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 write(2)							    write(2)




 NAME
      write, writev, pwrite - write on a file

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

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

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

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

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

 DESCRIPTION
      The write() function attempts to write nbyte bytes from the buffer
      pointed to by buf to the file associated with the open file
      descriptor, fildes.

      If nbyte is 0, write() will return 0 and have no other results if the
      file is a regular file; otherwise, the results are unspecified.

      On a regular file or other file capable of seeking, the actual writing
      of data proceeds from the position in the file indicated by the file
      offset associated with fildes.  Before successful return from write(),
      the file offset is incremented by	 the number of bytes actually
      written. On a regular file, if this incremented file offset is greater
      than the length of the file, the length of the file will be set to
      this file offset.	 If the O_SYNC flag of the file status flags is set
      and fildes refers to a regular file, a successful write() does not
      return until the data is delivered to the underlying hardware.  On a
      file not capable of seeking, writing always takes place starting at
      the current position. The value of a file offset associated with such
      a device is undefined.

      If the O_APPEND flag of the file status flags is set, the file offset
      will be set to the end of the file prior to each write and no
      intervening file modification operation will occur between changing
      the file offset and the write operation.

      If a write() requests that more bytes be written than there is room
      for (for example, the ulimit or the physical end of a medium), only as
      many bytes as there is room for will be written. For example, suppose
      there is space for 20 bytes more in a file before reaching a limit. A
      write of 512 bytes will return 20. The next write of a non-zero number
      of bytes will give a failure return (except as noted below) and the
      implementation will generate a SIGXFSZ signal for the process.

      If write() is interrupted by a signal before it writes any data, it
      will return -1 with errno set to EINTR.




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 write(2)							    write(2)




      If write() is interrupted by a signal after it successfully writes
      some data, it will return the number of bytes written.

      If the value of nbyte is greater than {SSIZE_MAX}, the result is
      implementation-dependent.

      After a write() to a regular file has successfully returned:

	   +  Any successful read() from each byte position in the file that
	      was modified by that write will return the data specified by
	      the write() for that position until such byte positions are
	      again modified.

	   +  Any subsequent successful write() to the same byte position in
	      the file will overwrite that file data.

      Write requests to a pipe or FIFO will be handled the same as a regular
      file with the following exceptions:

	   +  There is no file offset associated with a pipe, hence each
	      write request will append to the end of the pipe.

	   +  Write requests of {PIPE_BUF} bytes or less will not be
	      interleaved with data from other processes doing writes on the
	      same pipe.  Writes of greater than {PIPE_BUF} bytes may have
	      data interleaved, on arbitrary boundaries, with writes by
	      other processes, whether or not the O_NONBLOCK flag of the
	      file status flags is set.

	   +  If the O_NONBLOCK flag is clear, a write request may cause the
	      process to block, but on normal completion it will return
	      nbyte.

	   +  If the O_NONBLOCK flag is set, write() requests will  be
	      handled differently, in the following ways:

	      -	 The write() function will not block the process.

	      -	 A write request for {PIPE_BUF} or fewer bytes	will have
		 the following effect:	If there is sufficient space
		 available in the pipe, write() will transfer all the data
		 and return the number of bytes	 requested.  Otherwise,
		 write() will transfer no data and return -1 with errno set
		 to EAGAIN.

	      -	 A write request for more than {PIPE_BUF} bytes will case
		 one of the following:

		 a. When at least one byte can be written, transfer what it
		    can and return the number of bytes	written. When all
		    data previously written to the pipe is read, it will



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 write(2)							    write(2)




		    transfer at least {PIPE_BUF} bytes.

		 b. When no data can be written, transfer no data and return
		    -1 with errno set to EAGAIN.

      When attempting to write to a file descriptor (other than a pipe or
      FIFO) that supports non-blocking writes and cannot accept the data
      immediately:

	   +  If the O_NONBLOCK flag is clear, write() will block until the
	      data can be accepted.

	   +  If the O_NONBLOCK flag is set, write() will not block the
	      process. If some data can be written without blocking the
	      process, write() will write what it can and return the number
	      of bytes written. Otherwise, it will return -1 and errno will
	      be set to EAGAIN.

      Upon successful completion, where nbyte is greater than 0, write()
      will mark for update the st_ctime and st_mtime fields of the file, and
      if the file is a regular file, the S_ISUID and S_ISGID bits of the
      file mode may be cleared.

