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OpenSolaris 2009.06 - man page for write (opensolaris section 2)

write(2)				   System Calls 				 write(2)

NAME
       write, pwrite, writev - write on a file

SYNOPSIS
       #include <unistd.h>

       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 <sys/uio.h>

       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	bit has been set, write I/O operations on the file descriptor complete as
       defined by synchronized I/O file integrity completion.

       If fildes refers to a socket, write() is equivalent to send(3SOCKET) with no flags set.

       On a file not capable of seeking, writing always takes place starting at the current posi-
       tion.  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.

       For  regular files, no data transfer will occur past the offset maximum established in the
       open file description with fildes.

       A write() to a regular file is blocked  if  mandatory  file/record  locking  is	set  (see
       chmod(2)), and there is a record lock owned by another process  on the segment of the file
       to be written:

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

	   o	  If O_NDELAY and O_NONBLOCK are clear, write() sleeps until all  blocking  locks
		  are removed or the write() is terminated by a signal.

       If  a  write() requests that more bytes be written than there is room for--for example, if
       the write would exceed the process file size limit (see getrlimit(2) and  ulimit(2)),  the
       system  file  size  limit, or the free space on the device--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 returns 20. The next write() of a non-zero
       number of bytes gives a failure return (except as noted for pipes and FIFO below).

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

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

       If write() exceeds the process file size limit, the application generates a  SIGXFSZ  sig-
       nal, whose default behavior is to dump core.

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

	   o	  Any successful read(2) 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.

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

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

	   o	  There  is  no  file  offset  associated  with  a pipe, hence each write request
		  appends to the end of the pipe.

	   o	  Write requests of {PIPE_BUF} bytes or less are guaranteed not to 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 or O_NDELAY flags are
		  set.

	   o	  If O_NONBLOCK and O_NDELAY are clear, a write request may cause the process  to
		  block, but on normal completion it returns nbyte.

	   o	  If  O_NONBLOCK  and  O_NDELAY are set, write() does not block the process. If a
		  write() request for  PIPE_BUF  or  fewer  bytes  succeeds  completely   write()
		  returns nbyte. Otherwise, if O_NONBLOCK is set, it returns -1 and sets errno to
		  EAGAIN or if O_NDELAY is set, it returns 0.  A write() request for greater than
		  {PIPE_BUF} bytes  transfers what it can and returns the number of bytes written
		  or it transfers no data and, if O_NONBLOCK is set, returns -1 with errno set to
		  EAGAIN  or  if O_NDELAY is set, it returns 0.  Finally, if a request is greater
		  than PIPE_BUF bytes and all data previously written to the pipe has been  read,
		  write() transfers at least PIPE_BUF bytes.

       When  attempting  to write to a file descriptor (other than a pipe, a FIFO, a socket, or a
       STREAM) that supports nonblocking writes and cannot accept the data immediately:

	   o	  If O_NONBLOCK and O_NDELAY are clear, write() blocks	until  the  data  can  be
		  accepted.

	   o	  If  O_NONBLOCK  or O_NDELAY is set, write() does not block the process. If some
		  data can be written without blocking the process, write() writes  what  it  can
		  and  returns	the  number of bytes written. Otherwise, if O_NONBLOCK is set, it
		  returns -1  and sets errno to EAGAIN or if O_NDELAY is set,  it returns 0.

       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.

       For STREAMS files (see Intro(2) and streamio(7I)), 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 contained in the topmost STREAM module, and can  not  be  set  or
       tested from user level. If nbyte falls within the packet size range, nbyte bytes are writ-
       ten. If nbyte does not fall within the range and the minimum packet size value	is  zero,
       write()	breaks	the  buffer  into maximum packet size segments prior  to sending the data
       downstream (the last segment may be smaller than the maximum  packet size). If nbyte  does
       not  fall within the range and the minimum value is non-zero, write() fails and sets errno
       to ERANGE. Writing a zero-length buffer (nbyte is zero) to a STREAMS device sends  a  zero
       length message with zero returned. However, writing a zero-length buffer to a pipe or FIFO
       sends no message  and zero is returned. The user program may issue the  I_SWROPT  ioctl(2)
       to enable  zero-length messages to be sent across the pipe or FIFO (see streamio(7I)).

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

	   o	  If O_NDELAY and O_NONBLOCK are not set, and the STREAM cannot accept data  (the
		  STREAM  write  queue	is full due to internal flow control conditions), write()
		  blocks until data can be accepted.

	   o	  If O_NDELAY or O_NONBLOCK is set and the STREAM  cannot  accept  data,  write()
		  returns -1 and sets errno to EAGAIN.

	   o	  If  O_NDELAY or O_NONBLOCK is set and part of the buffer has already been writ-
		  ten when a condition occurs in which the STREAM cannot accept additional  data,
		  write() terminates and returns the number of bytes written.

