readv - read array from file

Syntax	#include <sys/uio.h> ssize_t readv(int fildes, const struct iovec *iov, int iovcnt);
Description	See read() . readv() behaves like read() but reads the input data from the file belonging to fildes into the iovcnt buffers which are specified as elements of the iov field: iov[0], iov[1], ..., iov[iovcnt-1]. 0 must be < iovcnt <= {IOV_MAX} The iovec structure contains the following elements: addr_t      iov_base; size_t      iov_len; Each iovec entry specifies the basic address and length of a storage area (buffer) in which data is to be put. readv() always fills a buffer completely before going on to the next one. If successful, readv() returns the number of bytes that were actually read and written to the buffer. If the end of file is reached, 0 is returned.
Return val.	integer >0	if successful. The number is the number of bytes that were actually read.
	0	if the end of file (EOF) was reached during reading.
	-1	if an error occurs. errno is set to indicate the error. The contents of the buffer are undefined.
Errors	readv() will fail if:
	EAGAIN	The O_NONBLOCK flag is set for the file descriptor and the process would be suspended by the read operation.
	Extension
	EAGAIN	The currently available amount of system memory for "raw" I/O is insufficient or there is no data in a terminal device file waiting to be read, and O_NONBLOCK is set or there is no message in a stream waiting to be read, and O_NONBLOCK is set. (End)
	EBADF	fildes is not a valid file descriptor open for reading.
	EBADMSG	The file is a STREAM file in control-normal mode, but the message waiting to be read contains a control section.
	EFAULT	iov points outside the allocated address space of the process.
	EINTR	The read operation was terminated due to the receipt of a signal, and no data was transferred.
	EINVAL	An attempt was made to read from a stream linked with a multiplexer or the sum of the iov-len values in the iov field caused a ssize_t overflow or iovcnt ≤ 0 or iovcnt > 16.
	EIO	A physical I/O error has occurred or the process is a member of a background process group attempting to read from its controlling terminal. The process is ignoring or blocking the  SIGTTIN signal or the process group is orphaned.
	EISDIR	fildes describes a directory that cannot be read with readv(). readdir() should be used instead.
	ENXIO	A request was made for a non-existent device or the request exceeded the capabilities of the device.
	ENOLINK	fildes is located on a remote computer to which the link is no longer active.
	A readv() from a STREAMS file will also fail if an error message is received at the stream head. In this case, errno is set to the value that is returned in the error message. If a hangup occurs in the stream currently being read, readv() continues running normally until the read queue of the stream head is empty. Thereafter 0 is returned.
Syntax	fcntl(), ioctl(), lseek(), open(), pipe(), stropts.h, sys/uio.h, unistd.h.