      If fildes refers to a STREAM, the operation of write() is determined
      by the values of the minimum and maximum nbyte range ("packet size")
      accepted by the STREAM. These values are determined by the topmost
      STREAM module. If nbyte falls within the packet size range, nbyte
      bytes will be written. If nbyte does not fall within the range and the
      minimum packet size value is 0, write() will break the buffer into
      maximum packet size segments prior to sending the data downstream (the
      last segment may contain less than the maximum packet size). If nbyte
      does not fall within the range and the minimum value is non-zero,
      write() will fail with errno set to ERANGE.  Writing a zero-length
      buffer ( nbyte is 0) to a STREAMS device sends 0 bytes with 0
      returned. However, writing a zero-length buffer to a STREAMS-based
      pipe or FIFO sends no message and 0 is returned. The process may issue
      I_SWROPT ioctl() to enable zero-length messages to be sent across the
      pipe or FIFO.

      When writing to a STREAM, data messages are  created with a priority
      band of 0.  When writing to a STREAM that is not a pipe or FIFO:

	   +  If O_NONBLOCK is clear, and the STREAM cannot accept data (the
	      STREAM write queue is full due to internal flow control
	      conditions), write() will block until data can be accepted.

	   +  If O_NONBLOCK is set and the STREAM cannot accept data,
	      write() will return -1 and set errno to [EAGAIN].

	   +  If O_NONBLOCK is set and part of the buffer has been written
	      while a condition in which the STREAM cannot accept additional



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 write(2)							    write(2)




	      data occurs, write() will terminate and return the number of
	      bytes written.

      In addition, write() and writev() will fail if the STREAM head had
      processed an asynchronous error before the call. In this case, the
      value of errno does not reflect the result of write() or writev() but
      reflects the prior error.

      The writev() function is equivalent to write(), but gathers the output
      data from the iovcnt buffers specified by the members of the iov
      array: iov[0], iov[1], ..., iov[iovcnt-1].  iovcnt is valid if greater
      than 0 and less than or equal to {IOV_MAX}, defined in <&lt&lt&lt;limits.h>&gt&gt&gt;.

      Each iovec entry specifies the base address and length of an area in
      memory from which data should be written. The writev() function will
      always write a complete area before proceeding to the next.

      If fildes refers to a regular file and all of the iov_len members in
      the array pointed to by iov are 0, writev() will return 0 and have no
      other effect.  For other file types, the behaviour is unspecified.

      If the sum of the iov_len values is greater than SSIZE_MAX, the
      operation fails and no data is transferred.

      The pwrite() function performs the same action as write(), except that
      it writes into a given position without changing the file pointer.
      The first three arguments to pwrite() are the same as write() with the
      addition of a fourth argument offset for the desired position inside
      the file.

 RETURN VALUE
      Upon successful completion, write() and pwrite() will return the
      number of bytes actually written to the file associated with fildes.
      This number will never be greater than nbyte.  Otherwise, -1 is
      returned and errno is set to indicate the error.

      Upon successful  completion, writev() returns the number of bytes
      actually written. Otherwise, it returns a value of -1, the file-
      pointer remains unchanged, and errno is set to indicate an error.

 ERRORS
      The write(), pwrite() and writev() functions will fail if:

	   [EAGAIN]	  The O_NONBLOCK flag is set for the file descriptor
			  and the process would be delayed in the write()
			  operation.

	   [EBADF]	  The fildes argument is not a valid file descriptor
			  open for writing.





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 write(2)							    write(2)




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

	   [EINTR]	  The write operation was terminated due to the
			  receipt of a signal, and no data was transferred.

	   [EIO]	  A physical I/O error has occurred.

	   [EIO]	  The process is a member of a background process
			  group attempting to write to its controlling
			  terminal, TOSTOP is set, the process is neither
			  ignoring nor blocking SIGTTOU and the process
			  group of the process is orphaned. This error may
			  also be returned under implementation-dependent
			  conditions.

	   [ENOSPC]	  There was no free space remaining on the device
			  containing the file.

	   [EPIPE]	  An attempt is made to write to a pipe or FIFO that
			  is not open for reading by any process, or that
			  only has one end open. A SIGPIPE signal will also
			  be sent to the process.

	   [ERANGE]	  The transfer request size was outside the range
			  supported by the STREAMS file associated with
			  fildes.

      The writev() function will fail if:

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

      The write(), pwrite() and writev() functions may fail if:

	   [EINVAL]	  The STREAM or multiplexer referenced by fildes is
			  linked (directly or indirectly) downstream from a
			  multiplexer.

