when to use readv()/writev() ?

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2 total messages Started by Marcus J. Ranum Thu, 08 Nov 1990 04:13

#1193

Author: Marcus J. Ranum
Date: Thu, 08 Nov 1990 04:13

20 lines
1411 bytes

	What are some cases when I'd want to use readv() instead of
read()? I understand the advantages of being able to break things
into several buffers (and possibly avoid having to move data around)
but does using writev() give me some kind of possible performance
improvement? Does writev() help block my data into popular buffer
sizes or something like that ?

	If I have 2 iovecs with 500 and 524 bytes respectively is the
operation performed analogous to write(fd,buf,1024) or is it simply
write(fd,buf1,500), write(fd,buf2,524) with the kernel just doing
the legwork?

	I see I still have to sum up the sizes of the contents of the
scatter/gather arrays so I can check the return of the write - doesn't
it get kind of icky when you have a partial writev()? Please let's
*NOT* get into a bOring 6-week long discussion about checking return
values. Suffice to say I do. :)

mjr.

Re: when to use readv()/writev() ?

#1194

Author: Chris Torek
Date: Thu, 08 Nov 1990 06:55

21 lines
1471 bytes

In article <1990Nov8.041357.29013@decuac.dec.com> mjr@decuac.dec.com
(Marcus J. Ranum) writes:
>	If I have 2 iovecs with 500 and 524 bytes respectively is the
>operation performed analogous to write(fd,buf,1024) or is it simply
>write(fd,buf1,500), write(fd,buf2,524) with the kernel just doing
>the legwork?

The latter (except, potentially, to a character device, but there are
no character devices in any Unix I have seen that do anything special
with multiple IO vectors).

>	I see I still have to sum up the sizes of the contents of the
>scatter/gather arrays so I can check the return of the write - doesn't
>it get kind of icky when you have a partial writev()?

Yes, it does.  This is one of the reasons I elected not to use writev()
in my stdio implementation (which has a stdio-level writev-like function
whose main raison d'etre is printf()).
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 405 2750)
Domain:	chris@cs.umd.edu	Path:	uunet!mimsy!chris

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