       The write() and writev() functions will fail if the STREAM head had processed an asynchro-
       nous 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.

   pwrite()
       The  pwrite()  function is equivalent to write(), except that it writes into a given posi-
       tion and does not change the file offset (regardless of	whether  O_APPEND  is  set).  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.

   writev()
       The writev() function performs the same action as 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]. The iovcnt buffer is valid if greater than 0 and	less  than  or	equal  to
       {IOV_MAX}. See Intro(2) for a definition of {IOV_MAX}.

       The iovec structure contains the following members:

	 caddr_t  iov_base;
	 int	  iov_len;

       Each  iovec  entry  specifies  the base address and length of an area in memory from which
       data should be written.	The writev() function always writes all data from an 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
       behavior is unspecified.

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

RETURN VALUES
       Upon successful completion, write() returns the number of bytes actually  written  to  the
       file  associated  with  fildes.	This number is never greater than nbyte. Otherwise, -1 is
       returned, the file-pointer remains unchanged, and errno is set to indicate the error.

       Upon successful completion, writev() returns the number of bytes actually written.  Other-
       wise,  it returns  -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	  Mandatory file/record locking is set, O_NDELAY or O_NONBLOCK is set, and  there
		  is a blocking record lock; an attempt is made to write to a STREAM that can not
		  accept data with the O_NDELAY or O_NONBLOCK flag set; or a write to a  pipe  or
		  FIFO	of PIPE_BUF bytes or less is requested and less than nbytes of free space
		  is available.

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

       EDEADLK	  The write was going to go to sleep  and cause a deadlock situation to occur.

       EDQUOT	  The user's quota of disk blocks on the file system containing the file has been
		  exhausted.

       EFBIG	  An  attempt  is made to write a file that exceeds the process's file size limit
		  or the maximum file size (see getrlimit(2) and ulimit(2)).

       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 file descrip-
		  tion associated with fildes.

       EINTR	  A signal was caught during the write	operation and no data was transferred.

       EIO	  The process is in the background and is attempting to write to its  controlling
		  terminal  whose  TOSTOP  flag  is  set, or the process is  neither ignoring nor
		  blocking SIGTTOU signals and the process  group of the process is orphaned.

       ENOLCK	  Enforced record locking was enabled and {LOCK_MAX} regions  are already  locked
		  in  the  system,  or the system record lock table was full and the write  could
		  not go to sleep  until the blocking record lock was removed.

       ENOLINK	  The fildes argument is on a remote machine and the link to that machine  is  no
		  longer active.

       ENOSPC	  During a write to an ordinary file, there is no  free space left on the device.

       ENOSR	  An  attempt  is  made  to  write  to a STREAMS with insufficient STREAMS memory
		  resources  available in the system.

       ENXIO	  A hangup occurred on the STREAM being written to.

       EPIPE	  An attempt is made to write to a pipe or a FIFO that is not open for reading by
		  any  process, or that has only one end open (or to a file descriptor created by
		  socket(3SOCKET), using type SOCK_STREAM that is no longer connected to  a  peer
		  endpoint).  A  SIGPIPE signal will also be sent to the thread. The process dies
		  unless special provisions were taken to catch or ignore the signal.

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

       The write() and pwrite() functions will fail if:

       EFAULT	 The buf argument points to an illegal address.

       EINVAL	 The nbyte argument overflowed an ssize_t.

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

       ESPIPE	 The fildes argument is associated with a pipe or FIFO.

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

       EINVAL	 The  STREAM  or  multiplexer  referenced  by fildes is linked (directly or indi-
		 rectly) 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.

       The writev() function may fail if:

       EINVAL	 The iovcnt argument was less than or equal to 0 or greater than  {IOV_MAX};  one
		 of  the  iov_len values in the iov array was negative; or the sum of the iov_len
		 values in the iov array overflowed an ssize_t.

USAGE
       The pwrite() function has a transitional interface for 64-bit file offsets.  See lf64(5).

ATTRIBUTES
       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Committed 		   |
       +-----------------------------+-----------------------------+
       |MT-Level		     |write() is Async-Signal-Safe |
       +-----------------------------+-----------------------------+
       |Standard		     |See standards(5). 	   |
       +-----------------------------+-----------------------------+

SEE ALSO
       Intro(2), chmod(2), creat(2), dup(2), fcntl(2), getrlimit(2), ioctl(2), lseek(2), open(2),
       pipe(2),  ulimit(2), send(3SOCKET), socket(3SOCKET), attributes(5), lf64(5), standards(5),
       streamio(7I)

SunOS 5.11				   29 Jan 2008					 write(2)


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