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Re: [Qemu-devel] [PATCH 2/3] block: Add support to "open" /dev/fd/X file
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] [PATCH 2/3] block: Add support to "open" /dev/fd/X filenames |
Date: |
Mon, 04 Jun 2012 10:36:33 -0600 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20120430 Thunderbird/12.0.1 |
On 06/04/2012 10:28 AM, Corey Bryant wrote:
>> But at least strtol lets you detect errors:
>>
>> char *tmp;
>> errno = 0;
>> fd = strtol(p,&tmp, 10);
>> if (errno || tmp == p) {
>> /* raise your error here */
>> }
>
> I don't think this is legitimate. errno can be set under the covers of
> library calls even if the strtol() call is successful.
Wrong. POSIX _specifically_ requires that strtol() leave errno
unchanged unless strtol() is reporting a failure, no matter what other
library calls (if any) are made under the covers by strtol().
In other words, pre-setting errno to 0, then calling strtol(), then
checking errno, _is_ the documented way to check for strtol() failures,
and a correct usage of strtol() MUST use this method. See also commit
6b0e33be88bbccc3bcb987026089aa09f9622de9. atoi() does not have this
same guarantee, which makes atoi() worthless at detecting errors in
relation to strtol().
>
> I was thinking if strtol returns 0 and errno is 0, perhaps we could
> assume success, but I don't think this is guaranteed either.
Actually, it _is_ guaranteed - if you pre-set errno to 0, then call
strtol(), then errno is still 0, then the result did not encounter an
error, so a result of 0 at that point means that you indeed parsed a 0.
>
> Maybe a combination of isdigit() then strtol() will give a better idea
> of success.
Not necessary.
--
Eric Blake address@hidden +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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- Re: [Qemu-devel] [PATCH 1/3] qmp/hmp: Add QMP getfd command that returns fd, (continued)
[Qemu-devel] [PATCH 3/3] Sample server that opens image files for QEMU, Corey Bryant, 2012/06/04
[Qemu-devel] [PATCH 2/3] block: Add support to "open" /dev/fd/X filenames, Corey Bryant, 2012/06/04
Re: [Qemu-devel] [PATCH 2/3] block: Add support to "open" /dev/fd/X filenames, Kevin Wolf, 2012/06/04