Re: [Xen-devel] [PATCH v2] tools/libxc: Implement writev_exact() in the same style as write_exact()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 08/07/14 10:27, David Vrabel wrote:
> On 07/07/14 11:18, Andrew Cooper wrote:
>> This implementation of writev_exact() will cope with an iovcnt greater than
>> IOV_MAX because glibc will actually let this work anyway, and it is very
>> useful not to have to work about this in the caller of writev_exact().  The
>> caller is still required to ensure that the sum of iov_len's doesn't overflow
>> a ssize_t.
>>
>> Promote the MAX() and MIN() macro definitions from xg_save_restore.h to
>> xc_private.h
> [...]
>> --- a/tools/libxc/xc_private.c
>> +++ b/tools/libxc/xc_private.c
>> @@ -854,6 +854,37 @@ int write_exact(int fd, const void *data, size_t size)
>>      return 0;
>>  }
>>  
>> +int writev_exact(int fd, const struct iovec *iov, int iovcnt)
>> +{
> [...]
>> +        /* writev() guarentees atomicity of individual iov[] elements.  
>> Sanity
>> +         * check that the returned len did lie on an iov[] element 
>> boundary. */
>> +        assert(len == 0);
> There's nothing in the writev(2) man page that says this.  I think you
> need to handle partial writes of an entry.

Final paragraph in the DESCRIPTION

"The data transfers performed by readv() and writev() are atomic: the
data written by writev() is written as a single block that is not
intermingled with output from writes in other processes"

By my reading, it cannot guarantee atomicity if it would split an iov[]
element.

>
>> --- a/tools/libxc/xc_private.h
>> +++ b/tools/libxc/xc_private.h
> [...]
>> +#ifndef MAX
>> +#define MAX(_a, _b) ((_a) >= (_b) ? (_a) : (_b))
>> +#endif
>> +#ifndef MIN
>> +#define MIN(_a, _b) ((_a) <= (_b) ? (_a) : (_b))
>> +#endif
> Do we really want to use these unsafe macros in more places?
>
> David

Perhaphs, perhaps not, but there are no typesafe ones to hand.

~Andrew

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