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write(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 com‐
       pletion.


       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 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 interven‐
       ing  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 gener‐
       ates a SIGXFSZ signal, 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 speci‐
                  fied by the write() for that position until such byte  posi‐
                  tions 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_NON‐
                  BLOCK 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  previ‐
                  ously  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 can‐
       not 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 con‐
       tained 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 written. 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  can‐
                  not  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  written  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 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.

   pwrite()
       The pwrite() function is equivalent to write(), except that  it  writes
       into  a  given position 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:

         void    *iov_base;
         size_t  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  opera‐
       tion fails and no data is transferred.

RETURN VALUES
       Upon  successful  completion, write() returns the number of bytes actu‐
       ally 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  actu‐
       ally  written.   Otherwise,  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 contain‐
                  ing 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 description 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 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.


       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 off‐
       sets.  See lf64(5).

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




       tab()  box;  cw(2.75i)  |cw(2.75i)  lw(2.75i)   |lw(2.75i)   ATTRIBUTE
       TYPEATTRIBUTE VALUE _ Interface StabilityCommitted _ MT-Levelwrite()
       is Async-Signal-Safe _ StandardSee 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                        24 Mar 2011                         write(2)
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