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 pointed 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.
RETURN VALUE
On success, the number of bytes written is returned (zero indicates nothing was written). It is not an error if this number is smaller than the number of bytes requested; this may happen for example because the disk device was filled. See also NOTES. On error, -1 is returned, and errno is set appropriately. 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 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 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.
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.
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 committed to disk. In fact, on some buggy implementations, it does not even guarantee that space has successfully been reserved for the data. 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.)
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 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
close(2), fcntl(2), fsync(2), ioctl(2), lseek(2), open(2), pwrite(2), read(2), select(2), writev(2), fwrite(3)
COLOPHON
This page is part of release 4.09 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/.
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