man2/write.2

   1 .\" This manpage is Copyright (C) 1992 Drew Eckhardt;
   2 .\"             and Copyright (C) 1993 Michael Haardt, Ian Jackson.
   3 .\" and Copyright (C) 2007 Michael Kerrisk <mtk.manpages@gmail.com>
   4 .\"
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  26 .\"
  27 .\" Modified Sat Jul 24 13:35:59 1993 by Rik Faith <faith@cs.unc.edu>
  28 .\" Modified Sun Nov 28 17:19:01 1993 by Rik Faith <faith@cs.unc.edu>
  29 .\" Modified Sat Jan 13 12:58:08 1996 by Michael Haardt
  30 .\"   <michael@cantor.informatik.rwth-aachen.de>
  31 .\" Modified Sun Jul 21 18:59:33 1996 by Andries Brouwer <aeb@cwi.nl>
  32 .\" 2001-12-13 added remark by Zack Weinberg
  33 .\" 2007-06-18 mtk:
  34 .\"     Added details about seekable files and file offset.
  35 .\"     Noted that write() may write less than 'count' bytes, and
  36 .\"     gave some examples of why this might occur.
  37 .\"     Noted what happens if write() is interrupted by a signal.
  38 .\"
  39 .TH WRITE 2 2016-03-15 "Linux" "Linux Programmer's Manual"
  40 .SH NAME
  41 write \- write to a file descriptor
  42 .SH SYNOPSIS
  43 .B #include <unistd.h>
  44 .sp
  45 .BI "ssize_t write(int " fd ", const void *" buf ", size_t " count );
  46 .SH DESCRIPTION
  47 .BR write ()
  48 writes up to
  49 .I count
  50 bytes from the buffer pointed
  51 .I buf
  52 to the file referred to by the file descriptor
  53 .IR fd .
  54
  55 The number of bytes written may be less than
  56 .I count
  57 if, for example,
  58 there is insufficient space on the underlying physical medium, or the
  59 .B RLIMIT_FSIZE
  60 resource limit is encountered (see
  61 .BR setrlimit (2)),
  62 or the call was interrupted by a signal
  63 handler after having written less than
  64 .I count
  65 bytes.
  66 (See also
  67 .BR pipe (7).)
  68
  69 For a seekable file (i.e., one to which
  70 .BR lseek (2)
  71 may be applied, for example, a regular file)
  72 writing takes place at the file offset,
  73 and the file offset is incremented by
  74 the number of bytes actually written.
  75 If the file was
  76 .BR open (2)ed
  77 with
  78 .BR O_APPEND ,
  79 the file offset is first set to the end of the file before writing.
  80 The adjustment of the file offset and the write operation
  81 are performed as an atomic step.
  82
  83 POSIX requires that a
  84 .BR read (2)
  85 that can be proved to occur after a
  86 .BR write ()
  87 has returned will return the new data.
  88 Note that not all filesystems are POSIX conforming.
  89 .SH RETURN VALUE
  90 On success, the number of bytes written is returned (zero indicates
  91 nothing was written).
  92 It is not an error if this number is smaller than the number of bytes
  93 requested; this may happen for example because the disk device was filled.
  94 See also NOTES.
  95
  96 On error, \-1 is returned, and \fIerrno\fP is set
  97 appropriately.
  98
  99 If \fIcount\fP is zero and
 100 .I fd
 101 refers to a regular file, then
 102 .BR write ()
 103 may return a failure status if one of the errors below is detected.
 104 If no errors are detected, or error detection is not performed,
 105 0 will be returned without causing any other effect.
 106 If
 107 \fIcount\fP is zero and
 108 .I fd
 109 refers to a file other than a regular file,
 110 the results are not specified.
 111 .SH ERRORS
 112 .TP
 113 .B EAGAIN
 114 The file descriptor
 115 .I fd
 116 refers to a file other than a socket and has been marked nonblocking
 117 .RB ( O_NONBLOCK ),
 118 and the write would block.
 119 See
 120 .BR open (2)
 121 for further details on the
 122 .BR O_NONBLOCK
 123 flag.
 124 .TP
 125 .BR EAGAIN " or " EWOULDBLOCK
 126 .\" Actually EAGAIN on Linux
 127 The file descriptor
 128 .I fd
 129 refers to a socket and has been marked nonblocking
 130 .RB ( O_NONBLOCK ),
 131 and the write would block.
 132 POSIX.1-2001 allows either error to be returned for this case,
 133 and does not require these constants to have the same value,
 134 so a portable application should check for both possibilities.
 135 .TP
 136 .B EBADF
 137 .I fd
 138 is not a valid file descriptor or is not open for writing.
 139 .TP
 140 .B EDESTADDRREQ
 141 .I fd
 142 refers to a datagram socket for which a peer address has not been set using
 143 .BR connect (2).
 144 .TP
 145 .B EDQUOT
 146 The user's quota of disk blocks on the filesystem containing the file
 147 referred to by
 148 .I fd
 149 has been exhausted.
 150 .TP
 151 .B EFAULT
 152 .I buf
 153 is outside your accessible address space.
