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NetBSD 6.1.5 - man page for pwritev (netbsd section 2)

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

NAME
     write, writev, pwrite, pwritev -- write output

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <unistd.h>

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

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

     #include <sys/uio.h>

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

     ssize_t
     pwritev(int d, const struct iovec *iov, int iovcnt, off_t offset);

DESCRIPTION
     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 specified by the members of the iov array: iov[0], iov[1], ...,
     iov[iovcnt-1].  pwrite() and pwritev() perform the same functions, but write to the speci-
     fied position in the file without modifying the file pointer.

     For writev() and pwritev(), 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 from which data
     should be written.  writev() and pwritev() will always write a complete area before proceed-
     ing to the next.

     On objects capable of seeking, the write() starts at a position given by the pointer associ-
     ated 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 undefined.

     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.

     If write() succeeds it will update the st_ctime and st_mtime fields of the file's meta-data
     (see stat(2)).

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

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

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

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

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

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

     [EINTR]		A signal was received before any data could be written to a slow device.
			See sigaction(2) for more information on the interaction between signals
			and system calls.

     [EINVAL]		The pointer associated with d was negative; or the total length of the
			I/O is more than can be expressed by the ssize_t return value.

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

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

     [EPIPE]		An attempt is made to write to a pipe that is not open for reading by any
			process; or an attempt is made to write to a socket of type SOCK_STREAM
			that is not connected to a peer socket.

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

     [EINVAL]		iovcnt was less than or equal to 0, or greater than {IOV_MAX}; or one of
			the iov_len values in the iov array was negative; or the sum of the
			iov_len values in the iov array overflowed a 32-bit integer.

     The pwrite() and pwritev() calls 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
     fcntl(2), lseek(2), open(2), pipe(2), poll(2), select(2), sigaction(2)

STANDARDS
     The write() function is expected to conform to IEEE Std 1003.1-1988 (``POSIX.1'').  The
     writev() and pwrite() functions conform to X/Open Portability Guide Issue 4, Version 2
     (``XPG4.2'').

HISTORY
     The pwritev() function call appeared in NetBSD 1.4.  The pwrite() function call appeared in
     AT&T System V Release 4 UNIX.  The writev() function call appeared in 4.2BSD.  The write()
     function call appeared in Version 2 AT&T UNIX.

CAVEATS
     Error checks should explicitly test for -1.  Code such as

	     while ((nr = write(fd, buf, sizeof(buf))) > 0)

     is not maximally portable, as some platforms allow for nbytes to range between SSIZE_MAX and
     SIZE_MAX - 2, in which case the return value of an error-free write() may appear as a nega-
     tive number distinct from -1.  Proper loops should use

	     while ((nr = write(fd, buf, sizeof(buf))) != -1 && nr != 0)

BSD					  April 3, 2010 				      BSD
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