write(2)



WRITE(2)                   Linux Programmer's Manual                  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 starting at 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 off-
       set 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.

       According to POSIX.1, if count is greater than SSIZE_MAX, the result is
       implementation-defined; see NOTES for the upper limit on Linux.

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 re-
       turn 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 re-
       turned without causing any other effect.   If  count  is  zero  and  fd
       refers  to a file other than a regular file, the results are not speci-
       fied.

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 implementa-
              tion-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  writ-
              ten; 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  ad-
              dress  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.   This
              error may relate to the write-back of data written by an earlier
              write(2), which may have been issued to  a  different  file  de-
              scriptor on the same file.  Since Linux 4.13, errors from write-
              back come with a promise that they may  be  reported  by  subse-
              quent.   write(2) requests, and will be reported by a subsequent
              fsync(2) (whether or not they were also reported  by  write(2)).
              An  alternate  cause  of EIO on networked filesystems is when an
              advisory lock had been taken out on the file descriptor and this
              lock  has been lost.  See the Lost locks section of fcntl(2) for
              further details.

       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  SIG-
              PIPE  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 in-
       teger data types specified by POSIX.1.

       A successful return from write() does not make any guarantee that  data
       has  been  committed  to  disk.  On some filesystems, including NFS, it
       does not even guarantee that space has successfully been  reserved  for
       the  data.   In  this case, some errors might be delayed until a future
       write(2), fsync(2), or even close(2).  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 re-
       turns 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 actu-
       ally 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  de-
       scription  (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 4.16 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/.

Linux                             2018-02-02                          WRITE(2)

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