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.

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 <string.h>

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

#define NUMBUFS 3

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

	struct iovec bufs[NUMBUFS];

	bufs[0].iov_base = (void*) buf1;
	bufs[0].iov_len = strlen(buf1);

	bufs[1].iov_base = (void*) buf2;
	bufs[1].iov_len = strlen(buf2);

	bufs[2].iov_base = (void*) buf3;
	bufs[2].iov_len = strlen(buf3);

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

	return 0;
}

References