The Devpts Filesystem¶
Each mount of the devpts filesystem is now distinct such that ptys and their indicies allocated in one mount are independent from ptys and their indicies in all other mounts.
All mounts of the devpts filesystem now create a
To retain backwards compatibility the a ptmx device node (aka any node
mknod name c 5 2) when opened will look for an instance
of devpts under the name
pts in the same directory as the ptmx device
As an option instead of placing a
/dev/ptmx device node at
it is possible to place a symlink to
to bind mount
/dev/ptmx. If you opt for using
the devpts filesystem in this manner devpts should be mounted with
chmod 0666 /dev/pts/ptmx should be called.
Total count of pty pairs in all instances is limited by sysctls:
kernel.pty.max = 4096 - global limit kernel.pty.reserve = 1024 - reserved for filesystems mounted from the initial mount namespace kernel.pty.nr - current count of ptys
Per-instance limit could be set by adding mount option
This feature was added in kernel 3.4 together with
In kernels older than 3.4 sysctl
kernel.pty.max works as per-instance limit.