In
computing
Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, ...
, vectored I/O, also known as scatter/gather I/O, is a method of
input and output
In computing, input/output (I/O, or informally io or IO) is the communication between an information processing system, such as a computer, and the outside world, possibly a human or another information processing system. Inputs are the signals ...
by which a single procedure call sequentially reads data from multiple
buffers and writes it to a single
data stream
In connection-oriented communication, a data stream is the transmission of a sequence of digitally encoded coherent signals to convey information. Typically, the transmitted symbols are grouped into a series of packets.
Data streaming has b ...
, or reads data from a data stream and writes it to multiple buffers, as defined in a
vector
Vector most often refers to:
*Euclidean vector, a quantity with a magnitude and a direction
*Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism
Vector may also refer to:
Mathematic ...
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
A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic.
Things called a process include:
Business and management
*Business process, activities that produce a specific se ...
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
In computing, a system call (commonly abbreviated to syscall) is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services (for example, acc ...
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
readv and
writev in
POSIX
The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines both the system- and user-level application programming inter ...
1003.1-2001 and the
Single UNIX Specification version 2. The
Windows API
The Windows API, informally WinAPI, is Microsoft's core set of application programming interfaces (APIs) available in the Microsoft Windows operating systems. The name Windows API collectively refers to several different platform implementations th ...
has analogous functions
ReadFileScatter and
WriteFileGather; however, unlike the POSIX functions, they require the alignment of each buffer on a
memory page
A page, memory page, or virtual page is a fixed-length contiguous block of virtual memory, described by a single entry in the page table. It is the smallest unit of data for memory management in a virtual memory operating system. Similarly, a p ...
.
Winsock
In computing, the Windows Sockets API (WSA), later shortened to Winsock, is an application programming interface (API) that defines how Windows network application software should access network services, especially TCP/IP. It defines a standar ...
provides separate
WSASend and
WSARecv 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
Vstr
the Vectored String API
for working efficiently can mitigate the difficulties.
Examples
The following example prints "Hello, Wikipedia Community!" to the standard output
In computer programming, standard streams are interconnected input and output communication channels between a computer program and its environment when it begins execution. The three input/output (I/O) connections are called standard input (stdin ...
. Each word is saved into a single buffer and with only one call to writev(), all buffers are printed to the standard output.
#include
#include
#include
#include
#include
int main(int argc, char *argv Zero_copy
0 (zero) is a number representing an empty quantity. In place-value notation such as the Hindu–Arabic numeral system, 0 also serves as a placeholder numerical digit, which works by multiplying digits to the left of 0 by the radix, usua ...