Section: Linux Programmer's Manual (2)
write - write to a file descriptor
ssize_t write(int fd, const void *buf, size_t count);
writes up to
bytes from the buffer starting at
to the file referred to by the file descriptor
The number of bytes written may be less than
if, for example,
there is insufficient space on the underlying physical medium, or the
resource limit is encountered (see
or the call was interrupted by a signal
handler after having written less than
For a seekable file (i.e., one to which
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
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
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
is greater than
the 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, and errno
to indicate the cause of the error.
Note that a successful
may transfer fewer than
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
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
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).
If count is zero and
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.
count is zero and
refers to a file other than a regular file,
the results are not specified.
The file descriptor
refers to a file other than a socket and has been marked nonblocking
and the write would block.
for further details on the
- EAGAIN or EWOULDBLOCK
The file descriptor
refers to a socket and has been marked nonblocking
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.
is not a valid file descriptor or is not open for writing.
refers to a datagram socket for which a peer address has not been set using
The user's quota of disk blocks on the filesystem containing the file
referred to by
has been exhausted.
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
is attached to an object which is unsuitable for writing;
or the file was opened with the
flag, and either the address specified in
the value specified in
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
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
be reported by subsequent.
be reported by a subsequent
(whether or not they were also reported by
An alternate cause of
on networked filesystems is when an advisory lock had been taken out
on the file descriptor and this lock has been lost.
for further details.
The device containing the file referred to by
has no room for the data.
The operation was prevented by a file seal; see
is connected to a pipe or socket whose reading end is closed.
When this happens the writing process will also receive a
(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
SVr4, 4.3BSD, POSIX.1-2001.
Under SVr4 a write may be interrupted and return
at any point,
not just before any data is written.
unsigned and signed integer data types specified by POSIX.1.
A successful return from
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
The only way to be sure is to call
after you are done writing all your data.
is interrupted by a signal handler before any bytes are written,
then the call fails with the error
if it is interrupted after at least one byte has been written,
the call succeeds, and returns the number of bytes written.
(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
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
was attempted should be considered inconsistent.
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
And 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
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.
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