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READ(2)			  FreeBSD System Calls Manual		       READ(2)

     read, readv, pread	-- read	input

     Standard C	Library	(libc, -lc)

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

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

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

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

     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 speci-
     fied by the members of the	iov array: iov[0], iov[1], ...,	iov[iovcnt-1].
     Pread() performs the same function, but reads from	the specified position
     in	the file without modifying the file pointer.

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

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

     Each iovec	entry specifies	the base address and length of an area in mem-
     ory where data should be placed.  Readv() will always fill	an area	com-
     pletely 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

     Upon successful completion, read(), readv(), and pread() return the num-
     ber of bytes actually read	and placed in the buffer.  The system guaran-
     tees 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	successful, the	number of bytes	actually read is returned.  Upon read-
     ing end-of-file, zero is returned.	 Otherwise, a -1 is returned and the
     global variable errno is set to indicate the error.

     Read(), readv(), and pread() will succeed unless:

     [EBADF]		D is not a valid file or socket	descriptor open	for

     [EFAULT]		Buf points outside the allocated address space.

     [EIO]		An I/O error occurred while reading from the file sys-

     [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.

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

     [EOPNOTSUPP]	The file descriptor is associated with a filesystem
			and file type that do not allow	regular	read opera-
			tions on it.

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

     [EINVAL]		Iovcnt was less	than or	equal to 0, or greater than

     [EINVAL]		One of the iov_len values in the iov array was nega-

     [EINVAL]		The sum	of the iov_len values in the iov array over-
			flowed a 32-bit	integer.

     [EFAULT]		Part of	the iov	points outside the process's allocated
			address	space.

     The pread() call may also return the following errors:

     [EINVAL]		The specified file offset is invalid.

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

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

     The read()	function call is expected to conform to	ISO/IEC	9945-1:1990
     (``POSIX.1'').  The readv() and pread() functions are expected to conform
     to	X/Open Portability Guide Issue 4, Version 2 (``XPG4.2'').

     The pread() function call appeared	in AT&T	System V Release 4 UNIX.  The
     readv() function call appeared in 4.2BSD.	A read() function call
     appeared in Version 6 AT&T	UNIX.

FreeBSD	11.1		       February	26, 1994		  FreeBSD 11.1


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