write - write to a file descriptor
ssize_t write(int fd, const void *buf, size_t
() writes up to count
bytes from the buffer starting at
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. On error, -1 is returned,
is set to indicate the cause of the error.
Note that a successful write
() may transfer fewer than count
bytes. Such partial writes can occur for various reasons; for example, because
there was insufficient space on the disk device to write all of the requested
bytes, or because a blocked write
() to a socket, pipe, or similar was
interrupted by a signal handler after it had transferred some, but before it
had transferred all of the requested bytes. In the event of a partial write,
the caller can make another write
() call to transfer the remaining
bytes. The subsequent call will either transfer further bytes or may result in
an error (e.g., if the disk is now full).
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 O_NONBLOCK flag.
- 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 writing.
- 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
- 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 inode. This error may
relate to the write-back of data written by an earlier write(),
which may have been issued to a different file descriptor on the same
file. Since Linux 4.13, errors from write-back come with a promise that
they may be reported by subsequent. write() requests, and
will be reported by a subsequent fsync(2) (whether or not
they were also reported by write()). An alternate cause of
EIO on networked filesystems is when an advisory lock had been
taken out on the file descriptor and this lock has been lost. See the
Lost locks section of fcntl(2) for further details.
- The device containing the file referred to by fd has no room for
- The operation was prevented by a file seal; see fcntl(2).
- 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. On some filesystems, including NFS, it does not
even guarantee that space has successfully been reserved for the data. In this
case, some errors might be delayed until a future write
(2), or even close
(2). The only way to be sure is to call
(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.
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.)
An error return value while performing write
() using direct I/O does not
mean the entire write has failed. Partial data may be written and the data at
the file offset on which the write
() was attempted should be considered
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.