write(2)

NAME

   write - write to a file descriptor

SYNOPSIS

   #include <unistd.h>

   ssize_t write(int fd, const void *buf, size_t count);

DESCRIPTION

   write()  writes  up  to  count bytes from the buffer pointed buf to the
   file referred to by the file descriptor fd.

   The number of bytes written may be less than  count  if,  for  example,
   there  is  insufficient space on the underlying physical medium, or the
   RLIMIT_FSIZE resource limit is encountered (see setrlimit(2)),  or  the
   call was interrupted by a signal handler after having written less than
   count bytes.  (See also pipe(7).)

   For a seekable file (i.e., one to which lseek(2) may  be  applied,  for
   example,  a  regular  file) writing takes place at the file offset, and
   the file offset is incremented by the number of bytes actually written.
   If  the  file was open(2)ed with O_APPEND, the file offset is first set
   to the end of the file before writing.   The  adjustment  of  the  file
   offset and the write operation are performed as an atomic step.

   POSIX  requires  that  a  read(2)  that  can be proved to occur after a
   write() has returned will return the  new  data.   Note  that  not  all
   filesystems are POSIX conforming.

RETURN VALUE

   On  success,  the  number  of bytes written is returned (zero indicates
   nothing was written).  It is not an error if  this  number  is  smaller
   than the number of bytes requested; this may happen for example because
   the disk device was filled.  See also NOTES.

   On error, -1 is returned, and errno is set appropriately.

   If count is zero and fd refers to a  regular  file,  then  write()  may
   return  a failure status if one of the errors below is detected.  If no
   errors are detected, or error detection is not  performed,  0  will  be
   returned  without  causing  any  other effect.  If count is zero and fd
   refers to a file other  than  a  regular  file,  the  results  are  not
   specified.

ERRORS

   EAGAIN The  file descriptor fd refers to a file other than a socket and
          has been marked nonblocking (O_NONBLOCK), and  the  write  would
          block.  See open(2) for further details on the O_NONBLOCK flag.

   EAGAIN or EWOULDBLOCK
          The  file  descriptor  fd refers to a socket and has been marked
          nonblocking   (O_NONBLOCK),   and   the   write   would   block.
          POSIX.1-2001  allows  either error to be returned for this case,
          and does not require these constants to have the same value,  so
          a portable application should check for both possibilities.

   EBADF  fd is not a valid file descriptor or is not open for writing.

   EDESTADDRREQ
          fd  refers to a datagram socket for which a peer address has not
          been set using connect(2).

   EDQUOT The user's quota of disk blocks on the filesystem containing the
          file referred to by fd has been exhausted.

   EFAULT buf is outside your accessible address space.

   EFBIG  An   attempt   was  made  to  write  a  file  that  exceeds  the
          implementation-defined maximum file size or the  process's  file
          size  limit,  or to write at a position past the maximum allowed
          offset.

   EINTR  The call was  interrupted  by  a  signal  before  any  data  was
          written; see signal(7).

   EINVAL fd  is attached to an object which is unsuitable for writing; or
          the file was opened with  the  O_DIRECT  flag,  and  either  the
          address  specified  in buf, the value specified in count, or the
          file offset is not suitably aligned.

   EIO    A low-level I/O error occurred while modifying the inode.

   ENOSPC The device containing the file referred to by fd has no room for
          the data.

   EPERM  The operation was prevented by a file seal; see fcntl(2).

   EPIPE  fd is connected to a pipe or socket whose reading end is closed.
          When this happens  the  writing  process  will  also  receive  a
          SIGPIPE  signal.   (Thus, the write return value is seen only if
          the program catches, blocks or ignores this signal.)

   Other errors may occur, depending on the object connected to fd.

CONFORMING TO

   SVr4, 4.3BSD, POSIX.1-2001.

   Under SVr4 a write may be interrupted and return EINTR  at  any  point,
   not just before any data is written.

NOTES

   The  types  size_t  and  ssize_t are, respectively, unsigned and signed
   integer data types specified by POSIX.1.

   A successful return from write() does not make any guarantee that  data
   has been committed to disk.  In fact, on some buggy implementations, it
   does not even guarantee that space has successfully been  reserved  for
   the  data.   The  only way to be sure is to call fsync(2) after you are
   done writing all your data.

   If a write() is interrupted by a signal handler before  any  bytes  are
   written, then the call fails with the error EINTR; if it is interrupted
   after at least one byte  has  been  written,  the  call  succeeds,  and
   returns the number of bytes written.

   On  Linux,  write()  (and  similar  system calls) will transfer at most
   0x7ffff000  (2,147,479,552)  bytes,  returning  the  number  of   bytes
   actually  transferred.   (This  is  true  on  both  32-bit  and  64-bit
   systems.)

BUGS

   According to POSIX.1-2008/SUSv4 Section XSI 2.9.7 ("Thread Interactions
   with Regular File Operations"):

       All of the following functions shall be atomic with respect to each
       other in the effects specified in POSIX.1-2008 when they operate on
       regular files or symbolic links: ...

   Among  the  APIs  subsequently  listed  are write() and writev(2).  And
   among the effects that should be atomic across threads (and  processes)
   are updates of the file offset.  However, on Linux before version 3.14,
   this was not the case:  if  two  processes  that  share  an  open  file
   description  (see open(2)) perform a write() (or writev(2)) at the same
   time, then the I/O operations were not atomic with respect updating the
   file  offset, with the result that the blocks of data output by the two
   processes might (incorrectly) overlap.  This problem was fixed in Linux
   3.14.

SEE ALSO

   close(2),  fcntl(2),  fsync(2), ioctl(2), lseek(2), open(2), pwrite(2),
   read(2), select(2), writev(2), fwrite(3)

COLOPHON

   This page is part of release 4.09 of the Linux  man-pages  project.   A
   description  of  the project, information about reporting bugs, and the
   latest    version    of    this    page,    can     be     found     at
   https://www.kernel.org/doc/man-pages/.

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