Date: Sat, 10 Jul 2010 17:28:56 -0400 From: Patrick Donnelly <batrick@batbytes.com> To: "C. P. Ghost" <cpghost@cordula.ws>, questions@freebsd.org Subject: Re: Writes to Hard Disk Going Beyond Capacity Message-ID: <AANLkTin3-mZTKPQrihEE4EBQCvMhkebcv9O06rLrlWeV@mail.gmail.com> In-Reply-To: <AANLkTinLsM2EEGMZEthZ95YDPnD7Ey20xbeiOR3sXWMp@mail.gmail.com> References: <AANLkTin9-MymICgQoSjFlKbLXN2Efm5_3r6X1lufqiCo@mail.gmail.com> <87iq4nsr3a.fsf@kobe.laptop> <AANLkTilAuZ3MkzDP5_f7ETk0wcvWR7OLYB8G4c_JVpj9@mail.gmail.com> <AANLkTilc1NdTqA91CTxINnAdtyE2ToViPdytTTnwkIm0@mail.gmail.com> <AANLkTinLsM2EEGMZEthZ95YDPnD7Ey20xbeiOR3sXWMp@mail.gmail.com>
next in thread | previous in thread | raw e-mail | index | archive | help
On Sat, Jul 10, 2010 at 2:38 PM, C. P. Ghost <cpghost@cordula.ws> wrote:
>> Are you aware of short writes?
>>
>> static int write_buf (int fd, const char *buf, size_t s)
>> {
>> =C2=A0ssize_t r =3D write(fd, buf, s);
>> =C2=A0if (r =3D=3D -1)
>> =C2=A0 =C2=A0fprintf(stderr, "write error: %s\n", strerror(errno));
>> =C2=A0return r >=3D 0;
>> }
>>
>> What if write(2) returns less than s, but not -1?
>
> Or, to be more precise, is it possible that write(2) returns 0 for
> some reason, perhaps because the device isn't ready and can't
> accept more data, so it says that it wrote 0 bytes, but that you
> are free to try again?
>
> On Solaris, the write(2) man page says:
>
> =C2=A0If a write() requests that more bytes be written than there is room
> =C2=A0for=E2=80=94for example, if the write would exceed the process file=
size
> =C2=A0limit (see getrlimit(2) and ulimit(2)), the system file size limit,
> =C2=A0or the free space on the device=E2=80=94only as many bytes as there=
is room
> =C2=A0for will be written. For example, suppose there is space for 20
> =C2=A0bytes more in a file before reaching a limit. A write() of 512-byte=
s
> =C2=A0returns 20. The next write() of a non-zero number of bytes gives a
> =C2=A0failure return (except as noted for pipes and FIFO below).
>
> http://docs.sun.com/app/docs/doc/816-5167/write-2?l=3Den&n=3D1&a=3Dview
>
> Have you tried your program on [Open]Solaris too? What happens there?
> Perhaps our write(2) isn't entirely IEEE Std 1003.1 compliant? Because
> write(2) there says:
>
> =C2=A0If a write() requests that more bytes be written than there is room
> =C2=A0for (for example, [XSI] [Option Start] the process' file size limit
> =C2=A0or [Option End] the physical end of a medium), only as many bytes a=
s
> =C2=A0there is room for shall be written. For example, suppose there is
> =C2=A0space for 20 bytes more in a file before reaching a limit. A write
> =C2=A0of 512 bytes will return 20. The next write of a non-zero number of
> =C2=A0bytes would give a failure return (except as noted below). (...)
>
> http://www.opengroup.org/onlinepubs/000095399/functions/write.html
>
> Hmmm... any C/POSIX standards lawyers/specialists here?
write returning 0 appears to be the problem. That is indeed strange
and I would guess it may be a bug?
--=20
- Patrick Donnelly
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?AANLkTin3-mZTKPQrihEE4EBQCvMhkebcv9O06rLrlWeV>