WRITE(2)                  OpenBSD Programmer's Manual                 WRITE(2)

NAME
     write, writev - write output



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

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

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

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].

     For writev(), 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() will always write a com-
     plete 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 po-
     sition.  The value of the pointer associated with such an object is unde-
     fined.

     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 request-
     ed; the return value must be noted, and the remainder of the operation
     should be retried when possible.

     Note that writev() will fail if the value of iovcnt exceedes the constant
     IOV_MAX.

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

ERRORS
     write() and writev() will fail and the file pointer will remain unchanged


     if:

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

     [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 pro-
                   cess's file size limit or the maximum file size.

     [EFAULT]      Part of iov or data to be written to the file points out-
                   side 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 con-
                   taining the file.

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

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

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

     In addition, write() may return the following error:

     [EINVAL]      nbytes was larger than SSIZE_MAX.

     Also, 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 was less than or equal to 0, or greater than
                   IOV_MAX.

     [EINVAL]      The sum of the iov_len values in the iov array overflowed
                   an ssize_t.

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

STANDARDS
     The write() function is expected to conform to IEEE Std1003.1-1988
     (``POSIX'').

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)

HISTORY
     The writev() function call appeared in 4.2BSD. The write() function call
     appeared in Version 6 AT&T UNIX.

OpenBSD 2.6                      July 28, 1998                               3