	   [ENXIO]	  A request was made of a non-existent device, or
			  the request was outside the capabilities of the
			  device.

	   [ENXIO]	  A  hangup  occurred  on  the STREAM being written
			  to.

      A write to a STREAMS file may fail if an error message has been
      received at the STREAM head. In this case, errno is set to the value
      included in the error message.




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 write(2)							    write(2)




      The writev() function may fail and set errno to:

	   [EINVAL]	  The iovcnt argument was less than or equal to 0,
			  or greater than {IOV_MAX}.

      The pwrite() function fails and the file pointer remains unchanged if:

	   [EINVAL]	  The offset argument is invalid, The value is
			  negative.  [ESPIPE] fildes is associated with a
			  pipe or FIFO.

 SEE ALSO
      chmod(2), creat(2), dup(2), fcntl(2), getrlimit(2), lseek(2), open(2),
      pipe(2), ulimit(2), <limits.h>, <stropts.h>, <sys/uio.h>, <unistd.h>.

 CHANGE HISTORY
      First released in Issue 1.

      Derived from Issue 1 of the SVID.

 Issue 4
      The following changes are incorporated for alignment with the ISO
      POSIX-1 standard:

	   +  The type of the argument buf is changed from char * to const
	      void*, and the type of the argument byte is changed from
	      unsigned size_t.

	   +  The DESCRIPTION section is changed:

	      -	 to indicate that writing at end-of-file is atomic

	      -	 to identify that {SSIZE_MAX} is now used to determine the
		 maximum value of nbyte

	      -	 to indicate the consequences of activities after a call to
		 the write() function

	      -	 To improve clarity, the text describing operations on pipes
		 or FIFOs when O_NONBLOCK is set is restructured.

      Other changes are incorporated as follows:

	   +  The header <&lt&lt&lt;unistd.h>&gt&gt&gt; is added to the SYNOPSIS section.

	   +  Reference to ulimit in the DESCRIPTION section is marked as an
	      extension.

	   +  Reference to the process' file size limit and the ulimit()
	      function are marked as extensions in the description of the
	      EFBIG error.



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 write(2)							    write(2)




	   +  The ENXIO error is marked as an extension.

	   +  The APPLICATION USAGE section is removed.

	   +  The description of EINTR is amended.

 Issue 4, Version 2
      The following changes are incorporated for X/OPEN UNIX conformance:

	   +  The writev() function is added to the SYNOPSIS.

	   +  The DESCRIPTION is updated to describe the reading of data
	      from STREAMS files, an operational description of the writev()
	      function is included, and a statement is added indicating that
	      SIGXFSZ will be generated if an attempted write operation
	      would cause the maximum file size to be exceeded.

	   +  The RETURN VALUE section is updated to describe values
	      returned by the writev() function.

	   +  The ERRORS section has been restructured to describe errors
	      that apply to both write() and writev() apart from those that
	      apply to writev() specifically. The EIO, ERANGE, and EINVAL
	      errors are also added.






























				    - 7 -	  Formatted:  August 2, 2006






 write(2)							    write(2)




				 HP-UX EXTENSIONS



 DESCRIPTION
      The iovec structure is defined in /usr/include/sys/uio.h.

      For ordinary files, if the O_DSYNC file status flag is set, the write
      does not return until both the file data and the file attributes
      required to retrieve the data are physically updated.  If the O_SYNC
      flag is set, the behavior is identical to that for O_DSYNC, with the
      addition that all file attributes changed by the write operation
      (including access time, modification time and status change time) are
      also physically updated prior to returning to the calling process.

      For block special files, if the O_DSYNC or the O_SYNC flag is set, the
      write does not return until the data is physically updated.  How the
      data reaches the physical media is implementation- and hardware-
      dependent.

      A write to an ordinary file is prevented if enforcement-mode file and
      record locking is set, and another process owns a lock on the segment
      of the file being written:

	   If O_NDELAY or O_NONBLOCK is set, the write returns -1 and sets
	   errno to EAGAIN.

	   If O_NDELAY and O_NONBLOCK are clear, the write does not complete
	   until the blocking record lock is removed.

