Does pppd lock slave pty devices?

rkyser · 08-07-2012, 02:53 PM

First of all, let me say that I have little experience with pppd and pseudo-terminals. So, if you say to yourself, "Huh, what is this guy talking about," I probably didn't say it correctly :-)

The setup:
I have a bluetooth application which gets a master and slave pty device pair. Then an instance of pppd is spawned, and given the following arguments:

debug
nodetach
logfd 2
dump
name SOMENAME@SOMEDOMAIN.COM
/dev/pts/0
115200
receive-all
ipcp-accept-remote
noipdefault
defaultroute
usepeerdns
:1.1.1.1

Finally, I have a test application which I want to use to open /dev/pts/0 (the slave device). I can open the /dev/pts/0 device, but I cannot read or write to it. I get a "Resource Temporarily Unavailable" error. (I've also tried to directly echo into it)

The question:
Does pppd lock (or make unavailable somehow) a slave device node that it is using? I have looked through the source code, and saw some stuff about locking, but I thought I would draw on the community experience before reverse engineering pppd :-)

Thank you.

colorpurple21859 · 08-08-2012, 08:46 AM

igoore this, had a a few different tabs opened and somehow posted on the wrong one. lol

rkyser · 08-08-2012, 08:57 AM

Doh! And I got all excited when I got an email about a reply!

rkyser · 08-08-2012, 10:05 AM

The slave node that I was attempting to read/write to was being modified by pppd. When pppd takes control of a tty/pty device, it changes the line discipline from N_TTY to N_PPP. This means that when when you open() and then read() or write() to the slave node, the PPP intermediate driver is being used instead of the TTY driver. So, a read() and write() boils down to a totally different function. Looking at the N_PPP driver I found the following. This answers my question as to why EAGAIN was being returned.

Code:

/*
 * Read does nothing - no data is ever available this way.
 * Pppd reads and writes packets via /dev/ppp instead.
 */
static ssize_t
ppp_asynctty_read(struct tty_struct *tty, struct file *file,
          unsigned char __user *buf, size_t count)
{
    return -EAGAIN;
}

/*
 * Write on the tty does nothing, the packets all come in
 * from the ppp generic stuff.
 */
static ssize_t
        ppp_asynctty_write(struct tty_struct *tty, struct file *file,
           const unsigned char *buf, size_t count)
{
    return -EAGAIN;
}