read()

NAME

read(), readv() - read input

SYNOPSIS

#include <unistd.h>


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


#include <sys/uio.h>


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

DESCRIPTION

The read(2) function attempts to read nbytes of data from the object referenced by the descriptor d into the buffer pointed to by buf. The readv(2) function 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].

For readv(2), 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. The readv(2) function will always fill an area completely before proceeding to the next.

On objects capable of seeking, the read(2) starts at a position given by the pointer associated with d (see lseek(2)). Upon return from read(2), 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(2) and readv(2) 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

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.

When the nbytes argument exceeds {SSIZE_MAX}, the return value is truncated to type ssize_t.

ERRORS

The read(2) and readv(2) calls will succeed unless:

[EAGAIN]

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

[EBADF]

D is not a valid file descriptor open for reading.

[ECONNRESET]

The connection was forcibly closed by one of the peers; this is normally an indication of a time-out or a reboot.

[EFAULT]

Buf points outside the allocated address space.

[EINTR]

A read from a slow device was interrupted before any data arrived by the delivery of a signal.

[EINVAL]

The pointer associated with d was negative.

[EIO]

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

In addition, readv(2) may return one of the following errors:

[EINVAL]

Iovcnt was less than or equal to 0, or greater than 16.

[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 points outside the process's allocated address space.

SEE ALSO

dup(2)

fcntl(2)

open(2)

pipe(2)

select(2)

socket(2)