write(2) System Calls Manual write(2)
NAME
write - write to a file descriptor
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <unistd.h>
ssize_t write(int fd, const void buf[.count], 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 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 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 off-
set 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 errno is set to indicate 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 re-
quested bytes, or because a blocked write() to a socket, pipe, or simi-
lar 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 trans-
fer the remaining bytes. The subsequent call will either transfer fur-
ther 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 re-
turn a failure status if one of the errors below is detected. If no
errors are detected, or error detection is not performed, 0 is 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 implementa-
tion-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 writ-
ten; 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 ad-
dress 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 issued to a different file descrip-
tor 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 alter-
nate 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 happens the writing process will also receive a SIG-
PIPE 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.
STANDARDS
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 in-
teger 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(), 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 re-
turns 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 actu-
ally 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 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, before Linux 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 to updating the file off-
set, with the result that the blocks of data output by the two pro-
cesses might (incorrectly) 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), select(2), writev(2), fwrite(3)
Linux man-pages 6.03 2022-12-04 write(2)
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