Re: [9fans] i/o on a hangup channel asymmetry

[Roman Shaposhnik <rvs@sun.com>]

On Jul 18, 2009, at 6:06 PM, erik quanstrom wrote:
> On Sat Jul 18 14:41:02 EDT 2009, rvs@sun.com wrote:
>> In the "mom, why sky is blue" department, here's a silly question:
>> is there any good reason that read(2) on a hangup channel returns
>> an error, while write(2) on a hangup channel terminates an
>> application
>> (by generating a note, of course, which can be ignored, but still)?
>
> hmm.  from a quick read of port/sysfile.c, i think the precise
> behavior
> might depend on the underlying device.  if devtab[m->c->type]->bread/
> bwrite
> are (ultimately) based on qbread and qbwrite, i read qio as saying
> that neither should
> generate a note.
>
> perhaps i've been asleep at the swtch, but i don't recall seing writes
> on closed channels terminate programs with a note.

Observe:

cpu% cat test2.c
#include <u.h>
#include <libc.h>
#include <stdio.h>

void notary(void *v, char* s)
{
     fprintf(stderr, "NOTE: %s\n", s);
     noted(NCONT);
}

int main()
{
     notify(&notary);

     while (1) {
        fprintf(stderr, "%d\n", write(1, "roman", 5));
        sleep(5000);
     }
     return 0;
}

cpu% 8c test2.c
cpu% 8l test2.8
cpu% { ./8.out | cat  } & { sleep 10 ; slay cat | rc }
5
roman5
romanNOTE:  sys: write on closed pipe pc=0x00001525
-1
NOTE:  sys: write on closed pipe pc=0x00001525
-1
NOTE:  sys: write on closed pipe pc=0x00001525
-1

Thanks,
Roman.