### 13.6 Fast Scatter-Gather I/O

Some applications may need to read or write data to multiple buffers, which are separated in memory. Although this can be done easily enough with multiple calls to `read` and `write`, it is inefficient because there is overhead associated with each kernel call.

Instead, many platforms provide special high-speed primitives to perform these scatter-gather operations in a single kernel call. The GNU C Library will provide an emulation on any system that lacks these primitives, so they are not a portability threat. They are defined in `sys/uio.h`.

These functions are controlled with arrays of `iovec` structures, which describe the location and size of each buffer.

— Data Type: struct iovec

> The `iovec` structure describes a buffer. It contains two fields:

> `void *iov_base`
>   Contains the address of a buffer.
> `size_t iov_len`
>   Contains the length of the buffer.

— Function: ssize_t readv (int filedes, const struct iovec *vector, int count)

> Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

> The `readv` function reads data from filedes and scatters it into the buffers described in vector, which is taken to be count structures long. As each buffer is filled, data is sent to the next.

> Note that `readv` is not guaranteed to fill all the buffers. It may stop at any point, for the same reasons `read` would.

> The return value is a count of bytes (not buffers) read, 0 indicating end-of-file, or -1 indicating an error. The possible errors are the same as in `read`.

— Function: ssize_t writev (int filedes, const struct iovec *vector, int count)

> Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

> The `writev` function gathers data from the buffers described in vector, which is taken to be count structures long, and writes them to `filedes`. As each buffer is written, it moves on to the next.

> Like `readv`, `writev` may stop midstream under the same conditions `write` would.

> The return value is a count of bytes written, or -1 indicating an error. The possible errors are the same as in `write`.

Note that if the buffers are small (under about 1kB), high-level streams may be easier to use than these functions. However, `readv` and `writev` are more efficient when the individual buffers themselves (as opposed to the total output), are large. In that case, a high-level stream would not be able to cache the data effectively.