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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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