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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  descrip‐
       tor, 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.  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(4I)), 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 mes‐
       sage 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(4I)).


       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 has
       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:


       EADI       The buffer pointed to by buf or a buffer  specified  in  the
                  iov array is enabled for ADI, and an ADI version mismatch is
                  detected while the system reads data from  the  buffer.  For
                  more information, see the adi(3C) man page.


       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  is going 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
                  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 is caught during the write operation and no data is
                  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 is enabled  and  {LOCK_MAX}  regions
                  are  already locked in the system, or the system record lock
                  table is full and the write cannot go  to  sleep  until  the
                  blocking record lock is 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 occurs 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, using  type
                  SOCK_STREAM that is no longer connected to a peer endpoint).
                  A SIGPIPE signal is also be sent to the thread. The  process
                  dies  unless special provisions are taken to catch or ignore
                  the signal.


       ERANGE     The transfer request size is 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 or
                 socket.



       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 is made of a non-existent device, or the request is
                 outside the capabilities of the device.


       ENXIO     A hangup occurs 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 is less than or equal  to  0  or  greater
                 than {IOV_MAX}; one of the iov_len values in the iov array is
                 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(7).

ATTRIBUTES
       See attributes(7) 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-Lev‐
       elwrite() is Async-Signal-Safe _ StandardSee standards(7).


SEE ALSO
       chmod(2), creat(2), dup(2), fcntl(2), getrlimit(2), Intro(2), ioctl(2),
       lseek(2), open(2),  pipe(2),  ulimit(2),  streamio(4I),  attributes(7),
       lf64(7), standards(7)



Oracle Solaris 11.4               27 Nov 2017                         write(2)
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