look for an instance 10of devpts under the name "pts" in the same directory as the ptmx device 11node. 12 13As an option instead of placing a /dev/ptmx device node at /dev/ptmx 14it is possible to place a symlink to /dev/pts/ptmx at /dev/ptmx or 15to bind mount /dev/ptx/ptmx to /dev/ptmx. If you opt for using 16the devpts filesystem in this manner devpts should be mounted with 17the ptmxmode=0666, or chmod 0666 /dev/pts/ptmx should be called. 18 19Total count of pty pairs in all instances is limited by sysctls: 20kernel.pty.max = 4096 - global limit 21kernel.pty.reserve = 1024 - reserved for filesystems mounted from the initial mount namespace 22kernel.pty.nr - current count of ptys 23 24Per-instance limit could be set by adding mount option "max=<count>". 25This feature was added in kernel 3.4 together with sysctl kernel.pty.reserve. 26In kernels older than 3.4 sysctl kernel.pty.max works as per-instance limit. 27