write - write to a file descriptor
ssize_t write(int fd, const void *buf, size_t
() writes up to count
bytes from the buffer pointed
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
resource limit is encountered (see setrlimit
or the call was interrupted by a signal handler after having written less than
bytes. (See also pipe
For a seekable file (i.e., one to which lseek
(2) may be applied, for
example, a regular file) writing takes place at the file offset, and the file
offset is incremented by the number of bytes actually written. If the file was
(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) that can be proved to occur after a
() has returned will return the new data. Note that not all
filesystems are POSIX conforming.
According to POSIX.1, if count
is greater than SSIZE_MAX
result is implementation-defined; see NOTES for the upper limit on Linux.
On success, the number of bytes written is returned (zero indicates nothing was
written). It is not an error if this number is smaller than the number of
bytes requested; this may happen for example because the disk device was
filled. See also NOTES.
On error, -1 is returned, and errno
is set appropriately.
is zero and fd
refers to a regular file, then
() may return a failure status if one of the errors below is
detected. If no errors are detected, or error detection is not performed, 0
will be returned without causing any other effect. If count
is zero and
refers to a file other than a regular file, the results are not
- The file descriptor fd refers to a file other than a
socket and has been marked nonblocking (O_NONBLOCK), and the write
would block. See open(2) for further details on the
- 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.
- fd is not a valid file descriptor or is not open for
- fd refers to a datagram socket for which a peer
address has not been set using connect(2).
- The user's quota of disk blocks on the filesystem
containing the file referred to by fd has been exhausted.
- buf is outside your accessible address space.
- 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.
- The call was interrupted by a signal before any data was
written; see signal(7).
- 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 file offset is not suitably aligned.
- A low-level I/O error occurred while modifying the
- The device containing the file referred to by fd has
no room for the data.
- The operation was prevented by a file seal; see
- 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.
The types size_t
are, respectively, unsigned and
signed integer data types specified by POSIX.1.
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
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.
On Linux, write
() (and similar system calls) will transfer at most
0x7ffff000 (2,147,479,552) bytes, returning the number of bytes actually
transferred. (This is true on both 32-bit and 64-bit systems.)
According to POSIX.1-2008/SUSv4 Section XSI 2.9.7 ("Thread Interactions
with Regular File Operations"):
All of the following functions shall be atomic
with respect to each other in the effects specified in POSIX.1-2008 when they
operate on regular files or symbolic links: ...
Among the APIs subsequently listed are write
() and writev
among the effects that should be atomic across threads (and processes) are
updates of the file offset. However, on Linux before version 3.14, this was
not the case: if two processes that share an open file description (see
(2)) perform a write
() (or writev
(2)) at the same
time, then the I/O operations were not atomic with respect updating the file
offset, with the result that the blocks of data output by the two processes
might (incorrectly) overlap. This problem was fixed in Linux 3.14.
This page is part of release 4.13 of the Linux man-pages
description of the project, information about reporting bugs, and the latest
version of this page, can be found at