1WRITE(2)                   Linux Programmer's Manual                  WRITE(2)


6       write - write to a file descriptor


9       #include <unistd.h>
11       ssize_t write(int fd, const void *buf, size_t count);


14       write() writes up to count bytes from the buffer starting at buf to the
15       file referred to by the file descriptor fd.
17       The number of bytes written may be less than  count  if,  for  example,
18       there  is  insufficient space on the underlying physical medium, or the
19       RLIMIT_FSIZE resource limit is encountered (see setrlimit(2)),  or  the
20       call was interrupted by a signal handler after having written less than
21       count bytes.  (See also pipe(7).)
23       For a seekable file (i.e., one to which lseek(2) may  be  applied,  for
24       example,  a  regular  file) writing takes place at the file offset, and
25       the file offset is incremented by the number of bytes actually written.
26       If  the  file was open(2)ed with O_APPEND, the file offset is first set
27       to the end of the file before writing.  The adjustment of the file off‐
28       set and the write operation are performed as an atomic step.
30       POSIX  requires  that  a  read(2)  that  can be proved to occur after a
31       write() has returned will return the  new  data.   Note  that  not  all
32       filesystems are POSIX conforming.
34       According to POSIX.1, if count is greater than SSIZE_MAX, the result is
35       implementation-defined; see NOTES for the upper limit on Linux.


38       On success, the number of bytes written  is  returned  (zero  indicates
39       nothing  was  written).   It  is not an error if this number is smaller
40       than the number of bytes requested; this may happen for example because
41       the disk device was filled.  See also NOTES.
43       On error, -1 is returned, and errno is set appropriately.
45       If  count  is  zero  and  fd refers to a regular file, then write() may
46       return a failure status if one of the errors below is detected.  If  no
47       errors  are  detected,  or  error detection is not performed, 0 will be
48       returned without causing any other effect.  If count  is  zero  and  fd
49       refers  to a file other than a regular file, the results are not speci‐
50       fied.


53       EAGAIN The file descriptor fd refers to a file other than a socket  and
54              has  been  marked  nonblocking (O_NONBLOCK), and the write would
55              block.  See open(2) for further details on the O_NONBLOCK flag.
58              The file descriptor fd refers to a socket and  has  been  marked
59              nonblocking   (O_NONBLOCK),   and   the   write   would   block.
60              POSIX.1-2001 allows either error to be returned for  this  case,
61              and  does not require these constants to have the same value, so
62              a portable application should check for both possibilities.
64       EBADF  fd is not a valid file descriptor or is not open for writing.
67              fd refers to a datagram socket for which a peer address has  not
68              been set using connect(2).
70       EDQUOT The user's quota of disk blocks on the filesystem containing the
71              file referred to by fd has been exhausted.
73       EFAULT buf is outside your accessible address space.
75       EFBIG  An attempt was made to write a file that exceeds the implementa‐
76              tion-defined maximum file size or the process's file size limit,
77              or to write at a position past the maximum allowed offset.
79       EINTR  The call was interrupted by a signal before any data  was  writ‐
80              ten; see signal(7).
82       EINVAL fd  is attached to an object which is unsuitable for writing; or
83              the file was opened with  the  O_DIRECT  flag,  and  either  the
84              address  specified  in buf, the value specified in count, or the
85              file offset is not suitably aligned.
87       EIO    A low-level I/O error occurred while modifying the inode.   This
88              error may relate to the write-back of data written by an earlier
89              write(2), which  may  have  been  issued  to  a  different  file
90              descriptor  on  the  same  file.   Since Linux 4.13, errors from
91              write-back come with a promise that they may be reported by sub‐
92              sequent.   write(2)  requests,  and will be reported by a subse‐
93              quent fsync(2) (whether  or  not  they  were  also  reported  by
94              write(2)).   An  alternate cause of EIO on networked filesystems
95              is when an advisory lock had been taken out on the file descrip‐
96              tor  and this lock has been lost.  See the Lost locks section of
97              fcntl(2) for further details.
99       ENOSPC The device containing the file referred to by fd has no room for
100              the data.
102       EPERM  The operation was prevented by a file seal; see fcntl(2).
104       EPIPE  fd is connected to a pipe or socket whose reading end is closed.
105              When this happens the writing process will also receive  a  SIG‐
106              PIPE  signal.  (Thus, the write return value is seen only if the
107              program catches, blocks or ignores this signal.)
109       Other errors may occur, depending on the object connected to fd.


112       SVr4, 4.3BSD, POSIX.1-2001.
114       Under SVr4 a write may be interrupted and return EINTR  at  any  point,
115       not just before any data is written.


118       The  types  size_t  and  ssize_t are, respectively, unsigned and signed
119       integer data types specified by POSIX.1.
121       A successful return from write() does not make any guarantee that  data
122       has  been  committed  to  disk.  On some filesystems, including NFS, it
123       does not even guarantee that space has successfully been  reserved  for
124       the  data.   In  this case, some errors might be delayed until a future
125       write(2), fsync(2), or even close(2).  The only way to be  sure  is  to
126       call fsync(2) after you are done writing all your data.
128       If  a  write()  is interrupted by a signal handler before any bytes are
129       written, then the call fails with the error EINTR; if it is interrupted
130       after  at  least  one  byte  has  been  written, the call succeeds, and
131       returns the number of bytes written.
133       On Linux, write() (and similar system  calls)  will  transfer  at  most
134       0x7ffff000  (2,147,479,552)  bytes, returning the number of bytes actu‐
135       ally transferred.  (This is true on both 32-bit and 64-bit systems.)


138       According to POSIX.1-2008/SUSv4 Section XSI 2.9.7 ("Thread Interactions
139       with Regular File Operations"):
141           All of the following functions shall be atomic with respect to each
142           other in the effects specified in POSIX.1-2008 when they operate on
143           regular files or symbolic links: ...
145       Among  the  APIs  subsequently  listed  are write() and writev(2).  And
146       among the effects that should be atomic across threads (and  processes)
147       are updates of the file offset.  However, on Linux before version 3.14,
148       this was not the case:  if  two  processes  that  share  an  open  file
149       description  (see open(2)) perform a write() (or writev(2)) at the same
150       time, then the I/O operations were not atomic with respect updating the
151       file  offset, with the result that the blocks of data output by the two
152       processes might (incorrectly) overlap.  This problem was fixed in Linux
153       3.14.


156       close(2),  fcntl(2),  fsync(2), ioctl(2), lseek(2), open(2), pwrite(2),
157       read(2), select(2), writev(2), fwrite(3)


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167Linux                             2018-02-02                          WRITE(2)