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  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.

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
              nonblocking (O_NONBLOCK), and the write would block.

       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 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  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.

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