READ(2)

NAME
read, readv - read input

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

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

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

DESCRIPTION
read 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].

For readv, 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. readv 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 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, read and readv 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.

If the file was opened with the GNO-specific flag O_TRANS, then newline translation will occur; any carridge return character (0x0d) read from descriptor d will be converted to a line feed (0x0a).

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.

ERRORS
read and readv will succeed unless:

    EBADF
        D is not a valid file or socket descriptor open for reading.

    EFAULT
        Buf points outside the allocated address space.

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

    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.

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

In addition, readv 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.

SEE ALSO
dup(2), fcntl(2), open(2), pipe(2), select(2), socket(2), socketpair(2)

STANDARDS
Read is expected to conform to IEEE Std 1003.1-1988 (POSIX).

HISTORY
The readv function call appeared in 4.2BSD. A read function call appeared in Version 6 AT&T UNIX.