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

     pwrite, write, writev -- write output

     Standard C Library (libc, -lc)

     #include <unistd.h>

     pwrite(int fildes, const void *buf, size_t nbyte, off_t offset);

     write(int fildes, const void *buf, size_t nbyte);

     #include <sys/uio.h>

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

     Write() attempts to write nbyte of data to the object referenced by the
     descriptor fildes 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() 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 memory
     ory from which data should be written.  Writev() will always write a com-plete complete
     plete area before proceeding to the next.

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

     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-user-id set-userid
     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
     requested; the return value must be noted, and the remainder of the oper-ation operation
     ation should be retried when possible.

     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.

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

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

     [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 fildes is negative.

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

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

     [EBADF]            fildes is not a valid file descriptor open for writ-ing. writing.

     [ECONNRESET]       A write is attempted on a socket that is not con-nected. connected.

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

     [EFBIG]            The file is a regular file, nbyte is greater than 0,
                        and the starting position is greater than or equal to
                        the offset maximum established in the open file
                        description associated with fildes.

     [EINTR]            A signal interrupts the write before it could be com-pleted. completed.

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

     [ENETDOWN]         A write is attempted on a socket and the local network
                        interface used to reach the destination is down.

     [ENETUNREACH]      A write is attempted on a socket and no route to the
                        network is present.

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

     [ENXIO]            A request is made of a nonexistent device, or the
                        request is outside the capabilities of the device.

     [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.

     The write() and writev() calls may also return the following errors:

     [EAGAIN]           See EWOULDBLOCK, below.

     [EWOULDBLOCK]      The file descriptor is for a socket, is marked O_NON-BLOCK, O_NONBLOCK,
                        BLOCK, and write would block.  The exact error code
                        depends on the protocol, but EWOULDBLOCK is more com-mon. common.

     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 is less than or equal to 0, or greater than

     [EINVAL]           One of the iov_len values in the iov array is nega-tive. negative.

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

     [ENOBUFS]          The mbuf pool has been completely exhausted when writ-ing writing
                        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.

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

     These include files are needed for all three functions.

     fcntl(2), lseek(2), open(2), pipe(2), select(2), compat(5)

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

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

BSD                              April 2, 1994                             BSD