Regarding the Pseudo terminal limit on Fedora linux kernel
C. Linus Hicks
lhicks at nc.rr.com
Wed Mar 2 16:03:08 UTC 2005
On Wed, 2005-03-02 at 17:47 +0530, Reddy V, Ravinder (Ravinder) wrote:
> Hi,
>
> Can anyone please let me know the system variable which holds the pseudo
> terminal limit in fedora linux.
> Also please let me know the filename where this variable is present, and how
> to change this variable value?
If you look in Documentation/devices.txt in the kernel sources, you will
see that device major numbers 136-143 reserved for Unix98 pseudo-TTY
devices. That makes 8 x 256 total available under the current device
allocation scheme.
However, if you look in include/linux/tty.h you see this:
#define NR_UNIX98_PTY_DEFAULT 4096 /* Default maximum for Unix98 ptys */
#define NR_UNIX98_PTY_MAX (1 << MINORBITS) /* Absolute limit */
and in include/linux/kdev_t.h
#define MINORBITS 20
Which gives a maximum of 1048576. So then looking in drivers/char/pty.c
/* Unix98 devices */
#ifdef CONFIG_UNIX98_PTYS
/*
* sysctl support for setting limits on the number of Unix98 ptys allocated.
* Otherwise one can eat up all kernel memory by opening /dev/ptmx repeatedly.
*/
int pty_limit = NR_UNIX98_PTY_DEFAULT;
static int pty_limit_min = 0;
static int pty_limit_max = NR_UNIX98_PTY_MAX;
And from sysctl:
kernel.pty.max = 4096
So you can set it either by echoing a value into:
/proc/sys/kernel/pty/max
Or by using
sysctl -w kernel.pty.max=<value>
If you want to set it permanently, add a line to /etc/sysctl.conf and it
will get loaded at boot time.
--
C. Linus Hicks <lhicks at nc dot rr dot com>
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