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

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

Table 28. Buffer-oriented synchronous random-access read device requirements

operation | type | semantics, pre/post-conditions
--- | --- | ---
`a.read_some_at(o, mb);` | `size_t` | Equivalent to:
    error_code ec;
    size_t s = a.read_some_at(o, mb, ec);
    if (ec) throw system_error(ec);
    return s;
`a.read_some_at(o, mb, ec);` | `size_t` | Reads one or more bytes of data from the device `a` at offset `o`. The mutable buffer sequence `mb` specifies memory where the data should be placed. The `read_some_at` operation shall always fill a buffer in the sequence completely before proceeding to the next. If successful, returns the number of bytes read 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 `mb` is `0`, the function shall return `0` immediately.