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write(2) System Calls Manual write(2)
NAME top
write - write to a file descriptor
LIBRARY top
Standard C library (libc, -lc)
SYNOPSIS top
#include <unistd.h>
ssize_t write(size_t count;
int fd, const void buf[count], size_t count);
DESCRIPTION top
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 wr...write(2) System Calls Manual write(2)
NAME top
write - write to a file descriptor
LIBRARY top
Standard C library (libc, -lc)
SYNOPSIS top
#include <unistd.h>
ssize_t write(size_t count;
int fd, const void buf[count], size_t count);
DESCRIPTION top
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 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 top
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 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).
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 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 top
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 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
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 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.
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 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.
STANDARDS top
POSIX.1-2008.
HISTORY top
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 top
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 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 inconsistent.
BUGS top
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 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.
SEE ALSO top
close(2), fcntl(2), fsync(2), ioctl(2), lseek(2), open(2),
pwrite(2), read(2), select(2), writev(2), fwrite(3)
COLOPHON top
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
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2025-08-11. If you discover any rendering problems in this HTML
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part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-06-28 write(2)
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