NAME¶

LIBRARY¶

SYNOPSIS¶

ssize_t
read(int fd, void *buf, size_t nbytes);

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

#include <sys/uio.h>

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

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

DESCRIPTION¶

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

struct iovec {
	void   *iov_base;  /* Base address. */
	size_t iov_len;    /* Length. */
};

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

On objects capable of seeking, the read() starts at a position given by the pointer associated with fd (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, read(), readv(), pread() 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.

RETURN VALUES¶

ERRORS¶

[EBADF]

The fd argument is not a valid file or socket descriptor open for reading.

[ECONNRESET]

The fd argument refers to a socket, and the remote socket end is forcibly closed.

[EFAULT]

The buf argument points outside the allocated address space.

[EIO]

An I/O error occurred while reading from the file system.

[EBUSY]

Failed to read from a file, e.g. /proc/<pid>/regs while <pid> is not stopped

[EINTR]

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.

[EINVAL]

The pointer associated with fd was negative.

[EAGAIN]

The file was marked for non-blocking I/O, and no data were ready to be read.

[EISDIR]

The file descriptor is associated with a directory residing on a file system that does not allow regular read operations on directories (e.g. NFS).

[EOPNOTSUPP]

The file descriptor is associated with a file system and file type that do not allow regular read operations on it.

[EOVERFLOW]

The file descriptor is associated with a regular file, nbytes is greater than 0, offset is before the end-of-file, and offset is greater than or equal to the offset maximum established for this file system.

[EINVAL]

The value nbytes is greater than INT_MAX.

In addition, readv() and preadv() may return one of the following errors:

[EINVAL]

The iovcnt argument was less than or equal to 0, or greater than IOV_MAX.

[EINVAL]

One of the iov_len values in the iov array was negative.

[EINVAL]

The sum of the iov_len values in the iov array overflowed a 32-bit integer.

[EFAULT]

Part of the iov array points outside the process's allocated address space.

The pread() and preadv() system calls may also return the following errors:

[EINVAL]

The offset value was negative.

[ESPIPE]

The file descriptor is associated with a pipe, socket, or FIFO.

SEE ALSO¶

STANDARDS¶

HISTORY¶