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.  On error,  -1  is
39       returned, and errno is set to indicate the error.
41       Note  that  a  successful  write() may transfer fewer than count bytes.
42       Such partial writes can occur for various reasons; for example, because
43       there was insufficient space on the disk device to write all of the re‐
44       quested bytes, or because a blocked write() to a socket, pipe, or simi‐
45       lar  was interrupted by a signal handler after it had transferred some,
46       but before it had transferred all of the requested bytes.  In the event
47       of  a partial write, the caller can make another write() call to trans‐
48       fer the remaining bytes.  The subsequent call will either transfer fur‐
49       ther bytes or may result in an error (e.g., if the disk is now full).
51       If  count is zero and fd refers to a regular file, then write() may re‐
52       turn a failure status if one of the errors below is  detected.   If  no
53       errors are detected, or error detection is not performed, 0 is returned
54       without causing any other effect.  If count is zero and fd refers to  a
55       file other than a regular file, the results are not specified.


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


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


123       The types size_t and ssize_t are, respectively, unsigned and signed in‐
124       teger data types specified by POSIX.1.
126       A  successful return from write() does not make any guarantee that data
127       has been committed to disk.  On some  filesystems,  including  NFS,  it
128       does  not  even guarantee that space has successfully been reserved for
129       the data.  In this case, some errors might be delayed  until  a  future
130       write(),  fsync(2),  or  even  close(2).  The only way to be sure is to
131       call fsync(2) after you are done writing all your data.
133       If a write() is interrupted by a signal handler before  any  bytes  are
134       written, then the call fails with the error EINTR; if it is interrupted
135       after at least one byte has been written, the call  succeeds,  and  re‐
136       turns the number of bytes written.
138       On  Linux,  write()  (and  similar  system calls) will transfer at most
139       0x7ffff000 (2,147,479,552) bytes, returning the number of  bytes  actu‐
140       ally transferred.  (This is true on both 32-bit and 64-bit systems.)
142       An  error  return  value while performing write() using direct I/O does
143       not mean the entire write has failed.  Partial data may be written  and
144       the  data  at the file offset on which the write() was attempted should
145       be considered inconsistent.


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


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


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177Linux                             2021-03-22                          WRITE(2)