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

NAME
     pwrite, write, writev -- write output

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <unistd.h>

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

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

     #include <sys/uio.h>

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

DESCRIPTION
     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 memory from which data should be
     written.  Writev() 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
     fildes, 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 pre-vents prevents
     vents 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
     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
     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 writing.

     [ECONNRESET]       A write is attempted on a socket that is not 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 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 capabili-ties capabilities
                        ties 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_NONBLOCK, and write would block.
                        The exact error code depends on the protocol, but EWOULDBLOCK is more 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 UIO_MAXIOV.

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

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

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

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

     These include files are needed for all three functions.

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

STANDARDS
     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'').

HISTORY
     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 appeared in Version 6 AT&T UNIX.

BSD                              April 2, 1994                             BSD

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