Vectored I/O

In computing, vectored I/O, also known as scatter/gather I/O, is a method of input and output by which a single procedure call sequentially reads data from multiple buffers and writes it to a single data stream (gather), or reads data from a data stream and writes it to multiple buffers (scatter), as defined in a vector of buffers. Scatter/gather refers to the process of gathering data from, or scattering data into, the given set of buffers. Vectored I/O can operate synchronously or asynchronously. The main reasons for using vectored I/O are efficiency and convenience.

Vectored I/O has several potential uses:
 * Atomicity: if the particular vectored I/O implementation supports atomicity, a process can write into or read from a set of buffers to or from a file without risk that another thread or process might perform I/O on the same file between the first process' reads or writes, thereby corrupting the file or compromising the integrity of the input
 * Concatenating output: an application that wants to write non-sequentially placed data in memory can do so in one vectored I/O operation. For example, writing a fixed-size header and its associated payload data that are placed non-sequentially in memory can be done by a single vectored I/O operation without first concatenating the header and the payload to another buffer
 * Efficiency: one vectored I/O read or write can replace many ordinary reads or writes, and thus save on the overhead involved in syscalls
 * Splitting input: when reading data held in a format that defines a fixed-size header, one can use a vector of buffers in which the first buffer is the size of that header; and the second buffer will contain the data associated with the header

Standards bodies document the applicable functions and  in POSIX 1003.1-2001 and the Single UNIX Specification version 2. The Windows API has analogous functions  and  ; however, unlike the POSIX functions, they require the alignment of each buffer on a memory page. Winsock provides separate  and   functions without this requirement.

While working directly with a vector of buffers can be significantly harder than working with a single buffer, using higher-level APIs for working efficiently can mitigate the difficulties.

Examples
The following example in the C programming language prints "Hello, Wikipedia Community!" to the standard output. Each word is saved into a single buffer and with only one call to, all buffers are printed to the standard output.