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READ(2) System Calls Manual READ(2)

read, readv, pread, preadvread input

#include <unistd.h>

read(int d, void *buf, size_t nbytes);

pread(int d, void *buf, size_t nbytes, off_t offset);

#include <sys/uio.h>

readv(int d, const struct iovec *iov, int iovcnt);

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

preadv(int d, const struct iovec *iov, int iovcnt, off_t offset);

() attempts to read nbytes of data from the object referenced by the descriptor d into the buffer pointed to by buf. readv() performs the same action, but scatters the input data into the iovcnt buffers specified by the members of the iov array: iov[0], iov[1], ..., iov[iovcnt-1]. () and preadv() perform the same functions, but read from the specified position offset in the file without modifying the file pointer.

For () and (), the iovec structure is defined as:

struct iovec {
	void	*iov_base;
	size_t	 iov_len;

Each iovec entry specifies the base address and length of an area in memory where data should be placed. () and () will always fill an area completely before proceeding to the next.

On objects capable of seeking, the () starts at a position given by the pointer associated with d (see lseek(2)). Upon return from read(), the pointer is incremented by the number of bytes actually read.

Objects that are not capable of seeking always read from the current position. The value of the pointer associated with such an object is undefined.

Upon successful completion, (), readv(), (), and preadv() return the number of bytes actually read and placed in the buffer. The system guarantees to read the number of bytes requested if the descriptor references a normal file that has that many bytes left before the end-of-file, but in no other case.

Note that () and () will fail if the value of iovcnt exceeds the constant IOV_MAX.

If successful, the number of bytes actually read is returned. Upon reading end-of-file, zero is returned. Otherwise, a -1 is returned and the global variable errno is set to indicate the error.

read(), readv(), pread(), and preadv() will fail if:

d is not a valid file or socket descriptor open for reading.
Part of buf points outside the process's allocated address space.
A read from a slow device (i.e. one that might block for an arbitrary amount of time) was interrupted by the delivery of a signal before any data arrived.
An I/O error occurred while reading from the file system.
The underlying file is a directory.

In addition, read() and readv() may return the following errors:

The file was marked for non-blocking I/O, and no data were ready to be read.
The file is a socket associated with a connection-oriented protocol and has not been connected.
The process is a member of a background process attempting to read from its controlling terminal, the process is ignoring or blocking the SIGTTIN signal or the process group is orphaned.

read() and pread() may return the following error:

nbytes was larger than SSIZE_MAX.

pread() and preadv() may return the following errors:

offset was negative.
d is associated with a pipe, socket, FIFO, or tty.

readv() and preadv() may return the following errors:

iovcnt was less than or equal to 0, or greater than IOV_MAX.
The sum of the iov_len values in the iov array overflowed an ssize_t.
Part of iov points outside the process's allocated address space.

dup(2), fcntl(2), open(2), pipe(2), poll(2), select(2), socket(2), socketpair(2)

The read(), readv(), and pread() functions conform to IEEE Std 1003.1-2008 (“POSIX.1”).

A read() system call first appeared in Version 1 AT&T UNIX; readv() in 4.1cBSD; pread() in AT&T System V Release 4 UNIX; and preadv() in OpenBSD 2.7.

Error checks should explicitly test for -1. Code such as

while ((nr = read(fd, buf, sizeof(buf))) > 0)

is not maximally portable, as some platforms allow for nbytes to range between SSIZE_MAX and SIZE_MAX - 2, in which case the return value of an error-free read() may appear as a negative number distinct from -1. Proper loops should use

while ((nr = read(fd, buf, sizeof(buf))) != -1 && nr != 0)
November 21, 2021 OpenBSD-7.4