 154 .TP
 155 .B EFBIG
 156 An attempt was made to write a file that exceeds the implementation-defined
 157 maximum file size or the process's file size limit,
 158 or to write at a position past the maximum allowed offset.
 159 .TP
 160 .B EINTR
 161 The call was interrupted by a signal before any data was written; see
 162 .BR signal (7).
 163 .TP
 164 .B EINVAL
 165 .I fd
 166 is attached to an object which is unsuitable for writing;
 167 or the file was opened with the
 168 .B O_DIRECT
 169 flag, and either the address specified in
 170 .IR buf ,
 171 the value specified in
 172 .IR count ,
 173 or the file offset is not suitably aligned.
 174 .TP
 175 .B EIO
 176 A low-level I/O error occurred while modifying the inode.
 177 .TP
 178 .B ENOSPC
 179 The device containing the file referred to by
 180 .I fd
 181 has no room for the data.
 182 .TP
 183 .B EPERM
 184 The operation was prevented by a file seal; see
 185 .BR fcntl (2).
 186 .TP
 187 .B EPIPE
 188 .I fd
 189 is connected to a pipe or socket whose reading end is closed.
 190 When this happens the writing process will also receive a
 191 .B SIGPIPE
 192 signal.
 193 (Thus, the write return value is seen only if the program
 194 catches, blocks or ignores this signal.)
 195 .PP
 196 Other errors may occur, depending on the object connected to
 197 .IR fd .
 198 .SH CONFORMING TO
 199 SVr4, 4.3BSD, POSIX.1-2001.
 200 .\" SVr4 documents additional error
 201 .\" conditions EDEADLK, ENOLCK, ENOLNK, ENOSR, ENXIO, or ERANGE.
 202
 203 Under SVr4 a write may be interrupted and return
 204 .B EINTR
 205 at any point,
 206 not just before any data is written.
 207 .SH NOTES
 208 The types
 209 .I size_t
 210 and
 211 .I ssize_t
 212 are, respectively,
 213 unsigned and signed integer data types specified by POSIX.1.
 214
 215 A successful return from
 216 .BR write ()
 217 does not make any guarantee that data has been committed to disk.
 218 In fact, on some buggy implementations, it does not even guarantee
 219 that space has successfully been reserved for the data.
 220 The only way to be sure is to call
 221 .BR fsync (2)
 222 after you are done writing all your data.
 223
 224 If a
 225 .BR write ()
 226 is interrupted by a signal handler before any bytes are written,
 227 then the call fails with the error
 228 .BR EINTR ;
 229 if it is interrupted after at least one byte has been written,
 230 the call succeeds, and returns the number of bytes written.
 231
 232 On Linux,
 233 .BR write ()
 234 (and similar system calls) will transfer at most
 235 0x7ffff000 (2,147,479,552) bytes,
 236 returning the number of bytes actually transferred.
 237 .\" commit e28cc71572da38a5a12c1cfe4d7032017adccf69
 238 (This is true on both 32-bit and 64-bit systems.)
 239 .SH BUGS
 240 According to POSIX.1-2008/SUSv4 Section XSI 2.9.7
 241 ("Thread Interactions with Regular File Operations"):
 242
 243 .RS 4
 244 All of the following functions shall be atomic with respect to
 245 each other in the effects specified in POSIX.1-2008 when they
 246 operate on regular files or symbolic links: ...
 247 .RE
 248
 249 Among the APIs subsequently listed are
 250 .BR write ()
 251 and
 252 .BR writev (2).
 253 And among the effects that should be atomic across threads (and processes)
 254 are updates of the file offset.
 255 However, on Linux before version 3.14,
 256 this was not the case: if two processes that share
 257 an open file description (see
 258 .BR open (2))
 259 perform a
 260 .BR write ()
 261 (or
 262 .BR writev (2))
 263 at the same time, then the I/O operations were not atomic
 264 with respect updating the file offset,
 265 with the result that the blocks of data output by the two processes
 266 might (incorrectly) overlap.
 267 This problem was fixed in Linux 3.14.
 268 .\" http://thread.gmane.org/gmane.linux.kernel/1649458
 269 .\"    From: Michael Kerrisk (man-pages <mtk.manpages <at> gmail.com>
 270 .\"    Subject: Update of file offset on write() etc. is non-atomic with I/O
 271 .\"    Date: 2014-02-17 15:41:37 GMT
 272 .\"    Newsgroups: gmane.linux.kernel, gmane.linux.file-systems
 273 .\" commit 9c225f2655e36a470c4f58dbbc99244c5fc7f2d4
 274 .\"    Author: Linus Torvalds <torvalds@linux-foundation.org>
 275 .\"    Date:   Mon Mar 3 09:36:58 2014 -0800
 276 .\"
 277 .\"        vfs: atomic f_pos accesses as per POSIX
 278 .SH SEE ALSO
 279 .BR close (2),
 280 .BR fcntl (2),
 281 .BR fsync (2),
 282 .BR ioctl (2),
 283 .BR lseek (2),
 284 .BR open (2),
 285 .BR pwrite (2),
 286 .BR read (2),
 287 .BR select (2),
 288 .BR writev (2),
 289 .BR fwrite (3)