### Buffer-oriented synchronous random-access write device requirements

In the table below, `a` denotes a synchronous random-access write device object, `o` denotes an offset of type `boost::uint64_t`, `cb` denotes an object satisfying constant buffer sequence requirements, and `ec` denotes an object of type `error_code`.

Table 31. Buffer-oriented synchronous random-access write device requirements

operation | type | semantics, pre/post-conditions
--- | --- | ---

`a.write_some_at(o, cb);`

`size_t`

Equivalent to:

    error_code ec;
    size_t s = a.write_some(o, cb, ec);
    if (ec) throw system_error(ec);
    return s;

`a.write_some_at(o, cb, ec);`

`size_t`

Writes one or more bytes of data to the device `a` at offset `o`.
The constant buffer sequence `cb` specifies memory where the data to be written is located. The `write_some_at` operation shall always write a buffer in the sequence completely before proceeding to the next.
If successful, returns the number of bytes written and sets `ec` such that `!ec` is true. If an error occurred, returns `0` and sets `ec` such that `!!ec` is true.
If the total size of all buffers in the sequence `cb` is `0`, the function shall return `0` immediately.