Editing Vectored I/O (section)

{{short description|Input/output method in computing}}
{{About|the I/O method|the vector addressing type|gather/scatter (vector addressing)}}

In [[computing]], '''vectored I/O''', also known as '''scatter/gather I/O''', is a method of [[Input/output|input and output]] by which a single procedure call sequentially reads data from multiple [[data buffer|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 (data structure)|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 (programming)|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 (computer science)|thread]] or [[Computer process|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 [[syscall]]s
* 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 <code>readv</code><ref>[http://www.opengroup.org/onlinepubs/009695399/functions/readv.html readv] {{Webarchive|url=https://web.archive.org/web/20080905083604/http://www.opengroup.org/onlinepubs/009695399/functions/readv.html |date=2008-09-05 }} in the Single Unix Specification</ref> and <code>writev</code><ref>[http://www.opengroup.org/onlinepubs/009695399/functions/writev.html writev] {{Webarchive|url=https://web.archive.org/web/20071217075647/http://www.opengroup.org/onlinepubs/009695399/functions/writev.html |date=2007-12-17 }} in the Single Unix Specification</ref> in [[POSIX]] 1003.1-2001 and the [[Single UNIX Specification]] version 2. The [[Windows API]] has analogous functions <code>ReadFileScatter</code> and <code>WriteFileGather</code>; however, unlike the POSIX functions, they require the alignment of each buffer on a [[memory page]].<ref>[http://msdn2.microsoft.com/en-us/library/aa365469.aspx ReadFileScatter] in MSDN Library</ref>  [[Winsock]] provides separate <code>WSASend</code> and <code>WSARecv</code> 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<ref>[http://www.and.org/vstr/ Vstr] {{Webarchive|url=https://web.archive.org/web/20170305020810/http://www.and.org/vstr/ |date=2017-03-05 }} the Vectored String API</ref>
for working efficiently can mitigate the difficulties.