From mboxrd@z Thu Jan  1 00:00:00 1970
From: michael@kjorling.se (Michael =?utf-8?B?S2rDtnJsaW5n?=)
Date: Mon, 15 Aug 2016 16:56:45 +0000
Subject: [TUHS] fstat(2) on pipes?
In-Reply-To: <1471274085.1099635.695741217.6472725F@webmail.messagingengine.com>
References: <20160815140450.C6CC618C0B3@mercury.lcs.mit.edu>
 <1471274085.1099635.695741217.6472725F@webmail.messagingengine.com>
Message-ID: <20160815165645.GE655@yeono.kjorling.se>

On 15 Aug 2016 11:14 -0400, from random832 at fastmail.com (Random832):
> On Mon, Aug 15, 2016, at 10:04, Noel Chiappa wrote:
>> But back to the original topic, it sounds like there's a huge amount
>> of variance in the semantics of doing fstat() on a pipe. V6 doesn't
>> special-case it in any way, but it sounds as if other systems do.
> 
> I expect that the single important thing, the only thing that most
> applications will rely on, is it returning successfully and indicating
> that the file type is fifo.

On Linux/glibc, based on the fstat(2) man page, it looks like the size
field is undefined for a FIFO:

> The st_size field gives the size of the file (if it is a regular
> file or a symbolic link) in bytes. The size of a symbolic link is
> the length of the pathname it contains, without a terminating null
> byte.

The mode field is used to hold the type of file:

> The following POSIX macros are defined to check the file type using
> the st_mode field:
> 
> ...
>     S_ISFIFO(m) FIFO (named pipe)?
> ...
> 
> The following flags are defined for the st_mode field:
> ...
>     S_IFIFO    0010000   FIFO
> ...

The above from Debian Wheezy.

-- 
Michael Kjörling • https://michael.kjorling.se • michael at kjorling.se
                 “People who think they know everything really annoy
                 those of us who know we don’t.” (Bjarne Stroustrup)