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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
       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.

       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.  On error, -1 is returned, and errno
       is set to indicate the cause of the error.

       Note that a successful write() may transfer fewer than count bytes.  Such  partial  writes
       can  occur  for  various reasons; for example, because there was insufficient space on the
       disk device to write all of the requested bytes, or because a blocked write() to a socket,
       pipe,  or  similar  was interrupted by a signal handler after it had transferred some, but
       before it had transferred all of the requested bytes.  In the event of  a  partial  write,
       the  caller can make another write() call to transfer the remaining bytes.  The subsequent
       call will either transfer further bytes or may result in an error (e.g., if  the  disk  is
       now full).

       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.  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  descriptor on the same file.  Since Linux 4.13, errors from
              write-back come with a promise that they may be reported by  subsequent.   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 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.   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 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.)

       An error return value while performing write() using direct I/O does not mean  the  entire
       write has failed. Partial data may be written and the data at the file offset on which the
       write() was attempted should be considered inconsistent.

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

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       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.