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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(), 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() requests, and will be reported by a subsequent fsync(2) (whether
              or not they were also reported by write()).  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(), 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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