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READ(2) System Calls Manual READ(2)

read, readv, pread, preadvread input

#include <unistd.h>



#include <sys/uio.h>


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


() 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.


Part of buf points outside the process's allocated address space.
The underlying file is a directory.

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

The file is a socket associated with a connection-oriented protocol and has not been connected.


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.


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.


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



September 30, 2016 OpenBSD-6.2