Typically when you open a file descriptor on a pipe, it’s either for
reading or writing, but not both.

However, when you open a named pipe, it is possible to specify the
mode O_RDWR; but does this work, or return an error? And if it doesn’t
return an error, what exactly does it do?

I checked some relevant man pages
<https://manpages.debian.org/fifo(7)>
<https://manpages.debian.org/pipe(7)>, but they completely avoid any
mention of O_RDWR mode.

So I tried it. And it works without error. You get back a single file descriptor, that you can use for both writing to the pipe and reading
from it. So if no other processes open the same named pipe, you can
write something to it (I suppose until the kernel buffer fills up),
and read the same thing back again.

Not very useful, on the face of it. But I guess it means you can (with appropriate coordination from both ends) switch the direction of data
flow at any point, without having to close and reopen the named pipe.
Basically O_RDWR gives you a half-duplex communication channel, where
O_RDONLY and O_WRONLY will give you the ends of a simplex
(unidirectional) channel.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 17.03.2025 15:22, hymie! wrote:

In our last episode, the evil Dr. Lacto had captured our hero,
Lawrence D'Oliveiro <ldo@nz.invalid>, who said:

Typically when you open a file descriptor on a pipe, it’s either for
reading or writing, but not both.

However, when you open a named pipe, it is possible to specify the
mode O_RDWR; but does this work, or return an error? And if it doesn’t
return an error, what exactly does it do?

I'm not exactly sure what you're asking here, but I just set this up yesterday.

I think the OP is not wondering about the usefulness of the pipe
communication spread between client and server processes. But it
rather gets more interesting if both directions are used by the
same process. As in, say, this code fragment

for (int i=0; i<=200000; i++)
{
sprintf(buf, " A %05d", i%10000);
if (write(myfifo, buf, 8) < 8) {
fprintf (stderr, "*** write A failed at i=%d\n", i);
exit (1);
}
sprintf(buf, " B %05d", i%10000);
if (write(myfifo, buf, 8) < 8) {
fprintf (stderr, "*** write B failed at i=%d\n", i);
exit (1);
}
if (read(myfifo, buf, 8) < 8) {
fprintf (stderr, "*** read failed at i=%d\n", i);
exit (2);
}
buf[8] = '\0';
printf ("%s\n", buf);
}

This will overflow the FIFO at some point - twice as much data
is written than read/processed. But there may be interesting
applications where some initial huge data in the pipe will get
processed, rewritten in a reduced form, and over time solved
the task (when iterated reduction is complete). - While this
would be (still) not convincing (since you could operate as
well just on a buffer as opposed to a FIFO) it might get more
interesting in case of co-operating processes.

Short answer; I thing it makes sense to not restrict the FIFO
interface unnecessarily.

Janis

I open a FIFO

char myfifoname[100];
int myfifo;
sprintf(myfifoname,"/tmp/program.%08d",getpid());
mkfifo(myfifoname, S_IRUSR | S_IWUSR);
myfifo = open(myfifoname,O_RDWR | O_NONBLOCK);

Then I spawn a bunch of forks which already have the FIFO open. The
forked children write to the FIFO

if ( (cpid[9]=fork()) == 0)
{
for (;;)
{
write(myfifo,"9a\n",strlen("9a\n"));
napms( (int) Random(50,250));
write(myfifo,"9b\n",strlen("9b\n"));
napms( (int) Random(50,250));
}
}

And the parent process reads from the FIFO

for (;;)
{
int r=read(myfifo,buf,10);
int window = buf[0]-'0';
if (buf[1] == 'a')
/* Do stuff on the provided window */
if (buf[1] == 'b')
/* Do stuff on the provided window */
}

In my case, I know there's always a single-digit number followed by a
single character.

--hymie! http://nasalinux.net/~hymie hymie@nasalinux.net

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

In our last episode, the evil Dr. Lacto had captured our hero,
Lawrence D'Oliveiro <ldo@nz.invalid>, who said:

Typically when you open a file descriptor on a pipe, it’s either for reading or writing, but not both.

However, when you open a named pipe, it is possible to specify the
mode O_RDWR; but does this work, or return an error? And if it doesn’t return an error, what exactly does it do?

I'm not exactly sure what you're asking here, but I just set this up
yesterday.

I open a FIFO

char myfifoname[100];
int myfifo;
sprintf(myfifoname,"/tmp/program.%08d",getpid());
mkfifo(myfifoname, S_IRUSR | S_IWUSR);
myfifo = open(myfifoname,O_RDWR | O_NONBLOCK);

Then I spawn a bunch of forks which already have the FIFO open. The
forked children write to the FIFO

if ( (cpid[9]=fork()) == 0)
{
for (;;)
{
write(myfifo,"9a\n",strlen("9a\n"));
napms( (int) Random(50,250));
write(myfifo,"9b\n",strlen("9b\n"));
napms( (int) Random(50,250));
}
}

And the parent process reads from the FIFO

for (;;)
{
int r=read(myfifo,buf,10);
int window = buf[0]-'0';
if (buf[1] == 'a')
/* Do stuff on the provided window */
if (buf[1] == 'b')
/* Do stuff on the provided window */
}

In my case, I know there's always a single-digit number followed by a
single character.

--hymie! http://nasalinux.net/~hymie hymie@nasalinux.net

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)