.\" This manpage is Copyright (C) 1992 Drew Eckhardt; .\" and Copyright (C) 1993 Michael Haardt, Ian Jackson. .\" and Copyright (C) 2007 Michael Kerrisk .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .\" Modified Sat Jul 24 13:35:59 1993 by Rik Faith .\" Modified Sun Nov 28 17:19:01 1993 by Rik Faith .\" Modified Sat Jan 13 12:58:08 1996 by Michael Haardt .\" .\" Modified Sun Jul 21 18:59:33 1996 by Andries Brouwer .\" 2001-12-13 added remark by Zack Weinberg .\" 2007-06-18 mtk: .\" Added details about seekable files and file offset. .\" Noted that write() may write less than 'count' bytes, and .\" gave some examples of why this might occur. .\" Noted what happens if write() is interrupted by a signal. .\" .TH WRITE 2 2014-05-04 "Linux" "Linux Programmer's Manual" .SH NAME write \- write to a file descriptor .SH SYNOPSIS .B #include .sp .BI "ssize_t write(int " fd ", const void *" buf ", size_t " count ); .SH DESCRIPTION .BR write () writes up to .I count bytes from the buffer pointed .I buf to the file referred to by the file descriptor .IR fd . The number of bytes written may be less than .I count if, for example, there is insufficient space on the underlying physical medium, or the .B RLIMIT_FSIZE resource limit is encountered (see .BR setrlimit (2)), or the call was interrupted by a signal handler after having written less than .I count bytes. (See also .BR pipe (7).) For a seekable file (i.e., one to which .BR lseek (2) may be applied, for example, a regular file) writing takes place at the current file offset, and the file offset is incremented by the number of bytes actually written. If the file was .BR open (2)ed with .BR 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 .BR read (2) which can be proved to occur after a .BR write () has returned returns the new data. Note that not all filesystems are POSIX conforming. .SH RETURN VALUE On success, the number of bytes written is returned (zero indicates nothing was written). On error, \-1 is returned, and \fIerrno\fP is set appropriately. If \fIcount\fP is zero and .I fd refers to a regular file, then .BR write () may return a failure status if one of the errors below is detected. If no errors are detected, 0 will be returned without causing any other effect. If \fIcount\fP is zero and .I fd refers to a file other than a regular file, the results are not specified. .SH ERRORS .TP .B EAGAIN The file descriptor .I fd refers to a file other than a socket and has been marked nonblocking .RB ( O_NONBLOCK ), and the write would block. .TP .BR EAGAIN " or " EWOULDBLOCK .\" Actually EAGAIN on Linux The file descriptor .I fd refers to a socket and has been marked nonblocking .RB ( 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. .TP .B EBADF .I fd is not a valid file descriptor or is not open for writing. .TP .B EDESTADDRREQ .I fd refers to a datagram socket for which a peer address has not been set using .BR connect (2). .TP .B EDQUOT The user's quota of disk blocks on the filesystem containing the file referred to by .I fd has been exhausted. .TP .B EFAULT .I buf is outside your accessible address space. .TP .B 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. .TP .B EINTR The call was interrupted by a signal before any data was written; see .BR signal (7). .TP .B EINVAL .I fd is attached to an object which is unsuitable for writing; or the file was opened with the .B O_DIRECT flag, and either the address specified in .IR buf , the value specified in .IR count , or the current file offset is not suitably aligned. .TP .B EIO A low-level I/O error occurred while modifying the inode. .TP .B ENOSPC The device containing the file referred to by .I fd has no room for the data. .TP .B EPIPE .I fd is connected to a pipe or socket whose reading end is closed. When this happens the writing process will also receive a .B SIGPIPE signal. (Thus, the write return value is seen only if the program catches, blocks or ignores this signal.) .PP Other errors may occur, depending on the object connected to .IR fd . .SH CONFORMING TO SVr4, 4.3BSD, POSIX.1-2001. .\" SVr4 documents additional error .\" conditions EDEADLK, ENOLCK, ENOLNK, ENOSR, ENXIO, or ERANGE. Under SVr4 a write may be interrupted and return .B EINTR at any point, not just before any data is written. .SH NOTES A successful return from .BR 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 .BR fsync (2) after you are done writing all your data. If a .BR write () is interrupted by a signal handler before any bytes are written, then the call fails with the error .BR EINTR ; if it is interrupted after at least one byte has been written, the call succeeds, and returns the number of bytes written. .SH BUGS According to POSIX.1-2008/SUSv4 Section XSI 2.9.7 ("Thread Interactions with Regular File Operations"): .RS 4 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: ... .RE Among the APIs subsequently listed are .BR write () and .BR 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 .BR open (2)) perform a .BR write () (or .BR 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. .\" http://thread.gmane.org/gmane.linux.kernel/1649458 .\" From: Michael Kerrisk (man-pages gmail.com> .\" Subject: Update of file offset on write() etc. is non-atomic with I/O .\" Date: 2014-02-17 15:41:37 GMT .\" Newsgroups: gmane.linux.kernel, gmane.linux.file-systems .\" commit 9c225f2655e36a470c4f58dbbc99244c5fc7f2d4 .\" Author: Linus Torvalds .\" Date: Mon Mar 3 09:36:58 2014 -0800 .\" .\" vfs: atomic f_pos accesses as per POSIX .SH SEE ALSO .BR close (2), .BR fcntl (2), .BR fsync (2), .BR ioctl (2), .BR lseek (2), .BR open (2), .BR pwrite (2), .BR read (2), .BR select (2), .BR writev (2), .BR fwrite (3) .SH COLOPHON This page is part of release 3.74 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%http://www.kernel.org/doc/man\-pages/.