      If the file being written is a pipe (or FIFO), the system-dependent
      maximum number of bytes that it can store is given by PIPSIZ (defined
      in <&lt&lt&lt;sys/inode.h>&gt&gt&gt;).  The minimum value of PIPSIZ on any HP-UX system is
      8192.  When writing a pipe, the following conditions apply:

	   If the O_NDELAY or O_NONBLOCK file status flag is set:

		If nbyte is less than or equal to PIPSIZ and sufficient room
		exists in the pipe or FIFO, the write() succeeds and returns
		the number of bytes written;

		If nbyte is less than or equal to PIPSIZ but insufficient
		room exists in the pipe or FIFO, the write() returns having
		written nothing.  If O_NONBLOCK is set, -1 is returned and
		errno is set to [EAGAIN].  If O_NDELAY is set, 0 is
		returned.

		If nbyte is greater than PIPSIZ and the pipe or FIFO is
		full, the write returns having written nothing.	 If
		O_NONBLOCK is set, -1 is returned and errno is set to
		[EAGAIN].  If O_NDELAY is set, 0 is returned.



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 write(2)							    write(2)




		If nbyte is greater than PIPSIZ, and some room exists in the
		pipe or FIFO, as much data as fits in the pipe or FIFO is
		written, and write() returns the number of bytes actually
		written, an amount less than the number of bytes requested.

	   If the O_NDELAY and O_NONBLOCK file status flags are clear:

		The write() always executes correctly (blocking as
		necessary), and returns the number of bytes written.

      For character special devices, if the stopio() call was used on the
      same device after it was opened, write() returns -1, sets errno to
      [EBADF], and issues the SIGHUP signal to the process.

      write() clears the potential and granted privilege vectors on the
      file.

      If the write is performed by any user other than the owner or a user
      who has appropriate privileges, write() clears the set-user-ID, set-
      group-ID, and sticky bits on all nondirectory files.  If the write is
      performed by the owner or a user who has appropriate privileges, the
      behavior is file-system dependent.  In some file systems, the write
      clears the set-user-ID, set-group-ID, and sticky bits on a
      nondirectory file.  In other file systems, the write does not clear
      these bits on a nondirectory file.

      For directories, write() does not clear the set-user-ID, set-group-ID,
      and sticky bits.

 ERRORS
      If write() or writev() fails, the file offset remains unchanged and
      errno is set to one of the following values.

	   [EAGAIN]	  Enforcement-mode file and record locking was set,
			  O_NDELAY was set, and there was a blocking record
			  lock.

	   [EDEADLK]	  A resource deadlock would occur as a result of
			  this operation (see lockf(2) and fcntl(2)).

	   [EDQUOT]	  User's disk quota block limit has been reached for
			  this file system.

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

	   [ENOLCK]	  The system record lock table is full, preventing
			  the write from sleeping until the blocking record
			  lock is removed.



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 write(2)							    write(2)




	   [ENOSPC]	  Not enough space on the file system.	The process
			  does not possess the limit effective privilege to
			  override this restriction.

      If writev() fails, the file offset remains unchanged and errno is set
      to one of the following values:

	   [EFAULT]	  iov_base or iov points outside of the allocated
			  address space.  The reliable detection of this
			  error is implementation dependent.

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

      If write() or writev() fails, the file offset is updated to reflect
      the amount of data transferred and errno is set to one of the
      following values.

	   [EFAULT]	  buf points outside the process's allocated address
			  space.  The reliable detection of this error is
			  implementation dependent.

 EXAMPLES
      Assuming a process opened a file for writing, the following call to
      write() attempts to write mybufsize bytes to the file from the buffer
      to which mybuf points.

	   #include <&lt&lt&lt;string.h>&gt&gt&gt;

	   int fildes;
	   size_t mybufsize;
	   ssize_t nbytes;
	   char *mybuf = "aeiou and sometimes y";
	   mybufsize = (size_t)strlen (mybuf);
	   nbytes = write (fildes, (void *)mybuf, mybufsize);

 WARNINGS
      Check signal(5) for the appropriateness of signal references on
      systems that support sigvector() (see sigvector(2)).  sigvector() can
      affect the behavior described on this page.

      Character special devices, and raw disks in particular, apply
      constraints on how write() can be used.  See specific Section 7 manual
      entries for details on particular devices.

 AUTHOR
      write() was developed by HP, AT&T, and the University of California,
      Berkeley.

 SEE ALSO
      mkfs(1M) creat(2), dup(2), fcntl(2), lockf(2), lseek(2), open(2),



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 write(2)							    write(2)




      pipe(2), sigvector(2), ulimit(2), ustat(2), signal(5).

 STANDARDS CONFORMANCE
      write(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1,
      POSIX.4

















































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