| NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON | The Linux Programming Interface |
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 file systems 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.
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).
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.
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.
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 3.32 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)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface