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Date: Sat, 16 Nov 1996 02:29:19 +1100
From: Bruce Evans <bde@zeta.org.au>
Message-Id: <199611151529.CAA16817@godzilla.zeta.org.au>
To: jgreco@brasil.moneng.mei.com, terry@lambert.org
Subject: Re: Sockets question...
Cc: hackers@FreeBSD.ORG, jdp@polstra.com, scrappy@ki.net
Sender: owner-hackers@FreeBSD.ORG
X-Loop: FreeBSD.org
Precedence: bulk

>% man 2 read
>(SunOS version)
>...
>READ(2V)                  SYSTEM CALLS                   READ(2V)
>
>     system guarantees to read the number of bytes  requested  if
>     the  descriptor references a normal file which has that many
>     bytes left before the EOF (end of file),  but  in  no  other
>     case.
>
>Key words, "but in no other case"..

FreeBSD says the same thing in the same words except for English
improvements.  It lies :-).  Short reads are possible even for
normal files because the read might be into a partially invalid
buffer.  ffs_read() returns the blocks that are successfully
read instead of unwinding the read and returning an EFAULT error.
Example:

---
#include <stdio.h>
#include <stdlib.h>

#define SIZE	0x10000

main()
{
    void *buf;
    int n;


    buf = malloc(0x10000);
    n = read(0, buf, 0x10000);
    printf("read %d\n", n);
}
---

This prints 65536 here.  A watertight example can be probably be
constructed using mmap().

Short writes to normal files are more likely.  They occur when the
disk fills up and for partially invalid buffers.  There are more
serious problems for the latter case.

Bruce