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

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

SYNOPSIS
     #include <&lt;unistd.h>&gt;

     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 <&lt;sys/uio.h>&gt;

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

     #include <&lt;sys/types.h>&gt;
     #include <&lt;sys/uio.h>&gt;

     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 speci-
     fied 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 spec-
     ified position offset 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 mem-
     ory from which data should be written.  writev() and pwritev() 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.  If
     a file was opened with the O_APPEND flag (see open(2)), calls to write()
     or writev() will automatically set the pointer to the end of the file
     before writing.

     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 superuser, 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 superuser.

     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 oper-
     ation should be retried when possible.

     Note that writev() and pwritev() will fail if the value of iovcnt exceeds
     the constant IOV_MAX.

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

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

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

     [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.

     [EINTR]            A write to a slow device (i.e. one that might block
                        for an arbitrary amount of time) was interrupted by
                        the delivery of a signal before any data could be
                        written.

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

     [EFAULT]           Part of buf points outside the process's allocated
                        address space.

     In addition, write() and writev() may return the following errors:

     [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.

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

     [ENETDOWN]         The destination address specified a network that is
                        down.

     [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.

     [EIO]              The process is a member of a background process
                        attempting to write to its controlling terminal,
                        TOSTOP is set on the terminal, the process isn't
                        ignoring the SIGTTOUT signal and the thread isn't
                        blocking the SIGTTOUT signal, and either the process
                        was created with vfork(2) and hasn't successfully exe-
                        cuted one of the exec functions or the process group
                        is orphaned.

     write() and pwrite() may return the following error:

     [EINVAL]           nbytes was larger than SSIZE_MAX.

     pwrite() and pwritev() may return the following error:

     [EINVAL]           offset was negative.

     [ESPIPE]           d is associated with a pipe, socket, FIFO, or tty.

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

     [EINVAL]           iovcnt was less than or equal to 0, or greater than
                        IOV_MAX.

     [EINVAL]           The sum of the iov_len values in the iov array over-
                        flowed an ssize_t.

     [EFAULT]           Part of iov points outside the process's allocated
                        address space.

     [ENOBUFS]          The system lacked sufficient buffer space or a queue
                        was full.

SEE ALSO
     fcntl(2), lseek(2), open(2), pipe(2), poll(2), select(2), termios(4)

STANDARDS
     The write(), writev(), and pwrite() functions conform to IEEE Std
     1003.1-2008 (``POSIX.1'').

HISTORY
     The pwritev() function call appeared in OpenBSD 2.7.  The pwrite() func-
     tion call appeared in AT&T System V Release 4 UNIX.  The writev() func-
     tion 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 negative number distinct from -1.
     Proper loops should use

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

BSD                            February 5, 2015                            BSD