DESCRIPTION

write allows you to communicate with other users, by copying lines from your terminal to theirs.

When you run the write command, the user you are writing to gets a message of the form:

Any further lines you enter will be copied to the specified user_zsingle_quotesz_s terminal. If the other user wants to reply, they must run write as well.

When you are done, type an end-of-file or interrupt character. The other user will see the message EOF indicating that the conversation is over.

You can prevent people (other than the superuser) from writing to you with the mesg(1) command. Some commands, for example nroff(1) and pr(1), may automatically disallow writing, so that the output they produce isn_zsingle_quotesz_t overwritten.

If the user you want to write to is logged in on more than one terminal, you can specify which terminal to write to by giving the terminal name as the second operand to the write command. Alternatively, you can let write select one of the terminals − it will pick the one with the shortest idle time. This is so that if the user is logged in at work and also dialed up from home, the message will go to the right place.

The traditional protocol for writing to someone is that the string `−o_zsingle_quotesz_, either at the end of a line or on a line by itself, means that it_zsingle_quotesz_s the other person_zsingle_quotesz_s turn to talk. The string `oo_zsingle_quotesz_ means that the person believes the conversation to be over.

SEE ALSO

mesg(1), talk(1), who(1)

HISTORY

A write command appeared in Version 6 AT&T UNIX.

AVAILABILITY

The write command is part of the util-linux package and is available from https://www.kernel.org/pub/linux/utils/util-linux/.

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 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 errno 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 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 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

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. 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

According to POSIX.1-2008/SUSv4 Section XSI 2.9.7 (Thread Interactions with Regular File Operations):

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)