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, or reads data from a data stream and writes it to multiple buffers, 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.

This article is about the I/O method. For the vector addressing type, see Gather-scatter (vector addressing).

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 readv[1] and writev[2] in POSIX 1003.1-2001 and the Single UNIX Specification version 2. The Windows API has analogous functions ReadFileScatter and WriteFileGather; however, unlike the POSIX functions, they require the alignment of each buffer on a memory page.[3] Winsock 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[4] for working efficiently can mitigate the difficulties.

Examples

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

#include <stdio.h>
#include <stdlib.h>

#include <sys/types.h>
#include <unistd.h>
#include <sys/uio.h>

int main(int argc, char *argv[])
{
	const char buf1[] = "Hello, ";
	const char buf2[] = "Wikipedia ";
	const char buf3[] = "Community!\n";

	struct iovec bufs[] = {
		{ .iov_base = (void *) buf1, .iov_len = sizeof buf1 - 1 },
		{ .iov_base = (void *) buf2, .iov_len = sizeof buf2 - 1 },
		{ .iov_base = (void *) buf3, .iov_len = sizeof buf3 - 1 },
	};

	if (-1 == writev(STDOUT_FILENO, bufs, sizeof bufs / sizeof *bufs))
	{
		perror("writev()");
		exit(EXIT_FAILURE);
	}

	return 0;
}
gollark: That's mostly just recursive.
gollark: I assumed you were talking about religion there initially given the phrasing, which I do consider to involve horrible punishment for dubious ethical reasons. But in general it seems to basically just mean "punishment", with connotations of "in accordance with some allegedly fair procedure(s)".
gollark: Secularly, people generally mean "punish people" when they talk about justice as far as I can tell.
gollark: Well, religiously, "justice" seems to mostly be "eternally torturing people".
gollark: I mean, I suppose you can define it that way, but then it becomes a less useful concept and OH BEE HE HAS COME HERE

References
  1. readv in the Single Unix Specification
  2. writev in the Single Unix Specification
  3. ReadFileScatter in MSDN Library
  4. Vstr the Vectored String API
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