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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 off-
       set);

       #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  sig-
            nal.


       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.

       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 mod-
            ified.

         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 pro-
            cesses, 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 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  writ-
            ten.  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  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 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 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.

   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 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  dead-
                       lock situation to occur.



       EDQUOT          The user's quota of disk blocks on the file system con-
                       taining 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 sys-
                       tem 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  insuffi-
                       cient  STREAMS  memory resources  available in the sys-
                       tem.



       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 sup-
                       ported 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 mul-
                       tiplexer.



       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()     allbox;     cw(2.750000i)|    cw(2.750000i)    lw(2.750000i)|
       lw(2.750000i).  ATTRIBUTE TYPEATTRIBUTE VALUE Interface  StabilityStan-
       dard MT-Levelwrite() is Async-Signal-Safe


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), streamio(7I)



SunOS 5.10                        1 Nov 2003                          write(2)