linux/Documentation/filesystems/devpts.txt
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   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
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