Re: [Qemu-devel] [PATCH] error: error_setg_errno(): errno gets preserved

[Markus Armbruster <armbru@redhat.com>]

Eric Blake <eblake@redhat.com> writes:

> On 07/27/2016 03:24 AM, Sascha Silbe wrote:
>> C11 allows errno to be clobbered by pretty much any library function
>> call, so in general callers need to take care to save errno before
>> calling other functions.
>> 
>> However, for error reporting functions this is rather awkward and can
>> make the code on the caller side more complicated than
>> necessary. error_setg_errno() already takes care of preserving errno
>> and some functions rely on that, so just promise that we continue to
>> do so in the future.
>> 
>> Signed-off-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
>> ---
>> 
>> Alternative approach to "error: error_setg_errno(): errno may be
>> clobbered" [1].
>
> I like this alternative better.
>
>
>> +++ b/include/qapi/error.h
>> @@ -170,6 +170,9 @@ void error_setg_internal(Error **errp,
>>   * Just like error_setg(), with @os_error info added to the message.
>>   * If @os_error is non-zero, ": " + strerror(os_error) is appended to
>>   * the human-readable error message.
>> + *
>> + * The value of errno (which usually can get clobbered by almost any
>> + * function call) will be preserved.
>>   */
>>  #define error_setg_errno(errp, os_error, fmt, ...)                      \
>>      error_setg_errno_internal((errp), __FILE__, __LINE__, __func__,     \
>
> Do we need/want to make the guarantee of preserving errno across any of
> the other functions and macros declared in error.h?

I guess we should for the ones that preserve errno, to make that
preservation actually useful.  These are: error_setv(),
error_setg_errno_internal(), error_append_hint().  Indirectly:
error_set_internal(), error_set(), error_setg_internal(), error_setg(),
error_setg_file_open_internal(), error_setg_file_open(), possibly
error_setg_win32_internal() and error_setg_win32().