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NAME    [Toc]    [Back]

     write, writev, pwrite -- write output

LIBRARY    [Toc]    [Back]

     Standard C Library (libc, -lc)

SYNOPSIS    [Toc]    [Back]

     #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);

DESCRIPTION    [Toc]    [Back]

     The write() system call attempts to write nbytes of data to the object
     referenced by the descriptor d from the buffer pointed to by buf.	The
     writev() system call performs the same action, but gathers the output
     data from the iovcnt buffers specified by the members of the iov array:
     iov[0], iov[1], ..., iov[iovcnt-1].  The pwrite() system call 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 {
		   void   *iov_base;  /* Base address. */
		   size_t iov_len;    /* Length. */

     Each iovec entry specifies the base address and length of an area in memory
 from which data should be written.  The writev() system call will
     always write a complete 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
     position.	The value of the pointer associated with such an object is

     If the real user is not the super-user, then write() clears the set-userid
 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
     requested; the return value must be noted, and the remainder of the operation
 should be retried when possible.

RETURN VALUES    [Toc]    [Back]

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

ERRORS    [Toc]    [Back]

     The write(), writev(), and pwrite() system calls will fail and the file
     pointer will remain unchanged if:

     [EBADF]		The d argument is not a valid descriptor open for

     [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 completed.

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

     [EROFS]		An attempt was made to write over a disk label area at
			the beginning of a slice.  Use disklabel(8) -W to
			enable writing on the disk label area.

     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]		The iovcnt argument was less than or equal to 0, or
			greater than IOV_MAX.

     [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.

     [ENOBUFS]		The mbuf pool has been completely exhausted when writing
 to a socket.

     The pwrite() system 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.

SEE ALSO    [Toc]    [Back]

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

STANDARDS    [Toc]    [Back]

     The write() system call is expected to conform to ISO/IEC 9945-1:1990
     (``POSIX.1'').  The writev() and pwrite() system calls are expected to
     conform to X/Open Portability Guide Issue 4.2 (``XPG4.2'').

HISTORY    [Toc]    [Back]

     The pwrite() function appeared in AT&T System V.4 UNIX.  The writev()
     system call appeared in 4.2BSD.  The write() function appeared in
     Version 6 AT&T UNIX.

FreeBSD 5.2.1			 April 2, 1994			 FreeBSD 5.2.1
[ Back ]
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