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WRITE(2)		    BSD	System Calls Manual		      WRITE(2)

     write, writev, pwrite -- write output

     Standard C	Library	(libc, -lc)

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

     write(int d, const	void *buf, size_t nbytes);

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

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

     Write() attempts to write nbytes of data to the object referenced by the
     descriptor	d from the buffer pointed to by	buf.  Writev() performs	the
     same action, but gathers the output data from the iovcnt buffers speci-
     fied by the members of the	iov array: iov[0], iov[1], ...,	iov[iovcnt-1].
     Pwrite() performs the same	function, but writes to	the specified position
     in	the file without modifying the file pointer.

     For writev(), 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 from which data should	be written.  Writev() will always write	a com-
     plete area	before proceeding to the next.

     On	objects	capable	of seeking, the	write()	starts at a position given by
     the pointer associated with d, see	lseek(2).  Upon	return from write(),
     the pointer is incremented	by the number of bytes which were written.

     Objects that are not capable of seeking always write from the current po-
     sition.  The value	of the pointer associated with such an object is unde-

     If	the real user is not the super-user, then write() clears the set-user-
     id	bit on a file.	This prevents penetration of system security by	a user
     who "captures" a writable set-user-id file	owned by the super-user.

     When using	non-blocking I/O on objects such as sockets that are subject
     to	flow control, write() and writev() may write fewer bytes than re-
     quested; the return value must be noted, and the remainder	of the opera-
     tion should be retried when possible.

     Upon successful completion	the number of bytes which were written is re-
     turned.  Otherwise	a -1 is	returned and the global	variable errno is set
     to	indicate the error.

     Write(), writev(),	and pwrite() will fail and the file pointer will re-
     main unchanged if:

     [EBADF]		D is not a valid descriptor open for writing.

     [EPIPE]		An attempt is made to write to a pipe that is not open
			for reading by any process.

     [EPIPE]		An attempt is made to write to a socket	of type
			SOCK_STREAM that is not	connected to a peer socket.

     [EFBIG]		An attempt was made to write a file that exceeds the
			process's file size limit or the maximum file size.

     [EFAULT]		Part of	iov or data to be written to the file points
			outside	the process's allocated	address	space.

     [EINVAL]		The pointer associated with d was negative.

     [ENOSPC]		There is no free space remaining on the	file system
			containing the file.

     [EDQUOT]		The user's quota of disk blocks	on the file system
			containing the file has	been exhausted.

     [EIO]		An I/O error occurred while reading from or writing to
			the file system.

     [EINTR]		A signal interrupted the write before it could be com-

     [EAGAIN]		The file was marked for	non-blocking I/O, and no data
			could be written immediately.

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

     [EDESTADDRREQ]	The destination	is no longer available when writing to
			a UNIX domain datagram socket on which connect(2) had
			been used to set a destination address.

     [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.

     [ENOBUFS]		The mbuf pool has been completely exhausted when writ-
			ing to a socket.

     The pwrite() 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.

     fcntl(2), lseek(2), open(2), pipe(2), select(2)

     The write() function call is expected to conform to ISO/IEC 9945-1:1990
     ("POSIX.1").  The writev()	and pwrite() functions are expected to conform
     to	X/Open Portability Guide Issue 4, Version 2 ("XPG4.2").

     The pwrite() function call	appeared in AT&T System	V Release 4 UNIX.  The
     writev() function call appeared in	4.2BSD.	 A write() function call ap-
     peared in Version 6 AT&T UNIX.

BSD				 April 2, 1994				   BSD


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