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write, writev, pwrite, pwritev - write output



      write, writev, pwrite, pwritev - write output


      Standard C Library (libc, -lc)


      #include <sys/types.h>
      #include <sys/uio.h>
      #include <unistd.h>
      write(int d, const void *buf, size_t nbytes);
      pwrite(int d, const void *buf, size_t nbytes, off_t offset);
      writev(int d, const struct iovec *iov, int iovcnt);
      pwritev(int d, const struct iovec *iov, int iovcnt, off_t offset);


      The write() system call attempts to write nbytes of data to the object
      referenced by the descriptor d from the buffer pointed to by buf.  The
      writev() system call 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].  The pwrite() and pwritev() system
      calls perform the same functions, but write to the specified position in
      the file without modifying the file pointer.
      For writev() and pwritev(), the iovec structure is defined as:
            struct iovec {
                    void   *iov_base;  /* Base address. */
                    size_t iov_len;    /* Length. */
      Each iovec entry specifies the base address and length of an area in mem‐
      ory from which data should be written.  The writev() system call 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.
      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 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 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(), pwrite() and pwritev() system calls will fail and
      the file pointer will remain unchanged if:
      [EBADF]            The d argument is not a valid descriptor open for
      [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
                         process’s file size limit or the maximum file size.
      [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 d was negative.
      [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.
      [EIO]              An I/O error occurred while reading from or writing to
                         the file system.
      [EINTR]            A signal interrupted the write before it could be com‐
      [EAGAIN]           The file was marked for non-blocking I/O, and no data
                         could be written immediately.
      [EROFS]            An attempt was made to write over a disk label area at
                         the beginning of a slice.  Use disklabel(8) -W to
                         enable writing on the disk label area.
      [EINVAL]           The value nbytes is greater than INT_MAX.
      In addition, writev() and pwritev() may return one of the following
      [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]           The iovcnt argument was less than or equal to 0, or
                         greater than IOV_MAX.
      [EINVAL]           One of the iov_len values in the iov array was nega‐
      [EINVAL]           The sum of the iov_len values in the iov array over‐
                         flowed a 32-bit integer.
      [ENOBUFS]          The mbuf pool has been completely exhausted when writ‐
                         ing to a socket.
      The pwrite() and pwritev() system calls may also return the following
      [EINVAL]           The offset value was negative.
      [ESPIPE]           The file descriptor is associated with a pipe, socket,
                         or FIFO.
      fcntl(2), lseek(2), open(2), pipe(2), select(2)


      The write() system call is expected to conform to ISO/IEC 9945-1:1990
      (“POSIX.1”).  The writev() and pwrite() system calls are expected to con‐
      form to X/Open Portability Guide Issue 4.2 (“XPG4.2”).


      The pwritev() system call appeared in FreeBSD 6.0.  The pwrite() function
      appeared in AT&T System V.4 UNIX.  The writev() system call appeared in
      4.2BSD.  The write() function appeared in Version 6 AT&T UNIX.


Based on BSD UNIX
FreeBSD is an advanced operating system for x86 compatible (including Pentium and Athlon), amd64 compatible (including Opteron, Athlon64, and EM64T), UltraSPARC, IA-64, PC-98 and ARM architectures. It is derived from BSD, the version of UNIX developed at the University of California, Berkeley. It is developed and maintained by a large team of individuals. Additional platforms are in various stages of development.