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.
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
has been marked non-blocking
(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'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.
This page is part of release 3.14 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/.
