1WRITE(2) Linux Programmer's Manual WRITE(2)
2
6 write - write to a file descriptor
7
9 #include <unistd.h>
10 ssize_t write(int fd, const void *buf, size_t count);
12
14 write() writes up to count bytes from the buffer starting at buf to the
15 file referred to by the file descriptor fd.
16 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).)
22 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.
29 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.
33 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.
36
38 On success, the number of bytes written is returned. On error, -1 is
39 returned, and errno is set to indicate the cause of the error.
40 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
44 requested bytes, or because a blocked write() to a socket, pipe, or
45 similar was interrupted by a signal handler after it had transferred
46 some, but before it had transferred all of the requested bytes. In the
47 event of a partial write, the caller can make another write() call to
48 transfer the remaining bytes. The subsequent call will either transfer
49 further bytes or may result in an error (e.g., if the disk is now
50 full).
51 If count is zero and fd refers to a regular file, then write() may
53 return a failure status if one of the errors below is detected. If no
54 errors are detected, or error detection is not performed, 0 will be
55 returned without causing any other effect. If count is zero and fd
56 refers to a file other than a regular file, the results are not speci‐
57 fied.
58
60 EAGAIN The file descriptor fd refers to a file other than a socket and
61 has been marked nonblocking (O_NONBLOCK), and the write would
62 block. See open(2) for further details on the O_NONBLOCK flag.
63 EAGAIN or EWOULDBLOCK
65 The file descriptor fd refers to a socket and has been marked
66 nonblocking (O_NONBLOCK), and the write would block.
67 POSIX.1-2001 allows either error to be returned for this case,
68 and does not require these constants to have the same value, so
69 a portable application should check for both possibilities.
70 EBADF fd is not a valid file descriptor or is not open for writing.
72 EDESTADDRREQ
74 fd refers to a datagram socket for which a peer address has not
75 been set using connect(2).
76 EDQUOT The user's quota of disk blocks on the filesystem containing the
78 file referred to by fd has been exhausted.
79 EFAULT buf is outside your accessible address space.
81 EFBIG An attempt was made to write a file that exceeds the implementa‐
83 tion-defined maximum file size or the process's file size limit,
84 or to write at a position past the maximum allowed offset.
85 EINTR The call was interrupted by a signal before any data was writ‐
87 ten; see signal(7).
88 EINVAL fd is attached to an object which is unsuitable for writing; or
90 the file was opened with the O_DIRECT flag, and either the
91 address specified in buf, the value specified in count, or the
92 file offset is not suitably aligned.
93 EIO A low-level I/O error occurred while modifying the inode. This
95 error may relate to the write-back of data written by an earlier
96 write(), which may have been issued to a different file descrip‐
97 tor on the same file. Since Linux 4.13, errors from write-back
98 come with a promise that they may be reported by subsequent.
99 write() requests, and will be reported by a subsequent fsync(2)
100 (whether or not they were also reported by write()). An alter‐
101 nate cause of EIO on networked filesystems is when an advisory
102 lock had been taken out on the file descriptor and this lock has
103 been lost. See the Lost locks section of fcntl(2) for further
104 details.
105 ENOSPC The device containing the file referred to by fd has no room for
107 the data.
108 EPERM The operation was prevented by a file seal; see fcntl(2).
110 EPIPE fd is connected to a pipe or socket whose reading end is closed.
112 When this happens the writing process will also receive a SIG‐
113 PIPE signal. (Thus, the write return value is seen only if the
114 program catches, blocks or ignores this signal.)
115 Other errors may occur, depending on the object connected to fd.
117
119 SVr4, 4.3BSD, POSIX.1-2001.
120 Under SVr4 a write may be interrupted and return EINTR at any point,
122 not just before any data is written.
123
125 The types size_t and ssize_t are, respectively, unsigned and signed
126 integer data types specified by POSIX.1.
127 A successful return from write() does not make any guarantee that data
129 has been committed to disk. On some filesystems, including NFS, it
130 does not even guarantee that space has successfully been reserved for
131 the data. In this case, some errors might be delayed until a future
132 write(), fsync(2), or even close(2). The only way to be sure is to
133 call fsync(2) after you are done writing all your data.
134 If a write() is interrupted by a signal handler before any bytes are
136 written, then the call fails with the error EINTR; if it is interrupted
137 after at least one byte has been written, the call succeeds, and
138 returns the number of bytes written.
139 On Linux, write() (and similar system calls) will transfer at most
141 0x7ffff000 (2,147,479,552) bytes, returning the number of bytes actu‐
142 ally transferred. (This is true on both 32-bit and 64-bit systems.)
143 An error return value while performing write() using direct I/O does
145 not mean the entire write has failed. Partial data may be written and
146 the data at the file offset on which the write() was attempted should
147 be considered inconsistent.
148
150 According to POSIX.1-2008/SUSv4 Section XSI 2.9.7 ("Thread Interactions
151 with Regular File Operations"):
152 All of the following functions shall be atomic with respect to each
154 other in the effects specified in POSIX.1-2008 when they operate on
155 regular files or symbolic links: ...
156 Among the APIs subsequently listed are write() and writev(2). And
158 among the effects that should be atomic across threads (and processes)
159 are updates of the file offset. However, on Linux before version 3.14,
160 this was not the case: if two processes that share an open file
161 description (see open(2)) perform a write() (or writev(2)) at the same
162 time, then the I/O operations were not atomic with respect updating the
163 file offset, with the result that the blocks of data output by the two
164 processes might (incorrectly) overlap. This problem was fixed in Linux
165 3.14.
166
168 close(2), fcntl(2), fsync(2), ioctl(2), lseek(2), open(2), pwrite(2),
169 read(2), select(2), writev(2), fwrite(3)
170
172 This page is part of release 5.04 of the Linux man-pages project. A
173 description of the project, information about reporting bugs, and the
174 latest version of this page, can be found at
175 https://www.kernel.org/doc/man-pages/.
176Linux 2019-10-10 WRITE(2)