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man_2_write.txt
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WRITE(2) Linux Programmer's Manual WRITE(2)
NAME
write - write to a file descriptor
SYNOPSIS
#include <unistd.h>
ssize_t write(int fd, const void *buf, size_t count);
DESCRIPTION
write() writes up to count bytes from the buffer starting at 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 insuffi‐
cient 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 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) that can be proved to occur after a write() 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, the result is implementation-
defined; see NOTES for the upper limit on Linux.
RETURN VALUE
On success, the number of bytes written is returned. On error, -1 is returned, and er‐
rno 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 par‐
tial 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).
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, or error de‐
tection is not performed, 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 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.
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).
EDQUOT The user's quota of disk blocks on the filesystem containing the file referred
to by fd has been exhausted.
EFAULT buf is outside your accessible address space.
EFBIG An attempt was made to write a file that exceeds the implementation-defined max‐
imum 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 file offset is not suitably aligned.
EIO 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 is‐
sued 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.
ENOSPC The device containing the file referred to by fd has no room for the data.
EPERM The operation was prevented by a file seal; see fcntl(2).
EPIPE fd is connected to a pipe or socket whose reading end is closed. When this hap‐
pens 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 sig‐
nal.)
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
The types size_t and ssize_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 commit‐
ted 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 de‐
layed until a future write(), fsync(2), or even close(2). 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.
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 en‐
tire write has failed. Partial data may be written and the data at the file offset on
which the write() was attempted should be considered inconsistent.
BUGS
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(2). 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 open(2)) perform a write() (or writev(2)) at the
same time, then the I/O operations were not atomic with respect updating the file off‐
set, with the result that the blocks of data output by the two processes might (incor‐
rectly) overlap. This problem was fixed in Linux 3.14.
SEE ALSO
close(2), fcntl(2), fsync(2), ioctl(2), lseek(2), open(2), pwrite(2), read(2), se‐
lect(2), writev(2), fwrite(3)
COLOPHON
This page is part of release 5.05 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be
found at https://www.kernel.org/doc/man-pages/.
Linux 2019-10-10 WRITE(2)