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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 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 insuf-
       ficient 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 file systems are POSIX conform-
       ing.

RETURN VALUE
       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.

ERRORS
       EAGAIN The file descriptor fd refers to a file other than a  socket  and  has  been
              marked non-blocking (O_NONBLOCK), and the write would block.

       EAGAIN or EWOULDBLOCK
              The  file  descriptor fd refers to a socket and has been marked non-blocking
              (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  possibili-
              ties.

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

       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  posi-
              tion past the maximum allowed offset.

       EINTR  The  call  was interrupted by a signal before any data was written; see sig-
              nal(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  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.

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
       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  guar-
       antee  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.

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 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2009-02-23                          WRITE(2)

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