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 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 off-
       set, 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  sys-
       tems 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 appli-
	      cation 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).

       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 inter-
       rupted 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 3.27 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/.

Linux								    2010-08-29								  WRITE(2)