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 to the file referenced by the file descriptor fd from the buffer starting at buf. 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 conforming.

RETURN VALUE

On success, the number of bytes written are 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

Non-blocking I/O has been selected using O_NONBLOCK and the write would block.

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' 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.

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.

SEE ALSO

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


This manpage is Copyright (C) 1992 Drew Eckhardt;
                              1993 Michael Haardt, Ian Jackson.

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
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Formatted or processed versions of this manual, if unaccompanied by
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Modified Sat Jul 24 13:35:59 1993 by Rik Faith <faith@cs.unc.edu>
Modified Sun Nov 28 17:19:01 1993 by Rik Faith <faith@cs.unc.edu>
Modified Sat Jan 13 12:58:08 1996 by Michael Haardt
  <michael@cantor.informatik.rwth-aachen.de>
Modified Sun Jul 21 18:59:33 1996 by Andries Brouwer <aeb@cwi.nl>
2001-12-13 added remark by Zack Weinberg