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WRITE(2)                            Linux Programmer's Manual                            WRITE(2)

       write - write to a file descriptor

       #include <unistd.h>

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

       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 current 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) which can be proved to occur after a write() has returned
       returns the new data.  Note that not all filesystems are POSIX conforming.

       On success, the number of bytes written is returned (zero indicates nothing was  written).
       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,  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.

       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.

              The  file  descriptor fd refers to a socket and has been marked nonblocking (O_NON‐
              BLOCK), 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.

              fd refers to a datagram socket for which a peer address has not been set using con‐

       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 max‐
              imum 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 speci‐
              fied in count, or the current 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.

       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.

       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.

       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.

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

           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.

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

       This page is part of release 3.74 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2014-05-04                                   WRITE(2)

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