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Linux/Documentation/filesystems/devpts.txt

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

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