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SubjectInconsistency between PTY read() return values
Hi all,

Some time ago I noticed this small inconsistency between the return
values of read() calls on pseudo-terminal master devices while working
on the FreeBSD TTY code.

Consider the following example:

| #include <fcntl.h>
| #include <stdio.h>
| #include <stdlib.h>
| #include <unistd.h>
|
| int
| main(int argc, char *argv[])
| {
| int f1, f2;
| char buf[20];
| ssize_t r;
|
| f1 = posix_openpt(O_RDWR|O_NOCTTY);
| grantpt(f1);
| unlockpt(f1);
|
| f2 = open(ptsname(f1), O_RDWR);
| close(f2);
|
| r = read(f1, buf, sizeof buf);
| printf("%zd\n", r);
| if (r == -1)
| perror("read");
|
| return (0);
| }

The code is very simple. It acquires a pseudo-terminal, opens the slave
device and closes it. After that it tries to perform a read() on the
pseudo-terminal master device. On at least Solaris 7 to 10, Mac OS X
10.5 and FreeBSD 6 to HEAD, this code just prints 0. On Linux
(at least 2.6.22-2.6.28) I see the following:

-1
read: Input/output error

I looked through the standards and it seems the POSIX onlinepubs don't
mention the behaviour of pseudo-terminal master file descriptors at all.
This means it would even be valid of we return 0x1337 or something.

But still, I think it's a little inconsistent. Most programmers would
expect pseudo-terminal master file descriptors to behave somewhat
similar to TTYs, even though they don't need to be. When you try to
perform a read() on the TTY after the master file descriptor gets
closed, you get an end-of-file. You only get an EIO when trying to write
to this descriptor.

Would it be hard to change the Linux TTY code to behave the same?

--
Ed Schouten <ed@FreeBSD.org>
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